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A motor imbalance caused by paralysis of one or several extraocular muscles must be clearly distinguished from comitant forms of strabismus whenever possible because correct identification of a paretic muscle or muscle groups is of paramount importance for the success of therapy. Moreover, an acquired paralysis may signal a condition that can affect the patient’s general health. Diagnosis of a paralysis of recent onset is not particularly difficult and is based on the presence of a motor deficiency in the field of action of the paralyzed muscle, diplopia, increase of the deviation when the patient fixates with the paralyzed eye, and in many instances, a compensatory anomaly of the head posture. However, diagnosis of a congenital or long-standing paralysis can present more of a clinical challenge. The emphasis in this chapter is on the diagnostic and differential diagnostic aspects of paralytic strabismus. For information on the neuro-ophthalmologic implications of paralytic strabismus, refer to the standard texts on this subject. The terms paretic and paralytic often are used interchangeably in clinical ophthalmology, even though paretic denotes only a partial or incomplete paralysis. The terms paralysis and palsy are synonyms. We distinguish between complete and partial paralysis whenever necessary in this chapter. Diagnosis and Clinical Characteristics Unlike nonparalytic comitant strabismus in which

the deviation may remain fairly stable or change only gradually in magnitude with time, paralytic strabismus is characterized by a more dynamic course of events. Onset is usually sudden and the patient immediately recognizes the problem as he or she becomes aware of diplopia. The clinical picture may change significantly, however, within a few weeks after the onset of either a paresis or paralysis, because of a profound effect on the innervational equilibrium of the entire oculomotor apparatus, which affects not only the paralyzed eye but also, under certain circumstances, the sound eye. A paralytic deviation undergoes several stages. The first stage is characterized by weakness of the paralyzed muscle followed, as a rule, by overaction of its direct antagonist. During this stage the maximal deviation is still in the field of action of the paralyzed muscle; for example, in a patient with a right superior oblique paralysis of recent onset, the largest amount of right hypertropia will be in the left lower field of gaze. This stage is followed by one in which overaction of the antagonist of the paralyzed muscle is the principal clinical feature. For instance, with paralysis of the right superior oblique, hypertropia will not be restricted to the field of action of the paralyzed muscle but may reach equal proportions in the entire left field of gaze or even increase when the patient is looking up and to the left. One of the most characteristic features during this stage is the fact that overaction and subsequent contracture of an antagonist muscle may overshadow defective action of the paralyzed muscle and persist long after the paralysis has subsided.

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The clinical application of the term contracture is with reference to increased resistance against passive stretching of the muscle. This loss of elasticity has been related to histologic alterations consisting of atrophy of muscle fibers and hyalinization of the normal muscle.1, 2 During the third stage the deviation will spread into all fields of gaze and become increasingly comitant. It may then no longer be possible to detect a paretic component, and the angle of strabismus may be of the same magnitude with either the paralyzed or sound eye fixating. This development has been referred to as spread of comitance. In a patient with right superior oblique paralysis, the right hypertropia would then be present in primary position and levoversion as well as in dextroversion. A paralytic deviation does not necessarily progress from the first to the last stage. Occasionally, and for unknown reasons, the antagonist of the paralyzed muscle does not overact, and the deviation remains limited to the field of action of the paralyzed muscle. In most patients, however, comitance spreads within a few weeks, months, or even years. Ductions and Versions If the paralysis is of recent onset, a careful study of ductions and versions, as outlined in Chapter 12, will readily disclose the weak, paralyzed muscle. A paralyzed medial or lateral rectus muscle is identified by its deficient action in adduction or abduction. A paralytic hyperdeviation is slightly more difficult to diagnose because it is necessary to differentiate between a pair of elevators or depressors in each eye. Once it has been determined that a right or left hypertropia is present in primary position, it must be established whether the deviation increases on dextroversion or levoversion. An increase of a left hyperdeviation in dextroversion, for instance, may be caused by paralysis of the left superior oblique or the right superior rectus muscles. When the paralysis is of recent onset, the diagnosis is made on the basis of incomplete duction in the field of action of the respective rectus or oblique muscles. However, to restrict the examination to ductions may mask a paresis, since the patient may overcome the muscle weakness by maximal innervational effort when fixating with the paretic eye. More revealing is the examination of versions, for under these circumstances the patient will show marked

overaction of the yoke muscle of the paretic muscle in the contralateral eye (secondary deviation) when fixating with the paretic eye. In patients who have a paresis of longer standing, the head tilt test (see p. 416) may be used to differentiate between a paretic elevator muscle in one eye and a paretic depressor muscle in the other. A contracture of the antagonist of the paralyzed muscle not only may obscure the nature of the primary defect in the paralyzed eye but also may affect motor balance of the fellow eye when the patient habitually fixates with the paralyzed eye. This is not an uncommon finding when the paralysis affects a strongly dominant eye. The antagonist of a paralyzed muscle will require less innervation to move the eye in its field of action since the normal tonus of its paralyzed opponent is decreased. Consequently, according to Hering’s law of equal innervation, the yoke muscle of the antagonist of a paretic or paralyzed muscle will receive less innervation than required and will be apparently underacting. This phenomenon, which Chavasse45, p.232 somewhat awkwardly designated inhibitional palsy of the contralateral antagonist, may present difficulties in identifying the paralyzed eye. For instance, in a patient with a left superior oblique paralysis who habitually fixates with the paretic eye and in whom overaction of the homolateral inferior oblique muscle has developed, less than the normal amount of innervation will be required when he or she is looking up and to the right. Since the innervation flowing to the right normal eye is determined by the left inferior oblique muscle, the right superior rectus will seem paretic (Fig. 20–1). The head tilt test is then used to determine which of the two muscles, left superior oblique or right superior rectus, is paretic (see Figs. 20–3, 20–4). Measurement of the Deviation Although evaluation of ductions and versions is sufficient to identify gross defects of ocular motility, a quantitative study of the angle of deviation in diagnostic positions of gaze with either eye fixating will reveal more subtle forms of paralysis. This examination is essential in establishing severity of the disturbance and in assessing whether further deterioration will occur or recovery will take place. These measurements are obtained using objective (prism and cover test) or subjective (diplopia fields) methods, as outlined in Chapter 12. Such tests, with the patient fixating

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FIGURE 20–1. Inhibitional palsy of the right superior rectus (upper left) in a patient with left superior oblique palsy who fixates with the paralyzed eye. Note normal right superior rectus action and marked overaction of the left inferior oblique under the translucent cover when the right eye is fixating (left), and the left eye is covered with a Spielmann translucent occluder (see Chapter 12).

first with one eye and then the other, are of fundamental diagnostic importance, since a difference between a primary deviation (nonparetic eye fixating) and a secondary deviation (paretic eye fixating) clearly distinguishes paralytic from nonparalytic strabismus; the secondary deviation is always greater than the primary deviation. According to Hering’s law of equal innervation (see Chapter 4), the innervation flowing to the yoke muscles of both eyes is always determined by the fixating eye. Thus the angle of deviation will vary depending on whether the patient fixates with the sound eye or the paretic eye. For instance, in a patient with a left superior rectus paralysis, when the right eye is fixating, the normal amount of innervation will maintain the right eye in primary position. A left hypotropia will be present, since the innervation flowing to the paretic eye is not sufficient to elevate it to the midline. If, however, the patient fixates with the left paretic eye, excess innervation will be required to move it into primary position and the same amount of innervation will flow to the yoke muscles of the right eye, causing it to elevate excessively (see Fig. 20–1). The derivation of the term yoke muscle was discussed on page 63. To this discussion we should like to add a historical note. The first to use this term in reference to synergistic muscles in paralytic strabismus was the neurologist Gowers who in 1888 wrote "it is as if a rein acted equally on a hard-mouthed and a tender-mouthed horse,

yoked together; the effort to make the former deviate would cause an excessive deviation of the latter."87 To assess the extent of functional impairment caused by double vision in patients with paralytic strabismus, it is helpful to chart the patient’s field of binocular fixation by means of a perimeter (see Fig. 20–30). Such records are invaluable not only for documenting subtle changes in terms of progression or improvement of a paralyzed muscle219 but also for medicolegal purposes as a record of the patient’s disability. Paralysis of the cyclovertical muscles invariably causes cyclotropia, and its diagnosis and measurement (see Chapters 12 and 18) provide important diagnostic clues. In patients who fixate with the paretic eye the cyclodeviation may appear in the normal eye—a phenomenon that occasionally causes confusion in diagnosis208 and has been referred to as paradoxical cyclotropia. Head Tilt Test The head tilt test is alluded to in Chapter 12. The physiologic basis of the head tilt test was explained by Hofmann and Bielschowsky112 and it has become universally known as the "Bielschowsky head tilt test." However, 30 years before Hofmann and Bielschowsky, Nagel191 noted that the combined action of the superior rectus muscle and

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the superior oblique muscle of one eye and of the inferior rectus and inferior oblique muscles in the fellow eye causes incycloduction and excycloduction. Nagel also hinted that with a cyclovertical paresis the deviation would be more noticeable with appropriate tipping of the head. This theory was fully confirmed on clinical grounds by Hofmann and Bielschowsky, who gave the following explanation for the head tilt phenomenon. If, for instance, in a patient with a right superior oblique paresis, the head is tilted to the right shoulder, nervous impulses will arise from the otolith apparatus and be sent to all muscles concerned when both eyes are rotating around their anteroposterior axis to the left. Thus excycloduction of the left eye is produced by co-contraction of both inferior muscles and incycloduction of the right eye by co-contraction of both superior muscles. However, since the paretic right superior oblique muscle is no longer capable of counteracting the elevating and adducting component of the right superior rectus muscle, the right eye will move upward (positive Bielschowsky test; Figs. 20–2, 20–3, 20–4, and 20–20). With the head tilted to the left, the cycloversional movement of both eyes to the right occurs without participation of the paretic muscle; hence, the visual axis will not become deviated. It goes without saying that owing to the orientation of the semicircular canals the test cannot be applied in the supine patient. The head tilt test is applicable in paresis of any of the cyclovertical muscles. However, there is less vertical difference between the two eyes upon tilting the head with paresis of vertical rectus muscles than with paresis of the oblique muscles because the vertical action of the unopposed oblique muscles is considerably less than that of the unopposed vertical rectus muscles. Following its original description by Hofmann and Bielschowsky112 the head tilt test has become firmly established in our diagnostic armamentarium. Several modifications have evolved with which the examiner can arrive at the correct diagnosis of the offending muscle.78, 99, 100, 103, 212 The test is especially useful in distinguishing a true from a simulated superior rectus paralysis in a patient with a contralateral superior oblique paralysis who habitually fixates with the paralyzed eye (inhibitional palsy of the contralateral antagonist; see Fig. 20–1). We follow the diagnostic scheme popularized by Parks212 by asking the following three questions: (1) Does the patient have a right or left hypertropia in primary position? (2) Does this deviation increase in dextroversion or levoversion? (3) Does it increase with the head tilted to the right

FIGURE 20–2. Physiologic basis of the Bielschowsky head tilt test. A, On tilting the head to either shoulder, reflex innervation of the cyclorotatory extraocular muscles occurs from stimulation of the otolith apparatus. Head inclination to the right shoulder causes incycloduction OD and excycloduction OS and inclination to the left shoulder elicits a cycloversion to the opposite (right) direction. The muscles involved in rotating the eyes around their anteroposterior axes are indicated by their initials. The compensation of the head inclination by cyclorotation of the eyes does not fully offset the angle of inclination. B, Muscles that act synergistically during cycloductions become antagonists when elevating and depressing the globes. Under normal conditions the vertical action of the rectus muscles exceeds that of the oblique muscles; conversely, the effect of the oblique muscles on cycloduction is greater than that of the vertical rectus muscles. C, When the head is tilted to the involved side in the case of a right superior oblique paralysis, the vertical and adducting action of the right superior rectus is unopposed. Contraction of this muscle in an attempt to incycloduct the eye results in elevation of that eye, thus increasing the vertical deviation (positive Bielschowsky test). (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.) or left shoulder? Using this three-step method, one can distinguish a paretic oblique or vertical rectus muscle in most instances. Figures 20–3 and 20–4 show the various responses that may be encountered during the head tilt test. Compensatory Anomalies of Head Position Ocular torticollis was first described in 1873 by Cuignet.53 The various forms of anomalous head

