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 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.
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.)
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 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.