Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

(subtends 6° or 12 PD), but suppress the nondominant eye for the distance Worth 4-dot (subtends 1.25°) because it falls within the suppression scotoma. Further descriptions of these sensory tests follow later in the chapter.

Patients with monofixation syndrome often have amblyopia. The amblyopia can be mild (1 or 2 Snellen lines difference) or quite severe (20/200). Even patients with 20/200 amblyopia can still maintain the monofixation syndrome with some peripheral fusion and gross stereopsis. Clinically, the monofixation syndrome is frequently encountered in patients with anisometropic amblyopia, unilateral partial cataract, and small-angle strabismus. Parks described a rare condition, primary monofixation syndrome, which he hypothesized was caused by a congenital lack of central fusion.3

Anomalous Retinal Correspondence

Normal retinal correspondence (NRC) is the binocular relationship in which the true anatomic foveas of each eye are functionally linked together in the occipital cortex. Anomalous retinal correspondence, or ARC, is an adaptation to a moderateangle infantile strabismus that allows the brain to accept parafoveal retinal images from the deviated eye and superimposes them with images fixing from the fixing eye. The angle of deviation associated with ARC is usually between 15 and 30 PD, too large to allow peripheral fusion or monofixation. Thus, ARC is a binocular sensory adaptation used to eliminate diplopia by accepting the eccentric image location in the deviated eye as the visual center. This adaptation is a cortical reorganization of retinal correspondence and establishes a new functional fovea called the pseudo-fovea that corresponds to the true fovea of the dominant fellow eye (Fig. 6-6A).8 By cortically establishing a pseudo-fovea at the site of the diplopic image in the deviated eye that corresponds with the true fovea of the fixing eye, the retinal images can be superimposed. ARC and the pseudo-fovea are only present under binocular conditions. When the fixing eye is occluded, the patient changes fixation to the true fovea of the previously deviated eye.

If the strabismus of a patient with ARC is partially or fully corrected by surgery or a prism, the image will be displaced off the pseudo-fovea onto the retina that is cortically perceived as being noncorresponding. Because the image is displaced off the pseudo-fovea, the patient will see double even if the image falls

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FIGURE 6-6A,B. Anomalous retinal correspondence with right esotropia.

(A) Left eye fixes with the fovea (F) and right eye fixes with the pseudofovea (PF). The PF corresponds with the esotropia and is located on the nasal retina. Patient perceives a single image as the pseudo-fovea (PF) of the right eye corresponds with the true fovea (F) of the left eye. (B) Placing a base-out prism to partially neutralize the esotropia. The patient fixes the left eye and sees double, as the image now falls temporal to the pseudo-fovea (PF). Images temporal to the pseudo-fovea (PF) will project to the opposite visual field and cause diplopia.

on the true anatomic fovea. This type of diplopia is called paradoxical diplopia. Remember that under binocular viewing the pseudo-fovea is the central orientation of the eye and images displaced off the pseudo-fovea will be perceived as falling on noncorresponding retina. Figure 6-6A shows a patient with 20 PD esotropia and ARC with a nasal pseudo-fovea, right eye. Note that after partial correction of the esotropia with a 15 PD baseout prism, the image is now temporal to the pseudo-fovea (Fig. 6-6B). This patient will have crossed diplopia because the image falls on retina that is temporal to the pseudo-fovea, and temporal retina projects to the opposite hemifield. The patient will experience the crossed diplopia so long as the image is temporal to the pseudo-fovea, even if the eyes are aligned so the image falls directly on the true fovea.

Adult patients with ARC will often experience some diplopia after correction of their strabismus. An easy way to predict if a strabismic patient has ARC and will have postoperative paradoxical diplopia is to neutralize the angle of deviation with a prism. If the patient has diplopia with prism neutralization of the deviation, then the patient has ARC and the patient should be informed that postoperative diplopia will occur after the eyes are straightened. Fortunately, paradoxical diplopia is usually not so bothersome as true diplopia associated with normal retinal correspondence and, in most cases, paradoxical diplopia will vanish within a few weeks after surgery. Only in rare circumstances is postoperative paradoxical diplopia so bothersome that it interferes with everyday activities. Even so, in rare instances, persistent postoperative paradoxical diplopia has required a reoperation to recreate the initial strabismus to eliminate paradoxical diplopia. In cases where preoperative prism neutralization creates paradoxical diplopia that bothers the patient, one can prescribe press-on prisms (prism adaptation) to see if the diplopia will subside over several weeks.