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FIGURE 20–3. Diagnosis of the paretic muscle in a patient with right hypertropia (RHT). By comparing the position of the patient’s eye with the drawing, one can identify the paretic muscle. A, RHT may be caused by paresis of the right superior oblique (RSO), right inferior rectus (RIR), left superior rectus (LSR), or left inferior oblique (LIO). Increased RHT in dextroversion (B), or levoversion (E), limits the number of possibly paretic muscles to two. The paretic muscle is identified by tilting the head to the right and left shoulders (C, D, F, and G). (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

posture, such as head turn, head tilt, and chin elevation or depression in patients with A and V patterns of strabismus or nystagmus, are discussed in Chapters 12, 19, and 23. In this chapter, it is necessary to mention only that most patients with paralytic strabismus habitually hold their heads in a position in which they can avoid the field of action of the paretic muscle. Horizontal, vertical, or torsional diplopia is thus eliminated and single binocular vision maintained. For instance, a head turn toward the side of the paralyzed eye will compensate for a right lateral rectus paralysis, and a head tilt to one shoulder with chin depression is characteristic of a superior oblique paralysis on the opposite side. Moses186 stated that to avoid the tilt of the vertical meridian and the vertical deviation in superior oblique paralysis, "the head will be held in that position which brings the vertical meridian of the normal eye parallel with that of the paralyzed eye," and that "this is accomplished

by tilting the head toward the opposite shoulder." One must keep in mind that the degree of "righting" of the eyes by tilting the head is much smaller than the degree of head tilt (see Chapter 4). Although it is true that the degree of cyclodeviation is greater with the head tilted to the shoulder on the side of the affected eye, the retinal meridians of the two eyes are by no means parallel with the head tilted to the opposite shoulder. Preferably, one should simply remember that there is a head position in which the paretic muscle receives a minimum of impulses to contract. This is the position in which a patient with a paralyzed muscle habitually holds the head. Not all patients with paralytic strabismus, however, achieve single binocular vision with an anomalous head position. If this were so, the head posture would be expected to normalize when either

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FIGURE 20–4. Diagnosis of the paretic muscle in a patient with left hypertropia (LHT). The threestep maneuver is performed as in Figure 20–3. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

eye is covered since diplopia is no longer present under such circumstances. The anomalous head posture often will persist when the fixating eye is covered and will disappear only by covering the paretic eye, which indicates that certain monocular benefits such as elimination of image disclination may accrue from an anomalous head posture.200 Furthermore, some patients unable to fuse by means of a compensatory head posture will turn or tilt their head in the opposite direction to increase the distance between the double images or to use their nose as an occluding device. It is also possible that the patient will assume an anomalous head posture to permit anomalous fusion on the basis of anomalous retinal correspondence.10 From the foregoing statements, it follows that even though anomalies of head posture should alert the ophthalmologist to the presence of paretic or paralytic strabismus, these signs are of limited significance in ascertaining the nature of the underlying condition. The direction of the head tilt is fairly consistent with paresis of the oblique muscles. The head is inclined toward the opposite side with involvement of the superior oblique and

toward the paretic side with involvement of the inferior oblique muscle, although a head tilt toward the paralyzed side (paradoxical torticollis) may occur occasionally with paralysis of the superior oblique muscle. The direction of compensatory head position varies more frequently with paralyses of the vertical rectus muscles, when the head may be tilted toward the involved or noninvolved side.150 It is important to distinguish between congenital nonocular and ocular torticollis. Congenital torticollis is caused by anomalous fusion or malformation of the cervical vertebrae or by fibrosis of the sternocleidomastoid muscle, possibly secondary to birth trauma. Needless to say, no amount of wearing collars, traction devices, or surgery on the sternocleidomastoid muscle can correct an ocular torticollis. It is disconcerting how often children undergo unnecessary physical therapy for many months and even orthopedic surgery in attempts to correct an ocular torticollis. We have seen children with classic congenital superior oblique paralysis who had scars over the sternocleidomastoid muscle from previous surgical

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TABLE 20–1. Differential Diagnosis: Congenital vs. Ocular Torticollis

attempts to correct the head tilt. A search of the leading texts of orthopedic surgery in which the treatment of congenital torticollis is discussed revealed the disturbing finding that oculomotor paralyses are rarely mentioned in the differential diagnosis of abnormal head posture. An unusual cause of nonocular torticollis is hiatus hernia.205, 257 Other sources of abnormal head positions (nystagmus, mechanical-restrictive forms of strabismus, uncorrected astigmatism, unilateral hearing loss) should always be kept in mind. Table 20–1 lists the principal differences between congenital and ocular torticollis. A head tilt of long standing, whether it be caused by a tight sternocleidomastoid muscle or by a congenital cyclovertical muscle paralysis, is often accompanied by facial asymmetry. The face toward the side of the head tilt is vertically compressed and the orbit is lower than on the opposite side275 (Fig. 20–5). It has been suggested that the facial asymmetry is caused by positional molding of the face from persistence of the head tilt during sleep.84 This is a possibility to be considered in nonocular congenital torticollis but we find it difficult to explain the facial asymmetry in patients with congenital paralysis of the superior oblique on the same basis. Clearly, there is as little need for an infant with a congenital superior oblique paralysis to maintain the head tilt during sleep as there is for the awake patient to tilt the head when vertical diplopia is eliminated by occluding one eye. Restriction of normal facial growth by persistent muscle pull on the facial bones toward one side115 appears to be a more plausible explanation for the asymmetry. The facial asymmetry caused by a head tilt must be differentiated from that occurring in plagiocephaly (see p. 438) where the forehead is flattened on the side opposite the head tilt.

The presence of facial asymmetry is an important clinical sign to distinguish a cyclovertical muscle paralysis with an onset at birth or early in life from one recently acquired. It is likely that facial asymmetry can be avoided by early surgery of the underlying condition but whether facial asymmetry can be reversed by early surgery is debatable. Another reason for early surgery is that secondary scoliosis and contracture of the neck muscles may develop as a result of an abnormal FIGURE 20–5. Marked facial asymmetry, vertical compression, and lowered orbit of the left side of the face in a patient with congenital right superior oblique paralysis. A marked head tilt to the left shoulder (more pronounced than shown on this photograph) was present since early childhood. (From Noorden GK von, Maumenee AE: Atlas of Strabismus. St Louis, Mosby–Year Book, 1967.)

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head posture.15, 61, 225 In such instances the head tilt may persist even after the paralyzed eye has been occluded or after the underlying cause has been eliminated surgically. Sensory Anomalies Sensory anomalies do not occur in association with paralysis of the extraocular muscles as frequently as they do with comitant forms of strabismus. The fact that most patients are able to maintain simultaneous binocular vision in a direction of gaze opposite to the field of action of a paretic muscle precludes development of deep-seated sensorial anomalies although exceptions to this rule have been reported.14 The incomitance of the deviation is not consistent with a stable angle of strabismus—one of the prerequisites of the establishment of sensorial adaptations. Sensory anomalies in paralytic strabismus usually are restricted to patients with congenital paralysis or paralysis of onset during early childhood. As pointed out on page 415, with the passage of time a paralytic deviation may become increasingly comitant and create conditions favorable to development of sensory anomalies, provided, of course, onset is at an age at which such adaptations are likely to occur. Suppression, amblyopia, or anomalous retinal correspondence is as likely to develop in such patients as it is in patients with nonparalytic strabismus. If the strabismus remains incomitant and onset is during childhood, diplopia in the paretic field of fixation may be prevented by regional suppression (see Chapter 13). Characteristically, this form of suppression will occur only when the patient moves the eyes into the paretic field of gaze and will be absent when the eyes are aligned. For instance, a patient with a right abducens paralysis acquired at 4 years of age and with esotropia may have perfectly normal binocular function in levoversion but suppress the right eye in primary position and dextroversion. Amblyopia in paralytic strabismus occurs only in patients unable to maintain simultaneous binocular vision in any direction of gaze and in whom paralysis occurs early in life. Amblyopia may cause complications in diagnosing paralytic strabismus since some patients with congenital paralysis may fixate with the paralyzed eye to increase separation of the images (secondary deviation). In such instances the nonparalyzed deviated eye may become amblyopic; thus, amblyopia in a patient

with paralytic strabismus does not necessarily mean that the amblyopic eye is also the paralyzed eye.  We have observed this pattern in several patients with congenital oculomotor paralysis who fixated with the paralyzed exotropic and hypotropic eye and, as a result, had developed a head turn and chin elevation of bizarre proportions (see also Kazarian and Flynn132). Under certain circumstances, limitation of ocular motility may influence visual acuity in different positions of gaze by changing the fixation behavior on a mechanical basis. For instance, in an eye with right abducens paralysis, visual acuity may be decreased when it is tested in abduction and be normal when tested in primary position or adduction (see Chapter 14). Obviously, the limitation of abduction prevents central fixation in that position, and visual acuity is limited by the functional capacity of that part of the retina in the nasal periphery with which the object is viewed. These motor influences on acuity and fixation behavior in strabismic patients were described first by von Graefe89 and in more recent years have been studied by other investigators.1, 28, 54, 55, 66, 119, 139, 198 Such studies led to a specific surgical approach for treatment of eccentric fixation. Past-Pointing Von Graefe first described anomalies of egocentric localization, referred to as past-pointing or false orientation, in patients with paralytic strabismus. If the patient is asked to point to an object in the field of action of the paralyzed muscle while the sound eye is covered, his finger will point beyond the object toward the field of action of the paralyzed muscle. During this test, it is important that the patient point rapidly toward the object to avoid visual correction of the error of localization while the hand is still moving toward the object. Better still, it helps if the hand of the patient is covered by a piece of cardboard during the test (Fig. 20–6). During the first part of the twentieth century the qualitative and quantitative study of this phenomenon was important in the diagnosis and differential diagnosis of paralytic strabismus. More recently, however, this test has become less popular and more reliable diagnostic methods have replaced it. However, since past-pointing occurs only with paralysis of the extraocular muscles of recent onset and tends to disappear gradually, this sign continues to be of clinical value in distinguishing between congenital and acquired

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FIGURE 20–6. Past-pointing in a patient with left sixth nerve paresis. A, Normal egocentric localization in dextroversion. B, Past-pointing to the left when viewing an object in the median plane, and C, in the paretic field of gaze.

paralysis. The theoretical aspects of the past-pointing phenomenon are far more interesting than its usefulness as a diagnostic tool. Egocentric localization of objects in space (see Chapter 2) is approximately correct as long as there is no discrepancy between the innervational effort to move the eye and the amplitude of the executed eye movement. Errors of egocentric localization such as pastpointing occur when motor innervation and the elicited eye movement are disproportionate. With left abducens paralysis, for instance, excessive innervation is required to counteract the unopposed antagonistic medial rectus muscle when the patient is holding the paralyzed eye in primary position or moving it toward abduction. As a result, the subjective impression created by excessive abduction innervation is that the object to be fixated lies to the left of the median plane in primary position and even farther to the left on attempted abduction (see Fig. 20–5). This classic theory of the mechanism of past-pointing22, 88, 102, 111 has not gone unchallenged, and other explanations have been proposed.3, 211, 262 However, von Noorden and coworkers201 confirmed experimentally that the classic theory of past-pointing as an error of subjective localization caused by disproportion between innervational input and motor output is still true. Interestingly, past-pointing is not limited to paralytic strabismus but has been described also in association with comitant deviations.7, 253, 258 Electromyography The works of Björk and Kugelberg,25 Huber and Lehner,116 Esslen and Papst,70 Breinin,32 Jampolsky, 123 and many others have established that electromyography is of limited value as a diagnostic tool in the field of paralytic strabismus. Electromyography is of value only in conjunction

with other diagnostic methods in establishing whether a paralysis of the extraocular muscles is of myogenic or neurogenic origin. For instance, in myogenic processes such as endocrine myopathy or chronic progressive ophthalmoplegia (see Chapter 21), the electromyogram is characterized by a disproportion between massive recruitment of motor units and inability to move the eye. In patients with myasthenia gravis, a disorder of the neuromuscular junction (see Chapter 21), increased electromyographic activity during intravenous administration of edrophonium chloride (Tensilon) will establish the diagnosis even in patients who may not show clinically observable improvement of ocular motility after the drug has been injected. In those with peripheral neurogenic oculomotor paralysis, the electromyogram will show partial or complete loss of motor units in spite of maximal volitional innervation, thus clearly differentiating such problems from a myogenic process. On the other hand, the electromyogram does not topographically differentiate between peripheral, nuclear, or supranuclear neurogenic lesions. In most instances, information obtained using other methods of examination, such as the forced duction test, and the specific clinical characteristics of the underlying disorder make electromyography unnecessary in the practice of clinical ophthalmology. However, electromyography is of value as a research tool and has added much to our knowledge of the physiology of vergence movements and of the Duane retraction syndrome (see Chapter 21). Neurogenic Paralysis vs. Myogenic or Structural Restriction of Eye Movements Differentiation between a neurogenic paralysis and the inability of the eye to move in certain directions of gaze because of structural anomalies

