Tests of Sensory Adaptation and Binocular Function
Sensory binocularity involves the use of both eyes together to form a unified perception. Ideally, testing of this function is performed before binocularity or ocular alignment is disrupted by occlusion. The sensory response to strabismus is diplopia, suppression, or ARC (see Chapter 5). While a variety of sensory tests demonstrate these adaptations, the Worth 4-dot and stereopsis tests are the ones most commonly used in clinical practice. Sensory tests must be performed in conjunction with cover tests to determine whether a fusion response is due to normal alignment or ARC. Also, the clinician should remember that no sensory test can perfectly replicate habitual viewing conditions; the more dissociative the test, the greater the risk that it does not reflect habitual binocular function.
The Red-Glass (Diplopia) Test
In a strabismic patient, placing a red glass or filter before the fixating eye while the patient views a white light stimulates the fovea of the fixating eye and an extrafoveal area of the fellow eye. If the patient sees only 1 light (either red or white), suppression is present (Fig 7-9A). A 5Δ or 10Δ base-up prism placed in front of the deviated eye can move the image out of the suppression scotoma, causing the patient to experience diplopia. With NRC, the white light will be localized below and to one side of the red light (Fig 7-9B). Incorrect localization of the white light, for example, directly below the red light, indicates ARC (Fig 7-9C).
In the absence of suppression, the following results are possible:
An esotropic patient without harmonious ARC experiences homonymous or uncrossed diplopia (see Chapter 5) (with the red glass over the left eye, the red light is perceived to the left of the white light—the same side as the red lens; Fig 7-9D). A patient with exotropia has heteronymous or crossed diplopia (with the red glass over the left eye, the red light is perceived to the right of the white light—the side opposite that of the red lens; Fig 7-9E). If the degree of separation between the 2 images is consistent with the magnitude of the deviation measured by cover testing, the patient has NRC.
If the patient sees the 2 lights superimposed so that they appear pinkish despite a measurable deviation, there is harmonious ARC.
If the patient sees 2 lights (with uncrossed diplopia in esotropia and with crossed diplopia in exotropia) but the image separation is less than expected based on the measured deviation, there is unharmonious ARC. Some investigators consider this to be a testing artifact.
Figure 7-9 Red-glass test findings in diplopia, suppression, and ARC (see text for explanation; see also Chapter 5, Fig 5-5).
(Modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:223.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.