General Diagnostic Evaluation
Many of the causes of unexplained vision loss resulting from media opacities or irregularities can be detected by a careful anterior segment examination. Slit lamp biomicroscopy allows detection of most anterior media opacities or irregularities in the optical interface between structures of the ante. rior segment. Special attention must be paid to the contour, as well as the clarity, of the anterior segment structures. Keratometry may be helpful in detecting irregularities in the anterior surface of the cornea. The placido disc and computerized corneal topography are also useful in evaluating the shape of the cornea. Retinoscopy may also allow identification of refractive irregularities in the cornea and lens. Posterior media opacities, such as vitreous hemorrhage, are easily detected by indirect ophthalmoscopy. The direct ophthalmoscope is a very useful tool for a semiquantitative estimate of the extent of vision loss from media opacities, The view of the optic nerve or macula using a direct ophthalmoscope is generally proportional to the sum of all opacities of the ocular media.
MACULAR TESTING THROUGH MEDIA OPACITIES
The presence of an opacity in the ocular media (e.g., a corneal opacity, cataract, or vitreous opacity) may prevent a thorough evaluation of the macula. Several ancillary tests are useful for evaluating macula function prior to corneal surgery, cataract extraction, or vitrectomy, when the macula is poorly seen by ophthalmoscopy alone.
Fluorescein angiography gives a general assessment of macular anatomy and may uncover macular dysfunction obscured by a media opacity. The optics of fundus cameras are similar to the indirect ophthalmoscope, and fluorescein angiography may show macular disease that is not readily apparent by ophthalmoscopy alone.
Other diagnostic tests are more useful when there is a dense media opacity. The entoptic phenomenon can be tested by placing a fiberoptic transilluminator over the closed lower or upper eyelid and moving the transilluminator slowly in a circular motion. The patient should be able to see readily the shadow of the perimacular retinal vessels, which resemble branches of a tree. This test is very useful and should be considered before resorting to other special diagnostic tests. Some patients with normal macular function, however, have difficulty understanding this test and may respond negatively despite normal macular function. The blue-field entoptic test is performed by directing a 530-nm blue light into the eye. The patient often visualizes the perimacular vessels more easily in this test than in the transilluminator test and may also see small dots traversing the perimacular capillaries. These dots have a surrounding light halo and are believed to be white blood cells within a column of red blood cells in the perimacular capillaries.
The entoptic tests have the advantage that they can be performed through very dense media opacities. The laser interferometer may allow a more quantitative measure of macular function through mild-to-moderate media opacities. The Potential Acuity Meter also may be used to quantify visual acuity through focal media opacities but generally cannot be used with media opacities severe enough to obscure the retina under indirect ophthalmoscopy.
The evaluation of unexplained vision loss must always include a careful refraction. Retinoscopy may be helpful as a screening tool because some patients give conflicting responses during subjective refraction. Retinoscopy may also uncover occult corneal or lenticular abnormalities that are potentially amenable to refractive correction with spectacles or contact lenses. Patients with nuclear sclerotic cataracts may develop progression of myopia and present with unexplained vision loss. Keratometry and computerized corneal topography may be useful to guide the retinoscopy and subjective refraction in some persons with high astigmatic errors or diseases that alter the normal spherical shape of the cornea (e.g., keratoconus and Terrien's marginal degeneration).
Corneal Opacities and Irregularities
Keratoconjunctivitis sicca and other corneal-surface diseases may lead to disruption of the precorneal tear film, thus altering the smooth air-cornea interface, causing optical irregularities and decreased visual acuity. The precorneal tear film and corneal surface should be evaluated by slit lamp biomicroscopy. Damage to the corneal epithelium from recurrent corneal erosions may also cause irregularity in the anterior surface of the cornea, leading to visual loss. Corneal erosions are easily identified by slit lamp examination after the instillation of fluorescein dye into the eye. A piano contact lens may be used to neutralize an irregular anterior corneal surface, with improvement in the visual acuity.
Abnormalities in the shape of the cornea may also lead to unexplained vision loss. Anterior keratoconus may not be evident on slit lamp biomicroscopy unless the central cornea is thinned, the anterior chamber is unusually deep, a Fleischer iron ring is present at the base of the cone, or the lid is distorted by the cornea on down-gaze (Munson's sign) (Fig. 12.1). Keratometry will reveal an unusually steep corneal contour, often with irregular astigmatism, in patients with this condition. Corneal topography will confirm the steep contour of the cornea. Retinoscopy of eyes with keratoconus will show a central area of irregular astigmatism with a more myopic refraction centrally than peripherally. Posterior keratoconus, a concave indentation of the posterior corneal surface is usually only present in the central 2 to 3 mm of the cornea. Posterior keratoconus is rare but may decrease visual acuity, owing to the abnormal interface between the posterior corneal surface and aqueous humor. The central cone in posterior keratoconus may also become opacified, leading to decreased acuity.
