In general, the neuro-ophthalmic examination for an adult with recent-onset diplopia is straightforward. Topping the list of possible causes is ocular motor cranial nerve palsy. If examination findings are not consistent with an isolated cranial neuropathy, the clinician must broaden the differential diagnosis to include a variety of entities. Although the list of possible causes is long, those most frequently encountered include ocular myasthenia gravis, thyroid eye disease, Miller Fisher syndrome, and Wernicke encephalopathy. Yet, there are 4 additional causes of diplopia that also should be considered especially when there exists a history of childhood-onset strabismus, when extraocular movements are full, and when neurologic evaluation including imaging is normal.
DIPLOPIA OCCURRING WITH CHANGES IN OCULAR ALIGNMENT
Suppression zones associated with childhood strabismus tend to be regional (i.e., mostly nasal hemiretina for esotropia and mostly temporal hemiretina for exotropia) and are not replaced with a new eye position in adulthood. Thus, diplopia may occur when there has been a change in the direction and/or magnitude of strabismus causing a drift of the retinal image outside of the suppression zone (1,2). Changes in ocular alignment can occur either spontaneously or after strabismus surgery, head trauma, systemic illness, or paresis or restriction of extraocular muscle(s). Changes in ocular alignment account for approximately 60% of cases of diplopia in adults with a history of childhood strabismus (3). Treatment involves reverting back to the former angle of strabismus where suppression occurs using prisms, botulinum toxin injection, or extraocular muscle surgery.
Diplopia caused by changes in ocular alignment in adults with a history of childhood strabismus can be precipitated by a change in the patient's refractive management. For example, diplopia can occur when prescribing progressive “no-line” bifocal lenses for presbyopic adults with V-pattern esotropia or A-pattern exotropia who are symptom-free in primary gaze (3,4). Progressive bifocal lenses require the patient to look further into downgaze when reading compared with conventional flat-top “line” bifocal lenses. Changing to nonprogressive bifocals, high-set flat-top bifocals, or single vision reading spectacles will alleviate the diplopia (3,4).
DIPLOPIA OCCURRING WITH FIXATION SWITCH
A switch in fixation from the preferred or fixating eye to the nonpreferred or deviating eye should be considered as a possible cause of diplopia in adults with a history of childhood strabismus (5,6). Patients having constant, nonalternating strabismus with strong fixation preference with or without amblyopia may not be able to transfer suppression from one eye to the other eye. Instead, they experience diplopia when fixation is switched to their nonpreferred eye. For example, a patient with left esotropia and suppression may experience diplopia when forced to fixate with the left eye. Fixation switch diplopia can be either intermittent or constant and in 1 study accounted for 13% of cases of recent-onset diplopia in adults having had childhood strabismus (3).
The most likely cause is reduced visual acuity in the preferred eye below that of the deviating eye due to an inaccurate refractive correction, a myopic refractive shift, cataract, or retinal pathology (3). Fixation switch diplopia can be iatrogenic and result from monovision (the optical correction for presbyopia of one eye used for distance and the other eye for near) with the use of contact lenses, refractive surgery, or intraocular lenses (3,7). Monovision should be used cautiously or avoided entirely in adults with history of childhood strabismus and a strong fixation preference (7).
Treatment includes establishing the original fixation pattern by prescribing the optimal refractive correction for each eye, treating cataract or retinal disease in the previously fixating eye, or reversing monovision. If these procedures are ineffective, using Bangerter filters before the originally nonfixating eye and then over time tapering the filter density should be attempted to re-establish the original fixation pattern where suppression occurs. The partially occlusive Bangerter filters aim to restore fixation in the originally fixating eye by degrading central vision and decreasing sensitivity in the originally nonfixating eye.
DIPLOPIA OCCURRING WITH TORSION
Torsion or cyclotorsion is a wheel-like movement of the eye around its anteroposterior axis, the superior pole of the vertical tilting inward (intorsion and incyclotorsion) or tilting outward (extorsion and excyclotorsion). When misalignment of the eyes due to paresis or restriction of a cyclovertical extraocular muscle(s) occurs in an adult with previous normal binocular vision, torsional diplopia may occur (8). Because the cyclovertical muscles contribute to vertical and horizontal eye movements, the diplopic image is perceived as being tilted or slanted in addition to being displaced vertically and/or horizontally. Superior oblique palsy is the most frequent cause of torsional diplopia, accounting for approximately 67% of all cases (9,10). Other possible causes include thyroid eye disease and skew deviation (8). Torsional diplopia can occur without measurable strabismus and be associated with optical (uncorrected astigmatism with an oblique axis) (11) and retinal (i.e., epiretinal membrane, retinal reattachment surgery) causes (12,13).