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directions of gaze because of structural anomalies involving the extraocular muscles, the conjunctiva, Tenon’s capsule, or all of these, is of pivotal diagnostic and therapeutic significance. For instance, paresis of the lateral rectus muscle may limit abduction of the eye. However, a mechanical restriction involving the medial rectus such as contracture; endocrine myopathy; fracture of the medial orbital wall with entrapment of the medial rectus muscle (see Chapter 21); contracture and scarring of the conjunctiva, Tenon’s capsule, or the medial aspect of the globe from a previous surgical procedure; or retroequatorial adhesions between sclera and the lateral rectus muscle (reverse leash effect) will also cause limitation of abduction. There are fundamental differences in the therapeutic approach to any of these conditions; with paresis of the lateral rectus muscle not accompanied by medial rectus contracture, surgery directed at strengthening the function of the paretic muscle combined with weakening the action of its antagonist is the preferable method of treatment. If, on the other hand, inability to abduct the eye is caused by structural anomalies involving the medial aspect of the globe, this operation is clearly contraindicated, for it would not only fail to improve abduction but would also cause retraction of the globe with narrowing of the palpebral fissure. The surgical aim in this situation must be primarily to remove the mechanical restriction of ocular motility. This example demonstrates the unequivocal need to separate mechanical from neurogenic causes of a paralysis before deciding on appropriate surgical management. The electromyogram may be of limited diagnostic value in such situations, but the equipment is rarely available nor can it be applied in pediatric patients. Other more preferable tests that can be readily performed in the physician’s office or under anesthesia will clearly indicate whether a deviation is caused by passive restriction of motility or by neurogenic paralysis. Forced Duction Test The forced duction test (referred to also as the traction test), although described early by Wolf276 (1900), Gifford82 (1924), and Jaensch121 (1929), has become popular only in our time as a simple and most useful method for diagnosing the presence of mechanical restriction of ocular motility. We anesthetize the conjunctiva with several drops of 4% lidocaine hydrochloride

(Xylocaine). This solution is prepared by a local pharmacy from the commercially available ampules for intravenous and intramuscular injection, sterilized, and dispensed in ophthalmic dropper bottles. Unlike other ophthalmic local anesthetics, this drug has no visible effect on the corneal epithelium. The eye is then moved with two-toothed forceps applied to the conjunctiva near the limbus in the direction opposite that in which mechanical restriction is suspected. For instance, to distinguish between lateral rectus paralysis and mechanical restriction involving the medial aspect of the globe, we apply the forceps at the 6- and 12-o’clock positions and move the eye passively into abduction. If no resistance is encountered, the motility defect is clearly caused by paralysis of the lateral rectus muscle. If resistance is encountered, mechanical restrictions do exist medially and contracture of the medial rectus muscle, conjunctiva, or Tenon’s capsule or myositis of the medial rectus muscle must be considered (Fig. 20–7). Occasionally, a reverse leash effect125 caused by retroequatorial adhesion of a rectus muscle may restrict passive ductions (Fig. 20–8). A reverse leash effect is also created by applying a posterior fixation suture to a muscle and will enhance the effect of this operation by weakening the muscle in its principal field of action.203 It is important not to press the globe into the orbit during the test since it may become negative and thus lead to wrong conclusions in the presence of mechanical restrictions. When topical anesthesia is used, the patient is requested to look at his or her hand held in the direction in which the eye is moved by the forceps. This will help control the influence of eye muscle innervation during the test, which otherwise might counteract passive movement of the globe and simulate mechanical restriction when none is present. In children and uncooperative adults the test must be performed under general anesthesia and before surgery. Various instrumentation to determine the degree of restriction quantitatively has been developed, 176, 222, 254, 256 but none of these methods have found general acceptance for routine clinical use. Guyton93 (see also Plager214, 215) modified the forced duction test (exaggerated traction test) to allow an estimation of superior and inferior oblique muscle tightness. To check for tightness of the superior oblique muscle the eye is grasped with toothed forceps at the 6- and 9-o’clock positions. For this test the eye must be pushed in the orbit as

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FIGURE 20–7. The forced duction test. A, Conjunctiva and episclera are grasped near the limbus with two fixation forceps. B, The eye is moved temporally and (C), nasally to test for mechanical restriction of ocular motility. Note that the eye must not be depressed into the orbit during the test to avoid false positives. (From von Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

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FIGURE 20–8. Reverse leash effect. Retroequatorial adhesion of superior rectus muscle causes a reverse leash effect with mechanical restriction of elevation as the ability of the muscle to stretch is limited. (Adapted from Jampolsky A: Surgical leashes and reverse leashes in strabismus surgical management. In Symposium on Strabismus: Transactions of the New Orleans Academy of Ophthalmology. St Louis, Mosby–Year Book, 1978, p 244.)

it is then elevated, adducted, and rocked back and forth by extorting and intorting the globe across the tendon. Tightness of the tendon becomes apparent when the globe seems to "jump" across the tendon during this maneuver. The condition of the inferior oblique muscle is tested in an analogous manner by pushing the eye down and nasally. The anesthesiologist is asked to avoid the use of succinylcholine chloride since generalized tightness of the extraocular muscles caused by this drug may simulate mechanical restriction of the globe. Estimation of Generated Muscle Force A. B. Scott237 has increased the scope of information to be gained from mechanical manipulation of the globe with forceps. He postulated that the active force generated by a contracting muscle can be estimated by stabilizing the eye with forceps while the patient tries to move the eye against this obstacle. This

concept had been used for many years by neurologists and orthopedists in assessing the function of other muscle systems (Fig. 20–9). Mechanical determination of muscle force can be useful in assessing the function of apparently paretic muscles with contracture of their antagonists. For instance, with lateral rectus paresis and secondary contracture of the medial rectus muscle the movement that carries the eye from adduction toward the primary position may be merely a passive one, caused by relaxation of the medial rectus muscle rather than lateral rectus muscle contraction but may also be due to active albeit reduced innervation of the lateral rectus muscle. Since a different surgical approach is required for each condition, a simple estimate of the muscle force generated by the lateral rectus muscle can be made by stabilizing the eye in adduction with forceps while the patient attempts to abduct the eye. The presence or absence of a tug on the forceps indicates whether a contraction of the lateral rectus muscle takes place (Fig. 20–10). Scott mentions the possible use of this test

FIGURE 20–9. Illustration from Möbius’ neurology textbook (1898),185 showing a mechanical device for measuring the strength of flexor muscles in the leg against mechanical resistance. The same principle underlies the estimation of generated force of a paretic extraocular muscle. (Courtesy of Dr. Amy Coburn, Houston, Texas.)

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FIGURE 20–10. Estimation of generated muscle force. A, A patient with a left lateral rectus paresis shows movement of the left eye from adduction toward primary position on levoversion and slightly beyond. This movement may be active (residual lateral rectus innervation) or passive (medial rectus relaxation). In the first instance, maximal recession of the medial rectus and resection of the paretic lateral rectus may be indicated. In the second situation, a muscle transposition procedure may be considered. B, The examiner senses a tug on the forceps as the paretic eye moves from adduction toward primary position. The tug is a sign of residual innervation of the lateral rectus muscle and incomplete paralysis. Absence of the tug is a sign of complete paralysis. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

in Duane’s syndrome, congenital elevator paralysis, and myasthenia gravis, and attempts have been made to quantitate this test and the forced duction test using mechanical devices.237, 254 Helveston and coworkers105 suggested that the generated muscle force could be estimated by comparing intraocular pressure in various positions of gaze and reported pressure increases as high as 50 mm Hg from compression of the globe by a nonrelaxing stiff muscle when attempts were made to move the eye into the field of its antagonist. Eye Movement Velocity The registration of eye movements by electrooculography may be useful only as an auxiliary diagnostic method in evaluating a paralytic condition of the extraocular muscles. François and Derouck79 were first to point out that the velocity of eye movements registered in this way may yield useful clinical information regarding severity of the paralysis and recognition of return of function during recovery. Similar studies were conducted by Mackensen166 and further pursued

and applied to clinical ophthalmology by A. B. Scott and coworkers,240 Metz,171–173 Metz and coworkers, 177–180 W. E. Scott,242 W. E. Scott and Nankin,245 and many others. These investigators showed that the systematic study of saccadic velocity may be useful in distinguishing between a mechanical and a paretic limitation of ocular motility. A study of saccadic velocity may be used also in conjunction with other diagnostic methods, such as the forced duction test or determination of generated muscle force, to assess the function of a paralyzed muscle. If paralysis exists, the velocity of eye movement into the field of action of the paralyzed muscle will be markedly decreased (Fig. 20–11), the normal rapid saccade being replaced by a slow, drifting eye movement. If the motility defect is caused by a mechanical obstacle, the saccadic velocity of an eye movement into the field of apparent paresis will be normal. Kirkham and coworkers138 used saccadic velocity measurements to distinguish between bilateral abducens paralysis and divergence paralysis (see also Chapter 22). An astute observer can detect the difference in eye movement velocities with the naked eye in

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FIGURE 20–11. Electro-oculographic registration of eye movements in a patient with left cranial nerve VI paresis. Positive deflection, gaze to right. Negative deflection, gaze to left. Arrows denote decreased saccadic velocity upon gaze into the paretic field of gaze. (From Mackensen G: Okulographie. In Hamburger FA, Hollwich F, eds: Augenmuskellähmungen, vol 46. Stuttgart, Ferdinand Enke Verlag, 1966.)

many cases, thereby eliminating the need for registering eye movement. Paralytic vs. Nonparalytic Strabismus Throughout the preceding discussions we have emphasized features that distinguish paralytic from nonparalytic strabismus. In paralysis of recent onset, such differentiation rarely presents diagnostic problems but may become increasingly difficult and at times even be impossible with paralysis or paresis of long standing. For easy reference, the most important clinical properties of each condition are summarized in Table 20–2. Congenital vs. Acquired Paralysis Once the diagnosis of a paralytic deviation has been established, it is of clinical importance to determine whether it is of recent onset or has been present for many years, perhaps even since birth. If the paralysis is of recent onset, a diligent search for its cause by a complete medical and neuroophthalmologic evaluation is mandatory. If the strabismus has been present for many years, the clinical management of the problem is

clearly in the sphere of the ophthalmologist and a general medical evaluation usually is not indicated. Determination of how long a paresis has been present is not always easy, since congenital or early acquired paralysis may be obscured by a compensatory head position or a strong fusion mechanism. Thus the patient may remain asymptomatic for decades before disturbing symptoms suddenly appear and medical help is sought. Old photographs are of great value, since they may reveal the presence of an anomalous head posture in early childhood (Fig. 20–12) and clearly eliminate the possibility that the paresis is of more recent onset (see Case 20–1). The most important clinical features used to distinguish congenital or old paralysis from one of more recent onset are summarized in Table 20–3. CASE 20–1   A 45-year-old man experienced vertical diplopia of sudden onset 2 days before examination. He denied ever having seen double before. One week before

TABLE 20–2. Differential Diagnosis: Paralytic vs. Nonparalytic Strabismus

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FIGURE 20–12. Head tilt to the right shoulder in photographs taken at various ages of a patient with left superior oblique palsy identifies the congenital nature of the condition.

this episode he had a rather severe attack of influenza. The past medical history was otherwise negative. Examination of ocular motility revealed a right hypertropia of 25 in the entire left field of gaze and 15 in primary position. The patient carried his head tilted to the left shoulder, and the right hypertropia increased to 30 on tilting it to the right shoulder. Examination of ductions and versions revealed marked overaction of the right inferior oblique and slight underaction of the right superior oblique muscles. These findings were compatible with a right superior oblique paresis and a secondary overaction of the antagonist. At our request,

on his return visit the patient brought a family photograph album. He was easily identified in his high-school class graduation picture as well as in several other group photographs because of his marked torticollis. Further examination revealed that he was able to overcome diplopia and to maintain comfortable single binocular vision with only 3 base-down before the OD. During the ensuing months the amount of prismatic power required increased, and the patient eventually underwent surgery. The functional result was excellent, and the anomalous head posture disappeared. Head

TABLE 20–3. Differential Diagnosis: Congenital and Old Paralysis vs. Recent Paralysis

