Figure 6-4 Rivalry pattern. A, Pattern seen by the left eye. B, Pattern seen by the right eye. C, Pattern seen with binocular

vision. (Reproduced with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:12.)

Diplopia

Double vision, or diplopia, usually results from an acquired misalignment of the visual axes that causes an image to fall simultaneously on the fovea of 1 eye and on a nonfoveal point in the other eye. The object that falls on these noncorresponding points must be outside Panum’s area to appear double. The same object is seen as having 2 locations in subjective space, and the foveal image is always clearer than the nonfoveal image of the nonfixating eye. The perception of diplopia depends on the age at onset, its duration, and the patient’s subjective awareness of it. The younger the child, the greater the ability to suppress, or inhibit, the nonfoveal image.

Central fusion disruption is intractable diplopia, the features of which are absence of suppression and absence of fusional amplitudes. These findings, when accompanied by active motor avoidance of bifocal stimulation, are referred to as horror fusionis. The angle of strabismus may be small or may vary. Central fusion disruption can occur in a number of clinical settings: after disruption of fusion for a prolonged period; after head trauma; and, in rare cases, in long-standing strabismus. Management can be challenging.

Sensory Adaptations in Strabismus

To avoid visual confusion and diplopia, the visual system can use the mechanisms of suppression and anomalous retinal correspondence (Fig 6-5). It is important to realize that pathologic suppression and anomalous retinal correspondence develop only in the immature visual system.

Figure 6-5 Retinal correspondence and suppression in strabismus. A, A strabismic patient 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 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.)

Suppression

Suppression is the alteration of visual sensation that occurs when the images from 1 eye are 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 result from strabismic misalignment of the visual axes or other conditions that result in discordant images in each eye, such as cataract or anisometropia. Such suppression can be seen as an adaptation of a visually immature brain 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 is the mechanism that eliminates diplopia by preventing awareness of the image that falls on the peripheral retina in the deviating eye that 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. Adults may be unable to develop peripheral suppression and therefore may be 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 predominate 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 present only when the eyes are in the deviated state and absent in all other states. Patients with intermittent exotropia, for instance, often experience suppression when the eyes are divergent but may experience highgrade stereopsis when the eyes are straight. In contrast, constant suppression 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).

Management of suppression

Therapy for suppression often includes the following:

proper refractive correction

occlusion or pharmacologic penalization, to treat amblyopia

alignment of the visual axes, to permit simultaneous stimulation of corresponding retinal elements by the same object

Orthoptic exercises may be attempted to overcome the tendency of the image from 1 eye to suppress the image from the other eye when both eyes are open. These exercises are designed to first make the patient aware of diplopia, then have the patient attempt to fuse the images—both on an instrument and in free space. However, suppression is a useful adaptation to avoid diplopia in patients who are unable to fuse, so eliminating it is problematic in a patient for whom subsequent fusion training is unsuccessful. The role of orthoptics in the treatment of suppression is therefore controversial.

Anomalous Retinal Correspondence

Anomalous retinal correspondence (ARC) is a condition wherein the fovea of the fixating eye has acquired an anomalous common visual direction with a peripheral retinal element in the deviated eye; that is, the 2 foveae have different visual directions. ARC is an adaptation that restores some degree of binocular cooperation despite manifest strabismus. Anomalous binocular vision is a functional state that is superior to total suppression. In the development of ARC, normal sensory development is replaced only gradually and not completely. The more long-standing the deviation, the more deeply rooted the ARC may become. The period during which ARC may develop probably extends through the first decade of life.

Paradoxical diplopia can occur when ARC persists after surgery. When esotropic patients whose eyes have been set straight or nearly straight report a crossed diplopic localization of foveal or parafoveal stimuli, they are experiencing paradoxical diplopia (Fig 6-6). Paradoxical diplopia is typically a fleeting postoperative phenomenon, seldom lasting longer than a few days to weeks, but in rare cases it can persist much longer.

Figure 6-6 Paradoxical diplopia. Diagram of esotropia and ARC, wherein the deviation is being neutralized with a base-out prism. A red glass and base-out prism are placed over the right eye. The prism neutralizes the deviation by moving the retinal image of the penlight temporally, off the pseudofovea (P) to the true fovea (F). Because the pseudofovea is the center of