Assessing torsion is advisable in an adult with recent-onset diplopia that cannot be corrected with prisms. Torsion is detected subjectively with the double Maddox rod test or objectively using fundus photography or binocular indirect ophthalmoscopy. With the former, the patient views a penlight at 40 cm in primary gaze through a red Maddox rod before one eye and a clear Maddox rod before the fellow eye. A small vertical prism before 1 eye may be used to assist with dissociation. Since the Maddox rods are placed with their axis vertically at 90°, the patient will see 2 horizontal lines, 1 red and 1 white. Without torsion, both lines are perceived by the patient as being oriented horizontally and parallel to the floor. Torsion is diagnosed when 1 or both lines are perceived as being tilted. The Maddox rod(s) before the affected eye(s) is rotated either clockwise or counterclockwise until both lines appear parallel. The amount of rotation from 90° is the magnitude of torsion, and the direction of rotation of the Maddox rod(s) differentiates excyclotorsion and incyclotorsion.
As indicated above, torsional diplopia cannot be treated with prisms. Amounts of torsion exceeding 10° can disrupt fusion entirely and the resulting diplopia misdiagnosed as being intractable (8,14). Using a haploscopic device such as the synoptophore (which allows for presentation at a different image to each eye), torsion can be neutralized along with any coexisting vertical/horizontal deviation, permitting assessment of fusion potential. When fusion is demonstrated, eradication of diplopia may be achieved with extraocular muscle surgery.
DIPLOPIA OCCURRING WITH RETINALLY INDUCED ANISEIKONIA
Aniseikonia, the binocular condition stemming from a significant difference between eyes in the perceived size and/or shape of the ocular images, is caused primarily by unequal refractive error of the 2 eyes or anisometropia, one eye requiring a different lens correction than the other. Clinically significant aniseikonia (greater than 2% image size disparity) occurs in approximately 1%–3.5% of the general population (15). Aniseikonia also may be due to retinal disorders such as epiretinal membrane, macular hole, macular edema, or after retinal detachment surgery and should be considered as a possible factor in development of diplopia (binocular and monocular) in adults with unilateral or asymmetric macular disease (12,13,16). With retinally induced aniseikonia, the degree of image size disparity is variable and results from forces causing compression or stretching of the retinal photoreceptors, the former producing a perceived image that is larger (macropsia) than it actually is and the latter producing a perceived image that is smaller (micropsia) than it actually is. Epiretinal membrane is the most common cause and usually induces macropsia, whereas macular hole, macular edema, and retinal detachment surgery usually induce micropsia (17). When excessive, the disparity in image size can disrupt fusion and be a contributing factor for diplopia.
Currently, there are 2 commercially available devices to measure aniseikonia: the New Aniseikonia Test (Handaya Co., Tokyo, Japan) and the Aniseikonia Inspector (Optical Diagnostics, Culemborg, the Netherlands) (18,19). With the former, the patient, while wearing red-green glasses, is presented a booklet with matched pairs of red-green targets (half-circles). Thus, one target is seen with one eye and the other target with the fellow eye. The patient indicates the pair of targets that appear equal in size, and the difference in the size of the chosen targets indicates the degree of aniseikonia. The Aniseikonia Inspector is the newer instrument and is computer-based. The patient, while wearing red-green glasses, views a series of direct comparison red-green rectangular targets presented for short viewing times and is asked which of the targets is larger. A psychometric function of the patient's responses with the degree of aniseikonia is generated. The Aniseikonia Inspector has the advantage of providing data for calculations for the necessary spectacle lens correction to alleviate the aniseikonia.
By modifying spectacle lens parameters such as prescribing steeper or flatter lens curvature, and/or thinner or thicker lenses, or using contact lenses, the image size disparity between the eyes can be reduced to better allow fusion.
A neuro-ophthalmic consultation is often a “the end of the line” in seeking resolution of a seemingly unsolvable visual disturbance. This certainly may apply to the complaint of diplopia, so that the clinical examination must be meticulous with no potential cause overlooked or forgotten.
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