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tilt early in life, the spread of comitance to the entire left field of gaze, and the highly developed vertical fusional amplitudes when the patient was first seen clearly indicated the presence of congenital or early acquired fourth cranial nerve (N IV) paresis that had recently decompensated, perhaps as a result of a reduction in general health following influenza. A neuro-ophthalmologic examination was not necessary in this circumstance. Orbital Imaging Techniques Demer and Miller60 introduced quantitative magnetic resonance morphometry of extraocular muscles for use in diagnostically complex cases of paralytic strabismus. A comparison of the images from normals and patients with paralysis of the oblique and lateral rectus muscles revealed atrophy and lack of contractibility of the paralyzed muscles. The reduction in muscle size is interpreted as denervation atrophy and in the case of a lateral rectus paralysis began within 6 weeks after onset to reach a complete stage within 1 year. Evaluation of Visual Impairment Caused by Diplopia The reader is referred to the Guides to the Evaluation of Permanent Impairment published by the American Medical Association.271 Paralysis of Individual Extraocular Muscles If paralysis and paresis are to be defined, respectively, as a complete and partial impairment of motor function caused by a lesion of the neuromuscular mechanism, a multitude of etiologic factors must be considered in each case. The lesion may be in the muscle, at the neuromuscular junction, in the peripheral nerve, in the nuclear region, or in the supranuclear oculomotor pathways. Myogenic paralysis is caused by a disease of the muscle itself (myositis or fibrosis), by mechanical obstacles to ocular motility such as scar formation following repeated muscle surgery, or by orbital fractures with entrapment of fat, fascia, or muscle (see Chapter 21). Impairment of motor function also may be caused by hypoplasia or congenital absence of an

extraocular muscle. The literature is replete with numerous case reports.63 A discussion of the prevalence and causes of paralysis of individual extraocular muscles or muscle groups is beyond the scope of this book. The reader is referred to recent textbooks on neuro-ophthalmology for detailed information. Noteworthy is the most recent and possibly largest study on the causes of paralysis of the oculomotor, trochlear, and abducens nerves, which includes 4373 acquired muscle paralyses from the Mayo Clinic.217 According to this report (Table 20–4) paralysis of cranial nerve VI occurred most frequently (43.8%), followed in order of frequency by cranial nerves III (28%) and IV (15%). Multiple nerves were involved in 13% of the cases. In another recent study from Germany20 that included 412 patients, palsies of the oculomotor nerve were most frequent, followed by cranial nerves VI and IV paralyses. In a pediatric population at the Mayo Clinic (160 cases) trauma was the leading cause of oculomotor, trochlear, and abducens paralysis, followed by neoplasm.145 Data of this sort reflect the type of practice from which they were obtained. Neurologists see different types of ocular paralyses than do ophthalmologists, and pediatric ophthalmologists have different experiences in this regard than general ophthalmologists. In our practice, which is exclusively concerned with strabismus in children and adults, cranial nerve IV palsies are seen by far most commonly, followed in order of frequency by cranial nerves VI and III paralyses. This experience is exactly opposite to that reported from the Mayo Clinic145 but practically identical to a recently published study of children in a defined population.114 Finally, it cannot be overemphasized that any paralysis of one or several extraocular muscles can be simulated by myasthenia gravis. TABLE 20–4. Causes of Paralysis of Cranial Nerves III, IV, and VI (%)*

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FIGURE 20–13. Paralysis of left superior rectus muscle. The patient fixates with the nonparetic eye. Note ptosis of the paralyzed eye and secondary overaction of the right inferior oblique in levoversion.

Cranial Nerve III Paralysis SUPERIOR RECTUS MUSCLE. An isolated paralysis of the superior rectus muscle in our experience is most commonly of congenital origin. It may also be secondary to trauma, for instance, after a bridle suture during cataract surgery in which instance the elevation deficit is usually only temporary. The paralyzed eye is affected primarily in elevation and abduction. Elevation is normal in adduction. However, when superior rectus palsy has been present for long periods, elevation from primary position and adduction may also become limited (double elevator palsy; see p. 442). The ipsilateral inferior rectus and the contralateral inferior oblique muscles overact, and a small excyclotropia usually is present. The paralyzed eye is hypotropic (Fig. 20–13) in primary position, and Bell’s phenomenon is absent. Ocular torticollis occurs frequently, but as mentioned, the position of the head is of little diagnostic significance. Even though in most patients the head is tilted toward the sound side, the opposite may occur. In persons with this type of muscle paralysis of recent onset, the face is turned upward, the chin is elevated, and the head usually is inclined toward the sound side. Superior rectus muscle paralysis is frequently but not always associated with weakness of the homolateral levator palpebrae muscle, particularly if the paralysis is congenital. Since the upper lid elevates with elevation of the globe and droops when the eye moves downward, a true ptosis caused by levator weakness must be differentiated from pseudoptosis secondary to the hypotropic position of the globe. Figure 20–14 shows a patient with paralysis of the right superior rectus

and pronounced ptosis of the right eye with the nonparetic eye fixating. When the paralyzed eye fixated, the ptosis disappeared and a marked hypertropia of the nonparalyzed eye (secondary deviation) occurred. The differential diagnosis of a superior rectus muscle paralysis includes mechanical causes that limit elevation of the eye, such as contracture, fibrosis, high myopia (heavy eye), myositis, endocrine orbitopathy, or a blow-out fracture of the orbital floor (see Chapter 21). Whenever elevation is restricted mechanically the forced duction test will be positive, and the restriction often involves the entire upper field of gaze. Rosenbaum and Metz220 reported an interesting structural anomaly simulating a superior rectus palsy. Surgical exploration revealed that the superior rectus tendon was inserted near the superior border of the lateral rectus muscle. Repositioning the superior rectus muscle to its normal anatomical position corrected the condition. FIGURE 20–14. Pseudoptosis of the right eye in a patient with right superior rectus paresis. A, When the nonparetic eye fixates, the paretic eye is hypotropic and ptosis is present. B, The ptosis disappears, and a left hypertropia is present on change of fixation to the paretic eye.

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FIGURE 20–15. Right medial rectus paralysis. Exotropia in primary position and overaction of left lateral rectus muscle.

MEDIAL RECTUS MUSCLE. An isolated paralysis of the medial rectus muscle without involvement of other muscles supplied by cranial nerve III is very rare. With this type of paralysis the greatest defect of ocular motility occurs when the affected eye moves into adduction. Since the action of the antagonistic lateral rectus muscle is unopposed, an exotropia usually is present in primary position (Fig. 20–15). The patient’s face turns toward the uninvolved side. The differential diagnosis of an isolated medial rectus paralysis includes internuclear ophthalmoplegia (see p. 441). A rare, bizarre phenomenon referred to as synergistic divergence, which consists of a congenital adduction deficit with simultaneous abduction of each eye on attempted gaze into the field of action of the paralyzed medial rectus muscles, has been described in patients with congenital medial rectus paralysis.14; 36; 45, p.246; 280 The etiology is unknown although innervational anomalies similar to those found in Duane’s syndrome have been implicated.52, 96 Extirpation of the ipsilateral lateral rectus muscle has been suggested to eliminate simultaneous abduction. Other substitution phenomena in congenital andacquired supranuclear disorders of eye movements have been described.39

INFERIOR RECTUS MUSCLE. An isolated paralysis of the inferior rectus muscle is often congenital in our experience. However, it may also occur following orbital trauma, especially after fracture of the orbital floor; from vascular disease; or in conjunction with myasthenia.197 The diagnosis is made on the basis of the prism and cover test in the diagnostic positions and on examination of ductions and versions. The deviation is greatest on attempts to look downward with the affected eye in abduction (Fig. 20–16). The unopposed action of the antagonistic superior rectus muscle causes the paretic eye to be incyclotropic and hypertropic in primary position. When the patient fixates with the paretic eye in primary position, pseudoptosis may occur in the sound eye, creating diagnostic problems. Ocular torticollis is a frequent occurrence but is not of diagnostic value since the head may be tilted to either side.197 INFERIOR OBLIQUE MUSCLE. Of all the extraocular muscles supplied by the oculomotor nerve, the inferior oblique muscle is least likely to become paralyzed. The onset is usually congenital but trauma has been mentioned as a cause. In primary position the affected eye may be hypotropic or the unaffected eye hypertropic, depending on whether the patient fixates with the nonparalyzed or paralyzed eye. The greatest

FIGURE 20–16. Left inferior rectus paralysis. The patient fixates with the paralyzed eye. Note right hypotropia and pseudoptosis in the primary position, and secondary overaction of the right superior oblique and left superior rectus muscles.

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FIGURE 20–17. Left inferior oblique paralysis. Note secondary overaction of the left superior oblique and right superior rectus muscles. The forced duction test in this patient showed no restriction on attempts to elevate the left eye in adduction.

deviation occurs when the patient attempts to elevate the adducted paretic eye33 (Fig. 20–17). Overaction of the unopposed ipsilateral superior oblique muscle causes incyclotropia. In all patients whom we have evaluated, onset was congenital. As in the case of superior oblique paralysis, the anomalous head posture is more characteristic than in paralyses of the vertical rectus muscles. As a rule the head is inclined toward the paralyzed side, and the face is turned toward the uninvolved side, but there are exceptions. The Bielschowsky head tilt test is positive on tilting the head toward the normal side. The forced duction test is necessary in making this diagnosis, since the prevalence of Brown syndrome (see Chapter 21) is far greater than paralysis of the inferior oblique muscle and since the defect of ocular motility is clinically similar. However, with Brown syndrome the involved eye is frequently depressed more severely in adduction than it is with inferior oblique paralysis. VERTICAL MUSCLE PARALYSES FOLLOWING CATARACT SURGERY. A sudden increase in vertical strabismus following cataract surgery has been reported only in recent years.42, 43, 59, 71, 91, 96, 118, 152, 189, 210 It consists of limitation of elevation or depression with diplopia with an onset on the day after surgery. This complication was exceedingly rare prior to the advent of local anesthesia with peribulbar injection and has occurred with increasing frequency shortly after introduction of this change in technique.71 Most authors agree that this sudden paralysis, sometimes accompanied by rapidly developing contracture of the antagonist or segmental contracture of the paretic muscle,42 is caused by

the myotoxic effect of the anesthetic agent or by accidental injection of the muscle belly or the nerve supplying the muscle. The inferior rectus muscle is most frequently involved, followed in order of frequency by the superior rectus and inferior oblique muscles. According to Esswein and von Noorden71 the following factors may contribute to this complication: the location of the injection, which is directly along the muscle belly56; the 1.5-in. needle, which disperses the anesthetic directly over or underneath the muscle; the high concentration used by some surgeons; and repeated injections.71 To avoid this complication these authors advocated avoiding the muscle belly by injecting slightly medial or lateral to it, to use the smallest quantity and lowest concentration of the medication that is necessary to obtain anesthesia and akinesia, to inject with a short, blunt-tipped needle, and to wait for at least 30 minutes for an effect before repeating the injection. Fortunately, the results of corrective muscle surgery are excellent in these patients and complete rehabilitation can be achieved in all but a small number.71, 189 Diplopia after cataract surgery caused by restriction of ocular motility in the immediate postoperative phase must be differentiated from double vision on account of sensory factors, such as loss of fusion from prolonged disruption of binocular vision in previously heterophoric patients or an increase in the angle of a preexisting strabismus.236 COMPLETE CRANIAL NERVE III PARALYSIS. When the oculomotor nerve is completely paralyzed, the position of the affected eye is determined by the function of the only two

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remaining intact muscles, the lateral rectus and the superior oblique muscles. Thus the paralyzed eye will be in a position of abduction, slight depression, and intorsion. Concurrent paralysis of the levator palpebrae and unopposed tonus of the orbicularis muscle will cause ptosis of the upper lid, and general relaxation of tonus of four of the six extraocular muscles may produce a small degree of proptosis. The motility of the affected eye will be limited to abduction, to small degrees of depression in abduction (which is limited by the minor contribution of the superior oblique muscle to depression in that position), to incycloduction, and to an adduction movement of the eye that does not go beyond the primary position (Fig. 20–18). With a complete cranial nerve III paralysis, the intrinsic muscles of the eye also are involved, causing the pupil to be dilated and nonreactive and a paralysis of accommodation. During the recovery of acquired third nerve paralysis, aberrant regeneration of nerve fibers may result in failure of the upper lid to follow the eye as it moves downward or in retraction of the upper lid in downward gaze (Fig. 20–19) or adduction.