Figure 12.1. Keratoconus with Munson's sign on down-gaze. (Courtesy of Ali Khodadoust.)
The increasing popularity of keratorefractive surgery has created new potential sources of corneal irregularity. Radial keratotomy may create irregular astigmatism by changing the normally spherical surface of the cornea. The shape of the cornea changes during the day and may exhibit changes over months or years, which may not be initially recognized as a changing refractive error. These patients typically complain of fluctuating or decreased acuity. Phototherapeutic keratectomy by the excimer laser also alters the spherical contour of the cornea and may create changes in the corneal contour. Excimer laser photokeratectomy may also result in opacification of Bowman's membrane with superficial corneal haze, but this is usually not visually significant. Irregular astigmatism may be unintentionally created by the excimer laser, which is difficult to correct during routine refraction with spectacle lenses. Computerized corneal topography is very helpful in uncovering abnormalities in the spherical contour of till cornea, which may be difficult to detect by other means, and it should be performed to those with unexplained visual loss and previous history of keratorefractive surgery.
Opacities in Bowman's membrane and the corneal stroma are usually easily seen and only occasionally result in unexplained vision loss. More commonly, it may be necessary to determine if the corneal opacity explains all of the vision loss. Discrete corneal opacities outside the center of the visual axim do not cause vision loss unless associated with irregular astigmatism, as may be found with a traumatic corneal scar. Discrete corneal opacities or diffuse corneal opacification within the central visual axis usually results in vision loss. The severity of the loss is generally predictable by the size and density of the corneal opacity in the visual axis. Central corneal opacities may also result from corneal dystrophies, such as macular or lattice dystrophy. Central corneal edema from corneal endothelial dysfunction may lead to mild or severe vision loss, depending on the severity of the corneal edema. The visual acuity may be retested in eyes with corneal edema after instillation of a topical anesthetic and topical glycerin, which transiently decreases the corneal edema. Deposits on the surface of the corneal endothelium (e.g,, keratic precipitates in eyes with anterior uveitis or extensive pigment deposition on the corneal endothelium) rarely cause decreased acuity.
Slit lamp biomicroscopy is the best means to assess decreased acuity resulting from corneal disease. Direct ophthalmoscopy is another useful technique for determining the extent of vision loss from corneal opacities. The view of the disc and retinal vessels in an eye with a well-dilated pupil is proportional to the extent of vision loss from the media opacity.
Circulating hyphema, or anterior-chamber Inflammation, may result in decreased visual acuity. This condition is easily detected by slit lamp biomicroscopy. Intraocular inflammation may also result in a translucent pupillary Membrane covering the anterior surface of the lens. Posterior synechiae between the iris and lens may be associated with pigment deposition on the anterior surface of the lens. The posterior synechiae may occasionally prevent pupillary dilation and obscure the central pupillary opening with pigment or fibrous tissue, leading to vision loss.
Lenticular Opacities and Irregularities
Cataracts are the most common cause of decreased acuity due to a media opacity. The severity of the central lenticular opacity is usually proportional to the severity of vision loss. In general, peripheral cortical lens opacities and small punctate cortical opacities cause minimal vision loss. Posterior subcapsular cataracts located in the visual axis tend to cause more significant vision loss from cataract. Cortical lens opacities and posterior subcapsular cataracts are easily seen by slit lamp biomicroscopy. Nuclear sclerosis is a much more common cause of unexplained vision loss from cataracts because this condition may induce increased myopia and interfere with the uniformity of the lens refractive power. Nuclear sclerosis may cause the lens to become a stronger plus lens centrally than peripherally. This causes the eye to become more myopic and may cause distortion in the image refracted by the lens onto the retina because of disparity between the central and peripheral lens power. Retinoscopy is useful for evalu-ating the optical uniformity of the lens in eyes with nuclear sclerosis. Direct ophthalmoscopy may also reveal the optical distortion induced by nuclear sclerosis when the examiner views the retina.
Subluxation of the lens may lead to unexplained vision loss if the center of the lens is displaced away from the visual axis. The subluxed lens may also be tilted, causing an astigmatic error due to induced astigmatism of oblique incidence. Subluxation of the lens may not be apparent if the lens is not ex-amined in a well-dilated pupil.