Bagolini striated lenses on a patient with a 20 PD esotropia and ARC are depicted in Figure 6-7A. The patient perceives a cross (normal response) even though there is an esotropia, because the line in the deviated eye passes through the pseudofovea. If a strabismic patient reports seeing a complete cross to Bagolini striated lenses, then they have ARC (Fig. 6-7B). This cortical reorganization of ARC is only present during binocular viewing and, when the dominant eye is covered, the patient reorients to the true anatomic fovea (Fig. 6-7C). ARC should not be confused with eccentric fixation. Remember, ARC is only

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FIGURE 6-7A–C. Anomalous retinal correspondence (ARC) as tested with Bagolini lenses. (A) Bagolini lenses stimulate the right fovea (F) and left pseudo-fovea (PF). Note that the pseudo-fovea (PF) is nasal to the true fovea (F). (B) Retinal location of the Bagolini striation when the fovea (F) of the right eye is being stimulated and the pseudo-fovea (PF) of the left eye is being stimulated. Patient’s perception is a cross, as the pseudo-fovea (PF) corresponds to the true fovea (F). (C) When the right eye is occluded, the patient now fixates with the true fovea (F) of the left eye. Note that the pseudo-fovea has disappeared. Patient perceives a single line, which stimulates the visual center.

present during binocular viewing, whereas eccentric fixation represents a monocular loss of vision (amblyopia) and is present during both monocular and binocular viewing.

ARC provides crude binocular vision with superimposition of retinal images; however, there is not true fusion. Patients with ARC do not have fusional vergence amplitudes, and they do not have stereoacuity. ARC can occur in association with intermittent strabismus. Some patients with intermittent exotropia, for example, have binocular vision with stereopsis when they are aligned but switch to ARC when they are tropic. In general, ARC is associated with good vision or only mild amblyopia.

Harmonious ARC is the term used for the situation as described previously where the position of the pseudo-fovea completely compensates for the angle of strabismus (see Fig. 6- 6). Described another way, the strabismic deviation equals the pseudo-foveal offset from the true fovea. The amount of pseudofoveal offset is termed the angle of anomaly, which is equal to the strabismic deviation (objective angle). Clinically, however, there are many cases in which the angle of strabismus does not exactly match the location of the pseudo-fovea so that the target image does not fall on the pseudo-fovea. This condition is called unharmonious ARC.

In Figure 6-8A, the angle of the strabismus measures 20 PD (objective angle), but the pseudo-fovea is only 15 PD from the true fovea (angle of anomaly 15 PD). Thus, the image is falling 5 PD nasal to the pseudo-fovea. A 5 PD base-out prism over the right eye places the image on the pseudo-fovea and eliminates the diplopia. The discrepancy between the location of the pseudo-fovea and the location of the target image is called the subjective angle; in Figure 6-8B, the subjective angle is 5 PD. Note that neutralizing the subjective angle eliminates diplopia associated with unharmonious ARC, but neutralizing more than the subjective angle results in paradoxical diplopia (Fig. 6-8C). In these cases of unharmonious ARC, it is likely that the angle of strabismus has changed (usually increased) after the development of the pseudo-fovea. Most patients with unharmonious ARC suppress the target image so as not to experience diplopia. Others, perhaps those who had a change in the deviation off the pseudo-fovea in late childhood or adulthood, do experience diplopia. Further discussion of unharmonious ARC and angle of anomaly is located under Amblyoscope, later in this chapter.

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FIGURE 6-8A–C. Unharmonious ARC in a patient with esotropia. The pseudo-fovea (PF) is not in alignment with the retinal image in the deviated eye. (A) Patient perceives uncrossed diplopia or suppresses the image in the deviated eye. (B) A base-out prism is used to place the image on the pseudo-fovea (PF). Patient perceives a superimposed single image. A red filter in front of the right eye causes the image to appear pink, a combination of the clear image (left eye) and the red image (right eye). (C) A 20 PD prism is placed base-out in front of the deviated eye to place the image on the true fovea (F). Patient now has paradoxical diplopia and sees the red image on the contralateral side, causing crossed diplopia.

Practically speaking, the differentiation between harmonious versus unharmonious ARC is not of great clinical importance; however, paradoxical diplopia after strabismus surgery is of clinical concern. Adult patients with long-standing strabis-

mus should be examined for ARC by neutralizing the deviation with a prism.