The retraction of the upper lid occasionally may be accompanied by contraction of the pupil. Because of its resemblance to Graefe’s sign in thyrotoxicosis, this phenomenon, notwithstanding its entirely different nature, is referred to as the pseudo-Graefe’s sign. The theory for this intriguing finding is that the nerve fibers originally connected with the inferior rectus muscle grow into the sheath of nerve fibers supplying the levator muscle so that the impulse to look down increases the tonus of the levator.23 Aberrant regeneration can be congenital or acquired182, 227 and may occur without a preceding oculomotor paralysis in patients with a slowly growing intracavernous meningioma or with a carotid aneurysm.160 Other abnormal connections between the various components of cranial nerve III have been observed during the recovery phase.23 The reader is referred to neuro-ophthalmologic texts for further details. One of the rarest and most interesting forms of cranial nerve III paralysis has been referred to as cyclic oculomotor paralysis. With this type of paralysis, some of the extraocular muscles over which the patient has lost all voluntary control contract spastically at more or less regular

FIGURE 20–18. This patient suddenly developed a complete paralysis of the left cranial nerve III from an aneurysm of the posterior communicating artery. A, There is complete ptosis of the left eye and the patient was unaware of double vision unless the left upper lid was manually lifted. B, Note the abducted position of the left eye with the right eye in primary position; the inability to elevate, adduct, or depress the left eye; and the dilation of the left pupil. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

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FIGURE 20–19. Aberrant regeneration in left third nerve paralysis. Note retraction of the left upper lid on downgaze and adduction. (Courtesy of Dr. John McCrary III, Austin, Texas.)

intervals. The sphincter of the iris, which is essentially unresponsive to all physiologic stimuli, together with the paralyzed ciliary muscle contracts synchronously at regular intervals with the muscles supplied by the oculomotor nerve. Cyclic oculomotor paralysis usually is congenital, and the various theories for this anomaly as well as the literature have been reviewed.37, 38, 72, 165 Several authors have drawn attention to the fact that congenital third nerve paralysis in children may be accompanied by diverse and often serious neurologic deficits.11, 94, 234, 263 Many of these can be associated with perinatal trauma. A neurologic evaluation is therefore advisable and an evaluation by neuroimaging to search for associated structural anomalies of the brain has been recommended by A. G. Lee and coworkers156 who published a guide to the indications for neuroimaging in isolated and nonisolated congenital and acquired oculomotor paralysis. Cranial Nerve IV Paralysis ETIOLOGY. In our practice superior oblique paralysis is the most common form of paralytic strabismus. In a review of 270 patients with superior oblique paralysis treated by us during a 10-year period, a congenital paralysis was encountered most often (39.5%). This was followed in order of frequency by traumatic (34%), idiopathic (23.2%), and neurologic (2.9%) paralyses.202 This etiologic distribution is similar to that reported by other strabismologists,67, 136, 142 but differs in a neuroophthalmologic practice where trauma and vascular disorders predominate and congenital paralysis is only infrequently diagnosed.217 Blunt head trauma, often only a

mild concussion without loss of consciousness, is among the most frequent causes, but direct injury to the trochlea143, 159 or to the tendon during blepharoplasty273 has also been reported. Congenital superior oblique paralysis may also follow an autosomal dominant mode of inheritance29 but this occurrence is rare. It is of historical interest that in the preantibiotic era iatrogenic damage to the trochlea during ethmoidectomy was probably the most common cause of a cranial nerve IV palsy. A superior oblique paralysis of sudden onset and without a history of trauma, while in most instances caused by spontaneous decompensation of a congenital paralysis, may also signal an intracranial process.17, 49, 151 Myasthenia gravis226 and multiple sclerosis120 may present as an isolated unilateral superior oblique paralysis with an insidious onset and may, therefore, easily be confused with a congenital paralysis. Of 221 cases with trochlear paralysis 6 patients had a unilateral recently acquired cisternal schwannoma of the trochlear nerve as diagnosed with neural imaging.72 None developed additional symptoms or signs of cranial nerve or central nervous system involvement. Helveston and coworkers108 drew attention to the fact that the superior oblique tendon is different in congenital as compared to acquired paralysis. A redundancy of the tendon or an abnormal posterior insertion of the tendon into Tenon’s capsule was noted in most congenital but not in acquired palsies, and Plager214, 215 confirmed the laxity of the tendon in congenital cases at the time of surgery by traction testing of the superior oblique. Slight ultrastructural differences in superior

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oblique tendons from patients with congenital and acquired paralyses have been observed108 and the question arose whether patients with the congenital variety have a true paralysis of that muscle. This speculation gained further substance by the findings of Tian and Lennerstrand260 who reported that the peak saccadic velocity of the eye during downward movement in adduction was more reduced in acquired than in congenital paralysis. On the other hand, magnetic resonance imaging (MRI) of the superior oblique muscle has shown a more pronounced volume reduction of the muscle in congenital vs. acquired cases.1, 231, 232 Whether this constitutes a denervation atrophy or represents a primary anomaly, as suggested by Sato,231 is not clear at this time. Be this as it may, the recognition of differences in the physical characteristics of the tendon in congenital and acquired cases has important therapeutic implications (see p. 450). We discussed in Chapter 18 that apparent oblique dysfunction, including decreased depression in adduction, may actually be caused by heterotopic muscle pulleys.47 MRI may be of help in differentiating true from pseudoparalysis of the superior oblique muscle.60 In the latter instance the volume and the contractile function of the muscle will be normal. SYMPTOMS. The symptoms of superior oblique palsy may consist of asthenopia, vertical diplopia, image tilting, and an anomalous head posture. The question arises whether the presence or absence of diplopia, with or without image tilting, is a helpful symptom in distinguishing between recently acquired and congenital paralysis. It is essential to determine the age of onset whenever possible since a recently acquired superior oblique palsy of nontraumatic origin requires a medical workup. Conversely, when the onset is clearly congenital, treatment, if indicated, may commence without further evaluation of the patient by costly and unnecessary procedures. Although vertical diplopia occurred in our series more commonly in patients with acquired superior oblique palsy, 25% of patients with congenital palsy also complained about diplopia. Therefore, the presence or absence of diplopia is not a reliable sign in determining the onset. However, image tilting as an isolated symptom or combined with vertical diplopia occurred only in acquired paralysis and thus emerges as a valid differential diagnostic criterion.202 We do not know of one instance

of congenital paralysis in which a patient was aware of image tilting under casual conditions of seeing. This should not distract from the fact that after dissociation of the eyes with Maddox rods, cyclotropia can be diagnosed in congenital cases as well because cyclofusion or other sensory or psychological mechanisms to eliminate image tilting under casual conditions of seeing are disrupted with this test (see Chapter 18). Excyclotropia, when measured with the Maddox double rod test, may occur in the nonparalyzed eye in patients who habitually fixate with their paralyzed eye because of a monocular sensorial adaptation to the cyclodeviation that has taken place in that eye.208 DIAGNOSIS. Diagnosis of superior oblique paralysis is based on the presence of a hypertropia, usually greatest in the nasal field of the involved eye, but not necessarily in the field of action of the paralyzed muscle. Overaction of the unopposed antagonistic inferior oblique commonly causes the hypertropia to be greatest in the field of action of that muscle. Kommerell and coworkers147 described two most unusual patients with the clinical signs of superior oblique palsy who were able to vary their vertical angle of strabismus at will. In view of the difficulties encountered by the student of ocular motility in diagnosing a superior oblique paralysis and in view of its being confused with a superior rectus paralysis of the fellow eye and its high prevalence, the principal diagnostic and clinical features in a patient with a left superior oblique paralysis are shown in detail in Figure 20–20. With a spread of comitance and with secondary contracture of the ipsilateral superior rectus muscle, the hypertropia may involve the entire lower field of gaze. This contracture of the superior rectus is easily diagnosed with the forced duction test and may cause pseudo-overaction of the superior oblique in the uninvolved eye (secondary deviation).124, 202, 248 Knapp142 and Knapp and Moore143 introduced a classification that describes the most common manifestations of superior oblique paralysis. Although modifications of this classification have been suggested108, 243, 244 we have adopted Knapp’s as being the practical one. Depending on the magnitude of hypertropia in the diagnostic positions of gaze, seven classes are distinguished. A description of each class and its prevalence in a group of 202 patients seen in our practice in

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FIGURE 20–20. Clinical findings in a patient with a long-standing traumatic left superior oblique paralysis. The head is tilted to the right shoulder and the face is slightly turned to the right (A). In primary position this patient had a left hypertropia of 20 prism diopters (F), increasing to 30 prism diopters in dextroversion (E), with the greatest deviation (35 prism diopters) when the patient was looking up and to the right (B). The hyperdeviation was also present in the left field of gaze (D, G) where it measured 10 prism diopters (spread of comitance). Note secondary overaction of the left inferior oblique muscle (B, E) and only minimal limitation of depression when looking down and to the right (H). The Bielschowsky head tilt test is diagnostic for a left superior oblique paralysis with increase of the left hypertropia on tilting the head to the left shoulder (K, L).

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whom preoperative diagnostic positions could be determined on the deviometer is presented in Table 20–5.202 The right eye is used here as an example. Examples: in class 1 the hypertropia is greatest when the right eye is elevated and adducted (27% of our patients). The deviation is categorized as class 3 when it is of equal magnitude in the entire paralyzed field of gaze (21% of our patients). Whereas the distribution of hypertropia in the different gaze positions may vary because of a spread of comitance, an exception exists in class 7. In these patients a classic superior oblique paralysis is associated with restriction of elevation in adduction (pseudo-Brown syndrome) and direct trochlear trauma is the cause. Knapp and Moore143 mentioned dog bites as a common cause, to which we add frontal sinus surgery. Auxiliary diagnostic features include a positive Bielschowsky head tilt test, which is nearly always present and a head tilt toward the nonparalyzed side, which is present in only approximately 70% of the patients.202 Burian and coworkers38 explained the absence of a head tilt by the patient’s inability to obtain single binocular vision by any vicarious head position, by the presence of large fusional amplitudes, or by reduced visual acuity in one eye. However, we have been unable to confirm this since visual acuity of each eye, the ability to fuse in primary position, and the magnitude of hypertropia and cyclotropia were the same in patients with and without a head tilt.202 Of special interest are patients with a paradoxical TABLE 20–5. Classification According to Amount of Hypertropia in Diagnostic Positions (N 202)

head tilt toward the paralyzed side. Whereas this occurred in only 7 of 270 patients,202 unawareness of the existence of a paradoxical head tilt may easily confound the diagnosis. A comparison of the clinical findings obtained in patients with a paradoxical head tilt and in those with a head tilt that "conformed to the rule" showed that in the former group intermittent and unstable fusion was present with the head tilted toward the uninvolved side, but alternating suppression and diplopia occurred when holding the head toward the paralyzed side.202 Thus these patients preferred a head position that disrupted fusion, caused a wide separation of the double images, and thus eliminated the discomfort that may have been associated with the constant effort to maintain single binocular vision in the presence of a superior oblique weakness. Unless a history of recent trauma or old photographs clearly establish the traumatic or congenital nature of a superior oblique paralysis, the clinician may be in a quandary in deciding whether to order neuroimaging. A recently established guide for the cost-effective evaluation of patients with superior oblique paralysis indicates that isolated congenital, old traumatic, or vasculopathic cases do not require neuroimaging.155 Patients with nonisolated palsies require directed neuroimaging studies based upon the results of the nonocular symptomatology. CONGENITAL ABSENCE OF THE SUPERIOR OBLIQUE MUSCLE. This mostly unexpected finding at the time of surgery presents a special challenge to the surgeon. Only in rare instances is this anomaly diagnosed prior to the operation. Helveston and coworkers107 noted the association of amblyopia and of a horizontal deviation in these cases. To this we added a large hypertropia in primary position, spread of comitance, and pseudo-overaction of the contralateral superior oblique muscle as additional clinical features.268 Neural imaging may demonstrate congenital absence of the superior oblique muscle preoperatively and thus facilitate planning of effective surgical correction. SPONTANEOUS LIMITATION OF ELEVATION (BROWN SYNDROME) FOLLOWING ACQUIRED SUPERIOR OBLIQUE PARALYSIS. The first patient with this intriguing anomaly following mostly traumatic superior oblique paralysis was described by Fox77 in 1981 and not more than 12