Irregularities in the surface of the lens are an uncommon cause of decreased acuity but may easily be missed if the contour of the lens is not examined carefully. Microspherophakia in persons with Weill-Marchesani syndrome and lentiglobus or lenticonus may result in myopia and irregularity in the optics of the lens. Anterior and posterior lenticonus are characterized by a central circular protrusion on the anterior and posterior surfaces of the lens, respectively. This may lead to an irregular astigmatism with progressive vision loss. Lenticonus may be associated with Alport syndrome, which is characterized by progressive renal failure and nerve deafness, with some patients also exhibiting vestibular dysfunction (Fig. 12.2).
Figure 12.2. Anterior lenticonus in Alport syndrome. The lenticonus was the major cause of vision loss in this eye. (Courtesy of All Khoclacloust.)
Vitreous opacities may be associated with unexplained vision loss, but these are usually easily detected by slit lamp biomicroscopy of the vitreous and indirect ophthalmoscopy. Although often considered a cause of decreased acuity, vitreous inflammation must be very dense in the visual axis to explain a significant decrease in acuity. Vitreous inflammation may result from an endogenous vitri-tis, such as pars planitis, or may be secondary to inflammation in the anterior segment or retina/choroid. Associated macular disease (e.g., cystoid macular edema or age-related macular degeneration) may be obscured by vitreous inflammation. Such associated macular disease is the more likely cause of vision loss rather than the vitritis, which often is erroneously considered the cause of vision loss. Large cell lymphoma (reticulum cell sarcoma) may cause vision loss if the vitreous cellular debris is severe or if there is a choroidal tumor within the macula. Rarely, neoplasms such as breast carcinoma, bronchogenic carcinoma, or cutaneous melanoma may metastasize to the vitreous, causing vitreous opacification. Seeding of a retinoblastoma into the vitreous over the macula or macular involvement by the retinoblastoma may cause decreased visual acuity in a child.
Vitreous hemorrhage may cause decreased acuity in a variety of vitreoretinal disorders. Vitreous hemorrhage may occur secondary to retinal vascular disease (e.g., proliferative diabetic retinopathy, sickle cell retinopathy, and venous occlusion). A second common cause of vitreous hemorrhage is a posterior vitreous detachment with or without an associated retinal tear or detachment. Vitreous hemorrhage may develop after ocular trauma due to bleeding from anterior segment structures (e.g., the root of the iris or ciliary body) or from bleeding in the posterior segment (e.g., a retinal tear or a choroidal rupture with a breakthrough vitreous hemorrhage). The density of the vitreous hemorrhage in the visual axis is the best predictor of visual acuity. In general, a diffuse vitreous hemorrhage causes less vision loss than a more focal vitreous hemorrhage located posteriorly over the macula. Focal vitreous hemorrhages often disperse in the first several weeks after onset, with an associated improvement in acuity if the hemorrhage is the primary cause of the decreased acuity.
Asteroid hyalosis results from deposition of calcium- and phosphate-containing lipid on vitreous fibrils (Fig. 12.3). This condition may make examination of the retina by if indirect ophthalmoscopy, direct ophthalmoscopy, and contact lens biomicroscopy difficult, but it rarely causes substantial visual loss. Persons with asteroid hyalosis generally have much better visual acuity than would be predicted by the examiner. Fluorescent angiography may useful in evaluating macula of a patient with asteroid hyalosis and decreased acuity, since the angiographic image of the retina is often better than the view by indirect ophthalmoscopy.
Figure 12.3. This patient had a visual acuity of 20/50 with asteriod hyalosis. A poorly defined choroidal neovascular membrane identified by fluorescein angiography was the actual cause of decreased acuity instead of the asterol hyalosis.
Posterior vitreous detachment may also be associated with focal opacities on the detached posterior hyaloid. The focal opacities on the posterior hyaloid usually occur at the site of previous attachment of the vitreous to the optic nerve. These opacities may be very troublesome to the patient but virtually never cause significant vision loss.
Preretinal hemorrhages can result from the same causes as vitreous hemorrhage. If the preretinal hemorrhage is trapped between the posterior hyaloid and the retina, it results in a more focal, dense opacity (Fig. 12.4). If the preretinal hemorrhage lies directly over the fovea, the visual acuity is usually decreased to the level of 20/400 or worse. Preretinal hemorrhages generally take longer to clear than intravitreal hemorrhages. Preretinal hemorrhages associated with trauma often obscure macular damage such as choroidal rupture or commotio retinae.
Figure 12.4. Preretinal hemorrhage between the posterior hyaloid and retina in an eye with proliferative diabetic retinopathy.