VARIABILITY OF THE DEVIATION
chapter 5 Sensory Adaptations to Strabismus
Suppression 135
Cnaraetenstics of Suppression 136 Testing for Suppression 139
History 140 Red Lens Test 140 WorthDotTest 140 Ambiyoscope Workup 141 Amblyopia 143
Classification 144 Strabismic Amblyopia 144 Anisometropic Amblyopia 144 Isoametropic Amblyopia 145 Image Degradation Amblyopia 145
Amblyopia as a
Developmental Disorder 145 Case History 148
Visual Acuity Testing 149 Snellen Charts 149 Bailey-Lovie Chart 151 Psychometric Charts 151 Tumbling E and Pieture Cards 153 Infant Visual Acuity Assessment 154 Visually Evoked Potentials 158
Interferometiy 159
Fixation Evaluation 159 Description oí Eccentric Fixation 160 Vísuoscopy 160
Haidinger Brush Testing 161 Refraction Procedures 163 Eye Disease Evaluation 163
Ophthalmoscopy 163 Visual Fíelds 164
Neutral-Density Filters 164 Tests of Retinal Function 165 Screening for Amblyopia 165 Anomalous Corresponderse 166 Classification 167 Characteristics 170
Horopter in Anomalous Retinal Correspondence 170 Horror Fusionis 172
Etiology of Anomalous Retinal Correspondence 174 Depth of Anomalous Retinal
Correspondence 175
Prevalence of Anomalous Retinal Correspondence 176
Testing 176
Dissociated Red Lens Test 176 Afterimages 176
Bifoveal Test of Cüppers 179 Major Ambiyoscope 182 Bagolini Striated Lenses 183 Color Fusión 185
Several anomalous conditions can develop sec-ondary to the onset of a deveiopmental strabismus, particularly of early origin. These ¡nclude Suppres-sion, amblyopia, and anomalous Correspondence.
These conditions and the appropriate testing meth-ods for them are discussed in this chapter.
Although ¡t ¡s customary to think in terms of the deviation causing these adaptive conditions, ¡t is also possible that the process may work in reverse.
In other words, the strabismus may be the end result rather than the cause of the anomalous sen-sory conditions.
SUPPRESSION
When a strabismus occurs, the affected individual may experience pathologic diplopia or confusión (or both). Suppression is the defense mechamsm
c.
o.s.
O.O. FUNDUS O.D
FIGURE 5-1—Confusión and diplopia ¡n an example of esotropía of the right eye and the resulting pathologic suppression. a. Cyclopean perception of confusión and pathologic homonymous diplopia. The fixation starlike object ¡s seen diplopically. The nonfixated circle falling on the fovea of the deviating right eye causes confu-sión. Although the circle could possibly be seen diplopically, it is not usually noticed, as the patient is not paying it any attention. b. Theoretical posterior view of the eyes showing the suppression zone that could result from the esotropic right eye. c. Theoretical ophthalmoscopic view of the right fundus, illustrating the shape and location of the suppression zone. (f = fovea; H = horizontal angle of deviation; O.D. = oculus dexter; O.S. = oculus sinister.)
that is usually attempted first by an individual to elimínate these perceptual annoyances. Suppression
¡s the lack of perception of normally visible objects in all or part of the field of visión of one eye, occurring only under binocular viewing conditions and attributed to cortical inhibition.1 In normal binocular visión, physiologic suppression naturally occurs, particularly, foralJ objects fallingoutsjdethe singleness horopter. The suppressed ¡rnageecán usually be brought to consciousness by directing attention to it. On the other hand, pathologic suppression ¡s a binocular anomaly. In the presence of strabis-mus, for example, a suppressed image ¡s not easily \perceived by merely directing one's attention to it. mere is, apparently, active cortical inhibition of the suppressed eye's ¡mage that ¡s not as subject to voli-tional control, von Noorden2 noted that even retinal rivalry disappears in strabismic patients. Retinal rivalry (see Chapter 1) and suppression both occur in the visual cortex, although they may be mediated by other neural processes.3
Suppression that occurs during infancy and early childhood can have a profound effect on the devel-opment of the full acuity potential of the affected eye and maturation of stereopsis. When the images of each eye are discordant due to strabismus or uncorrected anisometropia, there ¡s active cortical inhibition ¡n V1 related to the affected eye that slows or halts further sensory development.4 Stereopsis (binocular depth disparity detection) starts to develop ¡n normal infants at approximately 2.5-4.0 months of age and progresses rapidly.5 The onset of strabismus at this early time has the most disruptive effect.6 Amblyopia
can quickly develop ¡n the
turned eye if the child fails to develop an altérnate fixation pattern. Stereoacuity, however, can continué to develop in the presence of a constant strabismus, although not to the same degree as when the eyes are straight. In the case of anisometropia, suppression is directly related to the degree of the refractive difference between the eyes, and the development of stereopsis ¡s affected accordingly.7 Early identification of disorders of binocular visión that cause suppression and result ¡n amblyopia and reduced Stereoacuity ¡s a desirable public health goal, as ¡t makes the successful management of such conditions much easier.In cases of excessive heterophoria and intermit-tent strabismus (particularly intermittent exotro-pia), testing for suppression usually requires very sensitive suppression controls, such as the altérnate polarized letter test found on the Vectographic Slide (see Figure 3-10) and the Mentor B-VAT Binocular Vision Testing System.8Characteristics of SuppressionThe precise neurologic mechanism for suppression is not thoroughly known, but the phenomenon can be easily demonstrated by diagram. Figure 5-1 illus-trates the concept of diplopia and confusión and the resulting zone of suppression. The fixation target ¡s imaged on the fixating left eye. An esotropía of the right eye causes the target's ¡mage to fall on the nasal retina.