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other cases have been reported since.192, 235, 249 The onset of limitation of elevation in adduction is gradual, progressive, and may mimic Brown syndrome (see Chapter 21). The forced duction test was positive in some but not in all patients. The etiology is entirely speculative and fibrotic reaction of the superior oblique tendon or adjacent structures235 or a secondary contracture of the tendon192 have been mentioned. UNILATERAL VS. BILATERAL PARALYSIS. It is most important to carefully examine the patient for involvement of the fellow eye when superior oblique paralysis is of traumatic origin. Bilateral involvement was present in 19 (21%) of 92 traumatic cases observed in our clinic,202 which is in contrast to the 88% of cases reported by other observers.193 The severity of the paralysis is often asymmetrical, and the involvement of the second eye may not become apparent until the eye with the more severe defect has been operated on (masked bilateral superior oblique paresis).109, 117, 148, 153, 264 A right hypertropia in left gaze and a left hypertropia in right gaze, as well as a positive Bielschowsky test with the head tilted to either side, are the only signs we consider diagnostic for bilateral involvement because neither occurred in unilateral cases.202 However, absence of these two features does not exclude bilateral involvement. Several authors have stated that when excyclotropia is in excess of 10° to 15° a bilateral paralysis should be suspected.67, 149, 183 However, in reviewing 203 patients with unilateral and bilateral paralysis we were unable to confirm this widely held view: the mean excyclotropia was 7° (range, 1° to 25°) in patients with unilateral and 8° (range, 3° to 20°) in patients with bilateral paralysis.202 Kushner153 added bilateral objective excyclotorsion of the globes on fundus examination as another sign in distinguishing unilateral from bilateral paralysis. It has been claimed that bilaterality should be suspected when excyclotropia increases significantly in downward gaze.140 However, it is our experience that this occurs also in unilateral cases. Since the etiology of bilaterality of the paralyses is often traumatic the cyclotropia may be symptomatic. A significant V pattern (15 or more difference between upward and downward gaze), commonly accompanied by chin depression, occurred in 48% of our patients with bilateral, but also in 5% of patients with unilateral paralysis.202 The V pattern is caused by a decrease in the abducting effect of the superior oblique(s) in depression and overaction of the inferior oblique muscle(s) and

may be accompanied by chin depression. Finally, in patients with bilateral paralysis the vertical deviation in primary position is usually smaller than with unilateral involvement since the loss of the depressing function in one eye tends to balance the same loss in the fellow eye.136, 202 The differential diagnostic points between unilateral and bilateral involvement are summarized in Table 20–6. Ellis and coworkers68 made the interesting observation that surgical overcorrection of a unilateral superior oblique paralysis may masquerade as an apparent contralateral superior oblique paresis. PARALYSIS VS. PSEUDOPARALYSIS. Premature unilateral stenosis of the coronal sutures (plagiocephaly) may cause a pseudoparalysis of the superior oblique muscle with upshoot in adduction because of desagittalization of the planes of these muscles.10, 218a The retroplacement of the trochlea in these cases increases the angle between the reflected part of the superior oblique tendon and the plane of the inferior oblique muscle (Fig. 20– 21). This reduces the vertical effect of the superior oblique muscle while increasing its incyclotorsional effect and causes an imbalance of opposing muscle forces in favor of the inferior oblique muscle. Figure 20–22 shows the rather characteristic appearance of a patient with premature closure of the left coronal suture and marked hypertropia of the left eye. Limón de Brown and coworkers163 studied with anthropometric methods the relationship between orbital malpositioning and strabismus in plagiocephalic children and established a TABLE 20–6. Diagnosis of Bilateral Superior Oblique Paralysis

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FIGURE 20–21. Left plagiocephaly. Recession of the left trochlea reduces the length of the unreflected part of the left superior oblique (SO) muscle thus decreasing its contracting power and causing imbalance in relation to the contracting power of the ipsilateral inferior oblique (IO). Angle alpha is greater than angle beta, causing diminished vertical effect of SO. RE, Right eye; LE, left eye; a, distance between the sagittal axis of two muscles in orbit. (From Bagolini B, Campos EC, Chiesi C: Plagiocephaly causing superior oblique deficiency and ocular torticollis. A new clinical entity. Arch Ophthalmol 100:1093, 1982.) quantitative  correlation between the degree of orbital anomalies (vertical displacement, intorsion, and frontodisclination) and the hypertropia in the nasal field of the involved side. Cranial Nerve VI Paralysis The diagnosis of cranial nerve VI paralysis should not present any difficulties. The greatest esotropia occurs on attempts to abduct the paretic eye; with maximal innervational effort the palpebral fissure may widen in abduction. Most patients will

complain about double vision in lateral gaze and assume a compensatory face turn in the direction of the paralyzed muscle (Fig. 20–23). However, a face turn may be absent, and amblyopia presents a risk in young children with this condition.8 Because of the unopposed action of the antagonistic medial rectus muscle, esotropia will be present with the fixating eye in primary position. The differential diagnosis includes the Duane retraction syndrome (see p. 458) and, with congenital esotropia, pseudoabducens paralysis simulated by crossed fixation (see p. 200) and the nystagmus blocking syndrome (see p. 512). In bilateral paralysis both eyes may be in a position of adduction and the patient may have problems ambulating unless one eye is occluded (Fig. 20–24). Milder forms of bilateral cranial nerve VI paresis may occasionally be confused with divergence paresis (see p. 505). In the former the esotropia increases upon looking to the right or the left. Several recent studies have addressed the natural history of acquired abducens paralysis. In a retrospective study of 213 nontraumatic unilateral cases 78% experienced spontaneous recovery, 73% having recovered by 24 weeks.137 A prospective study investigated the natural history of acute traumatic unilateral and bilateral cranial nerve VI palsies.113 In a total of 33 patients spontaneous recovery occurred in 84% of the unilateral and in 25% of the bilateral palsies. BENIGN AND RECURRENT ABDUCENS PALSY OF CHILDHOOD. A benign and often recurrent24, 216, 255, 272 form of cranial nerve VI palsy occurs in children4, 144, 224, 266 (but has also been described in adults98), usually following upper respiratory infections or other forms of mild viral illness,

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FIGURE 20–23. Paralysis of the left lateral rectus muscle. Note face turn to the left (A) and inability to abduct the left eye beyond the midline (B). This patient had orthotropia in right gaze and had an esotropia of 15 in primary position which increased to 45 in left lateral gaze. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

immunizations, or impetigo.26 The patients usually recover in 3 to 4 months without developing any other neurologic signs or symptoms. Amblyopia prophylaxis is essential in children up to 4 years of age who acquire this condition. When pondering the diagnosis of benign cranial nerve VI paralysis we must remember that spontaneous remission of cranial nerve paralysis may also occur in children and adults with skull base tumors.267 MÖBIUS SYNDROME. Congenital bilateral abducens paralysis associated with facial diplegia and microglossia (Fig. 20–25) constitutes a syndrome named after the German neurologist Paul J. Möbius to whom we owe the first description of this entity in 1888.184 Unlike in the patient shown in the figure various degrees of esotropia may be present in primary position and the abducens paralysis may be incomplete or asymmetrical. The etiology seems to be multifactorial. An association with a midline defect,213 pituitary dwarfism,101 and hypogonadic hypogonadism,30 and a variety of brain stem anomalies80, 164 may exist that may be associated with involvement of other neural

structures.161 A vascular insult involving disruption of the embryonic subclavian artery supply during the sixth week of gestation presumably causing damage to the affected neural tissue has been proposed.229 Indeed, much evidence points toward a hypoxic or ischemic insult during early gestation and the syndrome has appeared in association with maternal benzodiazepine51 and thalidomide ingestion69 during pregnancy. Of 23 patients with Möbius syndrome from the Royal Alexandra Hospital for Children in Sydney, Australia, 10 had a history of a potentially noxious event in utero81 (see also Miller and Strömland181). However, genetic factors may also play a role, since autosomal dominant, autosomal recessive, and X-linked inheritance, as well as a deletion in chromosome 1317 have been reported in patients with the syndrome.57, 62, 229 Acquired Möbius syndrome was observed in a patient treated with surgery and radiation for a medulloblastoma of the rostral portion of the cerebellar vermis233 and a primary mesodermal dysplasia of the extraocular musculature has also been implicated.261

FIGURE 20–24. Bilateral sixth nerve (N VI) paralysis. This patient had a congenital right N VI palsy for which she had compensated with a face turn to the right. She then suffered a traumatic left N VI palsy and could no longer avoid diplopia by turning her head. Occlusion of the left eye enabled her to get around with a severe face turn until surgical correction was performed. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983.)

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FIGURE 20–25. Möbius syndrome. A, Note lack of facial innervation causing a masklike expression of the face and hypoplasia of the left side of the tongue. B, Normal elevation and depression with V-pattern esotropia in downward gaze but inability to abduct either eye. (Courtesy of James W. Shigley, C.R.A., Cullen Eye Institute, Houston, TX.)

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Miller and Strömland have reviewed the recent literature on Möbius syndrome, which they feel is better described as Möbius sequence, the term sequence being defined as a "cascade of secondary events after an embryonic insult from heterogenous causes."181 Surgery is useful in some patients with esotropia in primary position. Large recessions of both medial rectus muscles,131, 251, 270 a combined recession-resection operation,170 or muscle transpositions110, 170 have been recommended. For a more detailed discussion of the treatment of complete and partial abducens paralysis, see page 446. Skew Deviation A transient vertical divergence of the eyes, whereby one eye is elevated and the other depressed, may occur in association with brain stem, cerebellar, or vestibular disease. As a rule, the hypotropic eye is ipsilateral to the affected side; but variations occur, and this deviation has no consistent localizing or lateralizing value. Skew deviations are caused by damage to the tonic otolith-ocular pathways133 of the brain stem tegmentum, the cervicomedullary junction, or both.97 However, they can also be elicited in normal subjects by stimulation of the semicircular canals.128 The presence of ocular torsion in skew deviations associated with brain stem infarctions has been emphasized.31 Skew deviations are not always comitant but may vary in different positions of gaze.97 Upshoot of the adducted eye and downshoot of the abducted eye in skew deviation may easily be confused with overaction of the oblique muscles. The differentiation of skew deviations from a cyclovertical muscle palsy may be difficult and made possible only by associated signs of brain stem disease and the absence of midbrain and peripheral nerve disease.50, p.135 Lengthening of the superior oblique tendon167 and extraocular muscle injection with botulinum toxin194 have been reported to eliminate diplopia in skew deviations. Double Elevator Paralysis An apparent paralysis of both elevator muscles (superior rectus and inferior oblique muscles) is an unusual anomaly of ocular motility which was first described by Dunlap.64 When the patient fixates with the nonparetic eye, the paretic eye will take a hypotropic position and the upper lid may be

slightly ptotic (Fig. 20–26). Fixation with the paretic eye will cause a hypertropia of the nonparetic eye, and ptosis may disappear, provided the levator palpebrae is not involved. Elevation of the paretic eye from any position of gaze is severely restricted, hence the term double elevator palsy. Bell’s phenomenon is usually preserved206 (see Fig. 20–26) but may also be absent. The ductions of the paretic eye are normal in all other positions of gaze. The chin is usually elevated. This anomaly is often congenital and has been reported in identical twins.18 However, Jampel and Fells122 observed seven patients with an acquired form. All were adults with rapid onset of paresis or paralysis of elevation of one eye unaccompanied by ptosis. Diplopia was present in upward gaze, and in some patients associated anomalies of the pupils and other extraocular muscles were present. These authors postulated that monocular elevation paresis could be attributed to a unilateral lesion in the pretectum, probably as a result of occlusion of one of the fine blood vessels supplying this area (see also Lessel162 and Ford and coworkers76). The findings of Ziffer and coworkers,279 who reported upgaze saccadic velocity to be normal below but not above the midline also suggests a supranuclear elevation insufficiency. Acquired monocular elevation deficiency has also been reported in a child with a pituitary mass lesion (pineocytoma).189 These reports suggest that neuroimaging is advisable in all patients with acquired elevation deficiency of either eye. The etiology of this condition is obscure, especially if one accepts the view of Warwick,269 that the nerve fibers to the superior rectus muscle are crossed and those to the inferior oblique muscle are not and that both muscles are innervated by different branches from the oculomotor nerve (see Chapter 3). Indeed, one must consider the distinct possibility that the frequently used term double elevator paralysis is a misnomer and that generalized weakness of elevation is caused by a superior rectus palsy of long standing, the deviation having spread throughout the entire upward field of gaze and the inferior rectus having become contracted. In support of this is the view126 that the superior rectus muscle is the principal elevator not only in abduction and primary position but also in adduction.27 Against this theory and more in accord with a supranuclear lesion would be the finding that Bell’s phenomenon is often preserved (see Fig. 20–26). This would indicate

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FIGURE 20–26. Apparent paralysis of both elevating muscles of the right eye. In the primary position the palsied eye is hypotropic, and there is pseudoptosis when the left eye is fixating. Large left hypertropia (secondary deviation) appears when the patient fixates with the right eye (middle). There is limitation of elevation in all fields of gaze. Intact Bell’s phenomenon in this patient indicates the presence of superior rectus innervation. Forced ductions (top) were negative.

that the superior rectus muscle is not paralyzed. An absence of Bell’s phenomenon could be explained on the basis of inferior rectus muscle tightness. The results of MRI of the extraocular muscles in patients with apparent paralysis of both elevating muscles have added little to clarify the etiology. While one group of authors reported normal superior rectus volume in their patients,206 another study40 described the opposite: volume changes in that muscle that are characteristic of denervation atrophy. In the absence of information on the appearance of the inferior oblique muscles on MRI, the crucial question of whether this muscle is involved in the elevation deficiency has not been answered. The differential diagnosis of a double elevator paralysis includes inability to elevate the eye because of mechanical restriction involving the inferior aspect of the globe caused by a blow-out fracture of the orbital floor, congenital or acquired fibrosis, endocrine myopathy, anomalous insertion of the inferior rectus muscle,169 or an abnormal accessory muscle between the annulus of Zinn and the posterior part of the globe.265 A positive forced duction test and increased intraocular pressure in upward gaze may distinguish these conditions from double elevator palsy although it must also be considered that this muscle may become tight as a result of the elevation deficiency.