Large Regional Suppression

Children who have large-angle strabismus or severe unilateral retinal image blur develop a large suppression scotoma to eliminate the image disparity (Fig. 6-9). Patients with large-angle constant strabismus (e.g., congenital esotropia), will have essentially no binocularity, not even peripheral fusion or ARC. Large regional suppression, however, is not always constant and can be intermittent. Patients with large-angle strabismus and large fusional vergence amplitudes (e.g., intermittent exotropia) have intermittent strabismus and intermittent regional suppression. These patients switch from a state of binocular fusion to monocular vision and suppression. Another example of intermittent large regional suppression is seen in patients with congenital incomitant strabismus, where the eyes are straight in one field of gaze (Duane’s syndrome, or congenital superior oblique palsy). These patients have binocular fusion when their eyes are aligned with a compensatory face turn, but they suppress when they look into the field of gaze where they have strabismus. Patients with intermittent exotropia and Duane’s syndrome that have developed suppression do not have diplopia when they are tropic.

Horror Fusionis

Normal sensory and motor fusion, once established, is usually permanent. Binocular fusion, however, can be lost if severe and sustained abnormal visual stimulation is acquired. Long-term occlusion of one eye, especially if it is the dominant eye, can result in a loss of binocular fusion in some patients. If this loss of binocular fusion occurs late in visual development or adulthood, the patient will be too old to suppress. The inability to either fuse or suppress images results in intractable diplopia and is termed horror fusionis, or acquired disruption of central fusion. Causes of this rare syndrome include a unilateral acquired cataract occurring in older children and adults.2,4,6 In these cases, prolonged occlusion caused by a cataract appears to eliminate binocular fusion and, if the child is too old to suppress, diplopia results. An acquired cataract in the dominant eye of an adult with previous strabismus or amblyopia can also cause

FIGURE 6-9. Worth 4-dot in a patient with large regional suppression of the right eye. The two dots fall within the suppression scotoma, so the patient perceives three dots from the left eye.

horror fusionis. In these cases, prolonged occlusion of the dominant eye results in loss of preexisting suppression, leaving the patient with diplopia. In addition, horror fusionis can be caused by antisuppression therapy, such as forcing fixation with the nondominant eye in patients with strabismus. Antisuppression consists of training the strabismic patient to recognize the diplopia, which can be done by using dense red filters over the dominant eye to force fixation to the nondominant eye. Anti-

suppression is especially dangerous in patients with strabismus and poor fusion potential.

Prolonged Visual Plasticity

The dogma regarding the relatively short span of visual central nervous system plasticity has come into question. Veteran strabismologists know that some adult patients with acquired strabismus can eventually learn to ignore or suppress their double vision. Do these patients actually develop suppression or do they consciously ignore their diplopia? In a study of acquired strabismus in adults, this author used the pattern visual evoked potential (VEP) to document suppression of visual cortical activity in adult patients with acquired strabismus.10 Another example of prolonged plasticity is seen in adults with amblyopia, who can show significant visual acuity improvement after losing vision in their good eye.1,7

Sensory Tests

DIPLOPIA TESTS

Diplopia tests use one fixation target seen by both eyes. The target images fall on both foveas and corresponding retinal points if the eyes are aligned (Fig. 6-10). If strabismus is present, the target image falls on the fovea of the fixing eye and an extrafoveal point in the nonfixing eye (Fig. 6-11). A color filter is placed over one eye (usually red) or both eyes (usually red for right eye, green for left eye) to tint the image of each eye. By distinctly tinting the retinal images of each eye, the examiner can tell which image corresponds to which eye. Lenses that place a streak of light on the retina (Maddox rod and Bagolini lens) are also used to stimulate the retina.

Many diplopia tests disrupt fusion by obscuring, or even eliminating, peripheral fusion clues. Tests that disrupt fusion are referred to as dissociating tests. Table 6-1 lists different diplopia tests, with the most dissociating test listed first and the least dissociating test last. Note that under scotopic conditions tests that use filters, such as the Worth 4-dot test and red filter test, become extremely dissociating, because the only images seen by the patient are the test lights and peripheral fusion clues are lost.

Red Filter Test

Othotropia NRC

Penlight

One Pink Light

FIGURE 6-10. Red filter test in a normal patient with straight eye and normal retinal correspondence. Note that the image from the penlight falls on both foveas and the patient perceives a single binocular image.