To avoid visual confusion and diplopia, the visual system uses the mechanisms of suppression and anomalous retinal correspondence (Fig 5-5). Pathologic suppression and anomalous retinal correspondence develop only in the immature visual system.
Suppression is the alteration of visual sensation that occurs when an eye’s retinal image is inhibited or prevented from reaching consciousness during binocular visual activity. Physiologic suppression is the mechanism that prevents physiologic diplopia (diplopia elicited by objects outside Panum’s area) from reaching consciousness. Pathologic suppression may develop because of strabismic misalignment of the visual axes or other conditions resulting in discordant images in each eye, such as cataract or anisometropia. Such suppression can be regarded as an adaptation within the immature visual system to avoid diplopia. If a patient with strabismus and normal retinal correspondence (NRC) does not have diplopia, suppression is present, provided the sensory pathways are intact. In less obvious situations, several simple tests are available for clinical diagnosis of suppression (see Chapter 7).
The following classification of suppression may be useful for the clinician:
Central versus peripheral. Central suppression is the mechanism that keeps the foveal image of the deviating eye from reaching consciousness, thereby preventing visual confusion. Peripheral suppression, another mechanism, eliminates diplopia by preventing awareness of the image that falls on the peripheral retina in the deviating eye, which corresponds to the image falling on the fovea of the fixating eye. This form of suppression is clearly pathologic, developing as a cortical adaptation only within an immature visual system. When strabismus develops after visual maturation/in adults, peripheral suppression does not develop and the patient is thus unable to eliminate the peripheral second image without closing or occluding the deviating eye.
Nonalternating versus alternating. If suppression always causes the image from the dominant eye to be predominant over the image from the deviating eye, the suppression is nonalternating. This may lead to amblyopia. If the process switches between the 2 eyes, the suppression is described as alternating.
Facultative versus constant. Suppression may be considered facultative if it is present only when the eyes are deviated and is absent in all other states. Patients with intermittent exotropia, for instance, often experience suppression when the eyes are divergent but may experience high-grade stereopsis when the eyes are straight. In contrast, constant suppression denotes suppression that is always present, whether the eyes are deviated or aligned. The suppression scotoma in the deviating eye may be either relative (permitting some visual sensation) or absolute (permitting no perception of light).
Figure 5-4 Retinal correspondence and suppression in strabismus. A, A patient with right esotropia with normal retinal correspondence (NRC) and without suppression would have diplopia and visual confusion, which is the perception of a common visual direction for 2 separate objects (represented by the superimposition of the images of the fixated diamond and the circle, which is imaged on the fovea of the deviating eye). B, The elimination of diplopia and confusion by suppression of the retinal image of the deviating/esotropic right eye. C, The elimination of diplopia and confusion by anomalous retinal correspondence (ARC), an adaptation of visual directions in the deviated right eye.
(Adapted with permission from Kaufman PL, Alm A. Adler’s Physiology of the Eye. 10th ed. St Louis: Mosby; 2003:490.)
Management of suppression
Therapy for suppression often includes the following:
proper refractive correction
amblyopia therapy using occlusion or pharmacologic treatment
alignment of the visual axes, to permit simultaneous stimulation of corresponding retinal elements by the same object
Antisuppression orthoptic exercises may result in intractable diplopia and are not typically recommended.
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.