Double Depressor Paralysis An apparent paralysis of both depressor muscles of one eye (inferior rectus and superior oblique) occurs only infrequently. We have encountered it only in the congenital form but an acquired form has also been described.85 As with double elevator palsy, the etiology is obscure and is even more difficult to rationalize in terms of a central lesion, since both depressor muscles are innervated from different nuclei. Analogous to the mechanism of apparent double elevator palsy it is possible that so-called double depressor paralyses are caused by inferior rectus muscle paralysis of long standing and secondary superior rectus contracture. On the other hand, normal volume of the inferior rectus muscle was present on MRI in a patient with congenital double depressor paralysis who had a positive forced duction test on depressing the globe.207 Tightness of the superior rectus muscle on the basis of extraocular muscle fibrosis (see Chapter 21) may be another cause. When the nonparetic eye is fixating, the paretic eye is hypertropic in primary position (Fig. 20– 27). With the paretic eye fixating, the nonparetic eye may be hypertropic in primary position. Ductions are restricted in the entire lower field of gaze and normal in all other gaze positions. Mechanical causes that interfere with depression of the eye

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FIGURE 20–27. Apparent paralysis of the depressor muscles of the right eye. In the primary position the paralyzed eye is slightly hypertropic and there is inability to depress the eye from the primary position, adduction, or abduction.

must be excluded. We have seen one patient in whom an apparent double depressor paralysis developed on a mechanical basis following surgical exploration of a mucocele of the frontal sinus with subsequent extensive scarring of the upper fornix. Supranuclear and Internuclear Paralysis Lesions of the supranuclear oculomotor pathways or centers cause bilateral conjugate paralysis of associated muscle groups. Gaze paralyses or spastic conjugate deviations may occur in dextroversion, levoversion, elevation, or depression, depending on the site of the lesion. A discussion of these anomalies is beyond the scope of this text, except that in longstanding cases surgery on yoke muscles may shift the eyes toward primary position and reduce a compensatory face turn. Internuclear ophthalmoplegia must be considered in the differential diagnosis of isolated medial rectus paralysis. Internuclear ophthalmoplegia is caused by lesions in the medial longitudinal fasciculus. Adduction is abolished unilaterally or bilaterally (Fig. 20–28), and an asymmetrical nystagmus is present involving predominantly the

abducting eye. The nystagmus in these patients is a secondary response to the weakness of adduction and not caused directly by the central defect.203, 278 Disseminated sclerosis is the most common cause of bilateral internuclear ophthalmoplegia, and the unilateral type nearly always is caused by an infarct of a small branch of the basilar artery.50 Glaser83 pointed out that limitation of adduction and nystagmus of the abducting eye can be caused also by ocular myasthenia. We have described pseudointernuclear ophthalmoplegia in patients with paresis of both medial rectus muscles and a jerking nystagmus of the abducting eye following recession and retroequatorial posterior fixation of both medial rectus muscles.203 Therapy of Paralytic Strabismus In determining whether a patient with paralytic strabismus will require therapy, one must establish the extent to which the paralysis interferes with comfortable single binocular vision. The eyes rarely move more than 15° from the primary position during normal use, and diplopia in extreme positions of lateral or vertical gaze is tolerated by most patients. For instance, a patient with a paretic

FIGURE 20–28. Bilateral internuclear paralysis in a patient with demyelinating disease. Note limitation of adduction in both eyes. (Courtesy of Dr. John McCrary III, Austin, Texas.)

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right lateral rectus muscle may be comfortable with his eyes in primary position and in levoversion and experience diplopia only in dextroversion. A slight head turn toward the right may eliminate the need to move the eye into that position, in which case therapy obviously is not required. On the other hand, what is easily tolerated by one patient may not be acceptable to another. When one assesses a patient’s disability and determines whether treatment is needed, the occupational visual requirements must be considered on an individual basis. In determining medical disability special consideration must be given to construction workers and others working at great heights. Diplopia in a peripheral gaze position that may be easily tolerated by, for example, an office worker may prove to be a hazard in such cases. Thus indications for therapy are the presence of diplopia in the practical field of fixation (see Chapter 4) and inability to maintain single binocular vision without a conspicuous anomalous head posture. The inability to maintain single binocular vision without a conspicuous head posture not only is cosmetically distressing but also, as mentioned, may cause secondary structural changes in the cervical spine. Nonsurgical Therapy Therapy for incomitant paralytic strabismus is aimed at aligning the eyes in positions of gaze in which a deviation exists without disturbing single binocular vision elsewhere in the field of fixation. While surgery is necessary to achieve this goal in most instances, conservative methods should be considered in suitable cases. Prisms are most effective in treating comitant and in many instances incomitant paralytic strabismus of small amplitude. When a deviation is less than 10 we have found prismatic correction to be most effective in deleting diplopia. For larger deviations, prisms rarely are tolerated for prolonged periods and surgery becomes unavoidable. In some cases of comitant strabismus, segmental membrane (Fresnel) prisms may be considered and will eliminate diplopia. Segmental occluding devices that restrict the effect of occlusion or prisms to one position of gaze may also be feasible. When double vision is restricted to downward gaze, segmental occluding devices or segmental membrane prisms that restrict the effect of occlusion or prisms to one position of gaze occasionally may be feasible. When double vision is restricted to downward gaze, for instance,

in cranial nerve IV paresis, and the patient’s age, medical condition, or other reasons militate against surgery, occlusion of the lower third of the spectacles lens before the paretic eye with semiopaque adhesive tape is effective and readily accepted by most patients. The same method is useful when double vision is present in lateral gaze in patients with mild cranial nerve VI paresis (Fig. 20–29). In desperate situations in which single binocular vision cannot possibly be restored by any means, occlusion of one eye, preferably the sound eye, is a last resort to create visual comfort for the patient. Improvement in surgical techniques and, in recent years, the recognition and successful surgical management of mechanical restriction of ocular motility have made it possible to save the majority of patients with paralytic strabismus from permanently wearing a patch. Several methods have been advocated to prevent secondary contracture of the antagonist of a paretic muscle in lateral rectus paralysis that in some but not all cases takes place and that will present obstacles to later surgical alignment. The antagonistic muscle has been injected with a local anesthetic or with 15% alcohol. A. B. Scott238 advocated injection of the antagonist of a palsied muscle with botulinum toxin under electromyographic control. Surgical Therapy When conservative therapy fails or the deviation is of such magnitude that it may not even be considered, surgery becomes necessary. The timing of an operation depends on the nature of the underlying paralysis. If the paralysis is longstanding, surgery may be performed as soon as the diagnosis is established. If the paralysis is of recent onset, a 6- to 8-month waiting period is mandatory for the condition to be considered stable; spontaneous recovery of function rarely occurs after that length of time. During the waiting period the patient should be evaluated at frequent intervals and visual comfort maintained with prisms or unilateral occlusion. The determination of the binocular field of fixation (see p. 447) is of special value in following such patients. Figure 20–30 shows the gradual recovery of a traumatic superior oblique palsy. The following discussion contains an outline of the surgical management of paralytic strabismus.

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FIGURE 20–29. Sector nasal occlution in a patient with mild bilateral abducens paresis. Note limitation of abduction of both eyes. The patient fused in the primary position but developed uncrossed diplopia caused by secondary esotropia in lateral gaze. A, Nasal sector eliminates diplopia in dextroversion, and B, in levoversion.

For dosages and technical details of each operation, the reader is referred to Chapter 26. PARALYSIS OF RECTUS MUSCLES. The general principle of strabismus surgery—weakening the action of an overacting muscle and strengthening the action of an underacting muscle—also is applicable, with some exceptions, to the management of paralytic strabismus. Resections are indicated only in pareses where they may enhance the action of the weak muscle. In complete paralyses the effect of muscle resections will only be of a temporary nature. LATERAL RECTUS MUSCLE. A maximal recession-resection procedure suffices in most instances of incomplete abducens paralysis to restore a useful field of single binocular vision and to eliminate the head turn. The forced duction test (see Chapter 26) will determine whether contracture of the medial rectus muscle is present and the estimation of generated muscle force (see Chapter 26) whether the paralysis is complete or incomplete. For a complete paralysis of the lateral rectus a resection of that muscle will not only fail to improve the patient but will destroy the anterior ciliary arteries from that muscle. In both children and adults we prefer a transposition of the full inferior and superior rectus tendons to the insertion of the lateral rectus muscle, as suggested by Berens and Girard.19 In older patients we transpose only the temporal half of each vertical

rectus muscle, taking care to preserve at least one anterior ciliary artery in that part of the muscle that remains attached to the sclera. Carlson and Jampolsky44 also transpose only the temporal aspect of the superior and inferior rectus muscles and apply an adjustable suture to these muscle segments (see also Bechac and coworkers16). The Jensen muscle union (see Chapter 26) is less frequently performed now than only a few years ago since it has been shown that this procedure does not necessarily protect against anterior segment necrosis. Either procedure must be combined with a maximal recession of the medial rectus to be successful, provided contracture of that muscle has been shown to be present with the forced duction test. To preserve part of the blood supply to the anterior segment by avoiding the operation on the medial rectus muscle, injection of the medial rectus with botulinum toxin (see also Chapter 25) has been recommended.73, 74, 157, 168, 174, 175, 218, 221, 223, 239 Despite this precaution several cases of anterior segment ischemia have been reported after this procedure.104, 134 It may be argued that the ischemia could have occurred from detaching the vertical rectus muscles alone and was unrelated to the injection.134, 158 Whereas normal abduction can never be established by surgery in a complete abducens paralysis and adduction usually becomes restricted after the transposition or Jensen procedures, these operations are quite effective in moving the

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FIGURE 20–30. Progressive enlargement of the field of single binocular vision in a patient recovering from traumatic right superior oblique palsy. A small field of residual vertical diplopia remained 1 year after injury on gaze upward and left as a result of overaction of the ipsilateral inferior oblique muscle.

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adducted eye into primary position, in restoring a limited field of single binocular vision, and in eliminating or decreasing the head turn. Even a small abduction movement can occasionally be reestablished. This is caused by the springlike action of the transposed vertical muscles upon relaxation of the medial rectus rather than by active innervation of the lateral rectus. In bilateral complete abducens paralysis we operate on both eyes in the same session. MEDIAL RECTUS MUSCLE. Transposition of the vertical rectus muscles close to the upper and lower border of the medial rectus muscle is indicated. VERTICAL RECTUS MUSCLES. For paralysis of the superior or inferior rectus muscles, a simple resection-recession operation of the vertical rectus muscles usually is effective. If, in the case of paresis of the superior rectus muscle, the deviation is limited to upward gaze or, in the case of paresis of the inferior rectus, to downward gaze, 4-mm resection of the paretic muscle without recession of its antagonist may suffice. The question arises whether surgery should be performed on the fixating or nonfixating eye. With rare exceptions, if the horizontal or vertical rectus muscles are paralyzed, we prefer to operate on the paretic eye regardless of whether it is the dominant or nondominant eye. The amount of surgery that is necessary varies, of course, depending on whether the paretic eye (secondary deviation) or the nonparetic eye (primary deviation) habitually fixates. PARALYSIS OF INFERIOR OBLIQUE MUSCLE. The generalization to weaken the overacting muscle and strengthen the underacting muscle does not apply to inferior oblique paralysis, the reason being that resection or advancement of the inferior oblique, although technically possible, has consistently yielded unsatisfactory results in our hands. Instead, we recess the superior rectus and resect the inferior rectus muscle of the normal eye. In the presence of marked overaction of the unopposed superior oblique muscle a tenotomy of that muscle has been equally effective in our experience209 and that of others102, 245 and may be augmented by recession of the contralateral superior rectus muscle.135 This operation eliminates incyclotropia and an existing head tilt and may improve function of the paretic inferior oblique muscle; however, a gradually progressing

superior oblique palsy may develop in the eye operated on and require additional surgical treatment.209 COMPLETE CRANIAL NERVE III PARALYSIS. The surgical management of a complete cranial nerve III paralysis presents a formidable challenge and the therapeutic possibilities are limited. As may be expected, the sensorimotor outcome of treatment in children is poor.187 At the very best, the surgeon will succeed in moving the paretic eye into the primary position without restoring adduction, elevation, or depression to a significant degree. Before embarking on what often turns out to be a whole series of operations, a detailed discussion with the patient is necessary in which it must be pointed out that double vision is likely to persist in certain gaze positions and may, in fact, become more bothersome after surgery when the images are closer together. The surgeon must ascertain exactly what the patient expects from the operation. Some patients with a complete cranial nerve III paralysis and ptosis are clearly better off without surgery. This goes especially for older patients! In younger patients most surgeons are tempted to improve the situation. A maximal recession-resection of the horizontal rectus muscles will, at best, create only temporary improvement of the eye position. Eventually, the eye will drift back into an abducted position and a more radical approach will be called for. After trying several procedures, including maximal horizontal surgery with upward transposition of the muscle tendons or transposition of the superior oblique tendon to the insertion of the medial rectus muscle, according to Wiener274 the following operation has given the best results: tenotomy of the lateral rectus and superior oblique muscles combined with a transposition of the vertical rectus muscles to the insertion of the medial rectus muscle. Even though the treated eye will continue to be immobile, it will at least be centered and this operation should be considered especially in patients who fixate with the paralyzed eye and are thus forced to maintain an extreme head turn to the opposite side. Kaufmann129 reported a satisfactory surgical result in two patients who had a combined paralysis of cranial nerves III and IV. The lateral rectus muscle was split, its upper half was transposed to a retroequatorial point near the nasal superior vortex vein, and the lower half to a point near the nasal inferior vortex vein. The horizontal deviation was decreased by 15° to 20°. Other authors have

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suggested transposing the lateral rectus muscle to the medial side of the globe,120 fixating the globe with strips of fascia lata sutured onto the medial aspect of the globe and the nasal bone periostium,230 and decreasing surgically the elevation of the fixating eye.195 Kushner154 reported satisfactory outcomes in patients with paralysis of the inferior division of the third nerve after tenotomy of the superior oblique, transposition of the superior rectus to the medial rectus, and transposition of the lateral rectus to the insertion of the inferior rectus muscle. Taylor reported improvement after transposing the lateral rectus muscle to the medial portion of the globe.259 Young and coworkers277 suggested cutting the tendon at the medial border of the superior rectus muscle and reinserting it into the sclera 1.0 to 3.5 mm anterior to the medial border of the superior rectus insertion. This operation is combined with recession of the lateral rectus muscle and was reported to improve the exotropia and hypotropia in eight patients thus operated on. As may be expected, the prognosis for moving the eye at least into primary position by surgery becomes better when the cranial nerve III paralysis is incomplete and some recovery of medial rectus function has occurred. Surgery in such cases consists of a maximal recession of the lateral rectus muscle (at least 12 mm) and resection of the medial rectus muscle (at least 7 mm) with upward transposition of the tendons in case of an associated hypotropia. This may restore a small but useful field of single binocular vision even though double vision will persist in up- and downward gaze. If the eye remains in primary position after surgery, the upper lid may be suspended with an adjustable fascia lata sling in a second operation, unless a significant amount of hypotropia persists. In that case, attempts to elevate the lid are contraindicated, because exposure keratitis will invariably occur. In certain patients with cranial nerve III palsy, ptosis, exotropia, and aberrant regeneration, the lid position may improve when adduction is attempted because of abnormal yoking between the levator palpebrae on the paralyzed side and the contralateral lateral rectus muscle. Such patients may benefit from having the exotropia corrected by performing surgery on the horizontal recti of the normal eye.85, 204, 250 This operation

will work only if the patient prefers the nonparetic eye for fixation. Postoperatively, the patient must abduct the fixating eye to bring it into the primary position and this impulse, transmitted to the paretic eye, will, according to Hering’s law, elevate the drooping lid simultaneously. CRANIAL NERVE IV PARALYSIS. Surgical treatment of superior oblique paralysis presents no particular difficulties and is, as a rule, uniquely gratifying to the patient and surgeon. Our surgical methods depend on the classification (see Table 20–5) and are summarized in Table 20–7.202 This approach is similar, though not identical, to that advocated by Knapp and Moore,143 Helveston and coworkers (190 patients),106 Simons and coworkers (123 patients),246 Gräf and coworkers,90 and others. No stereotypic mode of treatment exists, and the surgeon must remain flexible because not all patients can be fitted into the classification proposed by Knapp and Moore or its modification by Helveston and coworkers.108 In patients of class 7 (Knapp) in whom there has been direct trauma to the trochlea, the involved area must be freed by surgical exploration and lysis of adhesions. Whether a recession of the contralateral inferior rectus or of the ipsilateral superior rectus is performed in patients belonging to classes 4 and 5 depends on the outcome of the forced duction test under general anesthesia. If the test is positive on attempts to depress the paralyzed eye, contracture of the superior rectus muscle must be suspected and that muscle is recessed 4 to 5 mm along with weakening the inferior oblique or tucking the superior oblique muscle, or both. Inclusion of the TABLE 20–7. Surgical Treatment of Superior Oblique Muscle Paralysis

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superior rectus muscle in the operation has resulted in excellent surgical results in such cases.9 If the traction test is negative, the contralateral inferior rectus is recessed by a similar amount and may be placed on an adjustable suture in suitable patients. When excyclotropia is the only complaint and a vertical deviation is absent, a superior oblique tendon transposition according to Harada and Ito (see Chapter 26) is indicated. Because of the laxity of the tendon in congenital paralysis a large tuck (12 mm or more) can be performed without difficulties. On the other hand, in acquired cases the tendon appears tight at the time of surgery and a tuck of more than 6 to 8 mm may not only be difficult to accomplish but is contraindicated for it may cause a postoperative Brown syndrome with double vision in upward gaze. Pre- and postoperative rotational forced duction testing of the tightness of the superior oblique tendon93, 214, 215 and, if necessary, surgical revision of the tuck have greatly reduced this complication in our experience. In bilateral cases we operate on both eyes at the same time when the paralysis is of equal severity on both sides. A tuck of both tendons is performed, to which we add a myectomy of the inferior oblique muscles in cases of marked overaction of these muscles. When the paralysis is unequal on both sides we operate first on the more severely involved eye. In a preliminary report Jampolsky127 advocated bilateral inferior oblique myectomy combined with 12-mm recessions of both superior rectus muscles to eliminate the chin depression and restore single binocular vision in the reading position. The effect of surgery according to the approaches outlined in Table 20–7 was evaluated in 112 patients with unilateral and bilateral superior oblique palsies.202 The mean follow-up was 19.8 months (range, 3 to 115 months). A cure, defined as the elimination of the signs or symptoms that caused the patient to seek medical help (diplopia, asthenopia, cosmetically disturbing hypertropia, torticollis), was achieved in 85% of the patients. In the remainder, additional surgery is planned or the patient did not return. It must be noted, however, that a total of 162 operations (1.45 operations per patient) were necessary to achieve a cure. Thus in discussing surgery with the patient it must be mentioned that although the prognosis is good, the

probability of having more than one operation is about 50%. Other authors have reported a lower rate.246 We realize that there are alternative approaches106, 246 to the surgical treatment of superior oblique paralysis but feel comfortable with the results achieved with the procedures outlined in Table 20–7. Congenital absence of the superior oblique muscle is often not suspected until the surgeon is unable to locate the superior oblique tendon. In that case the operation depends on the presence of preoperative inferior oblique muscle overaction.268 If such overaction was present and the hypertropia in the paralyzed field of gaze is less than 25 we myectomize the inferior oblique and recess the ipsilateral superior rectus muscle 3 to 4 mm. If the deviation in the paralyzed field is greater than 25 a recession of the contralateral inferior rectus muscle is added. When there is no preoperative upshoot in adduction we recess the contralateral inferior rectus muscle. If cyclotropia in downward gaze is the only complaint, nasal transposition of the inferior rectus muscle is performed.188 DOUBLE ELEVATOR AND DOUBLE DEPRESSOR PARALYSES. Not every case requires surgery, which is indicated only when there is hypotropia of the involved eye in primary position or chin elevation or both. The aim of surgery is to restore single binocular vision in primary position. Knapp142 introduced vertical transposition of the horizontal rectus muscles to the medial and lateral edge of the superior rectus muscle insertion (Knapp procedure; see Chapter 26) for the treatment of this condition. This operation has been successful in our hands and that of others41, 130 provided there is no contracture of the inferior rectus muscle. If contracture is present a recession of that muscle becomes also necessary. In cases where it is difficult to determine whether the hypotropia is maintained by a primary inferior rectus tightness alone or by an innervational deficit of the elevator muscle(s) it is best to perform the operation in two stages, beginning with recessing the inferior rectus muscle, after which the situation should be reassessed. The same approach is advocated in older patients to lessen the risk of anterior segment ischemia (see Chapter 26). A complication of surgery is limitation of depression of the eye operated on, with diplopia in downward gaze. In such instances we have performed a recession of the contralateral inferior rectus muscle on an adjustable suture and eliminated the

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problem. The long-term results of this procedure for double elevator paralysis have been described as "stable,"35 a description with which we concur. An analogous approach is used to treat a double depressor paralysis: both horizontal rectus muscles are transposed to the medial and lateral edge of the inferior rectus insertion (inverse Knapp procedure.)65 SUPRANUCLEAR GAZE PARALYSIS. An anomalous head posture, especially a face turn in the direction of the gaze palsy, may cause great discomfort to the patient. The general health of the patient permitting, such cases may be considered for surgery on the extraocular muscles. An operative approach similar to that in treating anomalous head posture in manifest congenital nystagmus (see Chapter 23) should be used and we have obtained satisfactory results after unconventionally large recessions of yoke muscle (modified Anderson operation; see p. 523). For instance, in a conjugate palsy to the left with a face turn to the right, we recess the right lateral rectus muscle 12 mm and the left medial rectus muscle 10 mm. This dosage may be modified in cases with a coexisting horizontal strabismus.48 Good surgical results have been reported not only in supranuclear but also in internuclear paralyses.34 Alternative Methods Ophthalmologists have for many years dreamed about and experimented with methods to restore function to or replace a paralyzed muscle with a prosthetic device. Electrical stimulation of a muscle via a subcutaneously implanted radiofrequency receiver46 and secondary muscular neurotization of the lateral rectus by implantation of a nerve or neuromuscular pedicle from the adjacent inferior oblique5, 12, 92 are but a few alternative treatment methods currently under investigation.13, 21 A. B. Scott and coworkers241 reported successful implantation of a silicone rubber band along the course of a paralyzed lateral rectus muscle. The band has been in place for 7 years and provides a spring against which the antagonistic medial rectus muscle can pull so that ocular alignment and a 20° field of single binocular vision is restored. Similar success was reported by the same authors with a superior oblique muscle prosthesis. Kolling146 reported encouraging results in two patients in whom a paralyzed lateral rectus muscle was augmented by an elastic silicone tube that was fixed to the orbital wall. REFERENCES 1. Adelstein F, Cüppers C: Zur Diagnoses des Strabismus paralyticus. Klin Monatsbl Augenheilkd 141:335, 1962. 2. Adelstein F, Cüppers C: Probleme der operativen Schielbehandlung. Ber Dtsch Ophthalmol Ges 69:580, 1969. 3. Adler FH: Pathologic physiology of convergent strabismus. Motor aspects of the nonaccommodational type. Arch Ophthalmol 33:362, 1945. 4. Afifi AK, Bell WE, Menezes AH: Etiology of lateral

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