Cyclopean projection shows the patient per-ceiving two images. When the diplopic ¡mage ¡s seen on the same side as the eye that deviates (e.g., right eye seeing the diplopic ¡mage in the right field),
Chapter 5 137
the diplopia ¡s called homonymous, or uncrossed. If, however, the diplopic ¡mage were to fall on the tem-poral retina of the deviating eye, heteronymous (crossed) diplopia would occur. For the redundant ocular ¡mage to be eliminated, the target point on the nasal retina of the right eye must be suppressed.
Jampolsky9 referred to this location as the "zero measure" point (point zero). This point and its adja-cent área must be suppressed to avoid diplopia.
Peripheral diplopia may occur if the deviation is larger than Ranum's fusional áreas ¡n the peripheral binocular field, but the combined influence of low resolution, suppression, and selective attention to the fixated target usual ly prevents the perception of double images ¡n these distant locations.
Whereas point zero (the target point, sometimes designated as T) usually ¡s suppressed, the fovea in the deviating eye ¡s suppressed even more intensely. If this were not the case, then two dis-similar images would be superimposed, as each fovea is pointing to a different location within the binocular visual field. This intolerable situation is called confusión. Suppression of the fovea of the deviated eye occurs more quickiy and deeply than at point zero because foveal visión is usually the location of attention. Clinically, strabismic individ-uáis typically do not report confusión, but many do have symptoms of diplopia.
It is probable that suppression begins first at the fovea when a horizontal deviation of the visual axes becomes manifest, as in Figure 5-1; later, point zero ¡s also suppressed. Afterward, a patho-logic zone of suppression encompasses the área between the fovea and point zero of the deviating eye. The vertical dimensión of this zone ¡s usually smaller than the horizontal dimensión. The shape of the zone resembles the letter D, according to Jampolsky,9 and the vertical demarcation at the fovea resembles a hemianoptic visual field defect.
Although this is a theoretical model of the sup-pression zone, clínica! findings suggest that these demarcations are not always so clear-cut. Pratt-Johnson and MacDonald10 showed that suppres-sion does not exclusively involve the nasal retina in esotropes and the temporal retina in exotropes, but it may extend in both directions regardless of the direction of the deviation. The shape and size of the suppression zone depends on the targets used and the way the test is performed. The sup-pression "scotoma" ¡s, therefore, considered rela-tive rather than absolute, appearing more extensive and deep in the hemiretina toward point zero. In
some cases, however (e.g., a large-angle strabis-mus with amblyopia of long standing), it appears that most or all of the binocular visual field of the deviating eye is pathologically suppressed.
How does the suppressing strabismic patient per-ceive visual objects in space? Such a patient does experience continuity of visual space across the visual field, similar to the individual having normal binocular visión (Figure 5-2a). However, there may be a slight decrease or increase in the horizontal size of the visual field, depending on whether the deviation is esotropic (see Figure 5-2b) or exotropic (see Figure 5-2c), respectively. Fortunately, a strabis-mic patient who is free of ocular pathology per-ceives no gaps (missing portions) ¡n the visual field.
Suppression of the turned eye occurs only within the binocular overlap área. Suppression ¡s not obvi-ous to the individual except indirectly, possibly because of deficient stereopsis; a vivid spatial sense of three-dimensionality often is missing, depending on the extent and depth of the suppression zone.
The extreme peripheral lateral fields of each eye are, however, normal. These temporal crescents, approx-imately 30 degrees on each side, cannot be sup-pressed. The crescents are neurally subserved only by monocular fibers from the nasal retina of each eye. The suppressed eye ¡s unresponsive to binocu-lar stimulation but ¡s responsive to the "monocubinocu-lar"
stimulation of the peripheral nasal retina.
Foveal suppression may also be found ¡n nonstra-bismic patients. Anisometropia may cause image size difference on the retina of each eye (aniseiko-nia) and also a difference ¡n clarity. Suppression ¡s, therefore, necessary to elimínate the confusión
aris-¡ng from the resulting superímpositíon of dissimilar ocular images (¡.e., one image being larger than the other). The suppression zone in such cases is rela-tively small and encircles only the fovea, as there is no extrafoveal point zero. Therefore, confusión, and not diplopia, ¡s the problem. Foveal suppression is found also ¡n patients with large heterophoria if fusional vergence compensaron is poor. The mech-an ism is not fully understood, but ¡t ¡s likely that ver-gence stress or fixation disparity can initiate a suppression response.
Suppression may be classified by size and inten-sity. In regard to size, suppression ¡s classified as being either central or peripheral. If a patient has central suppression, the edge of the suppression zone can extend to 5 degrees from the center of the fovea. Beyond this limit, suppression ¡s consid-ered to be peripheral (Table 5-1). It must be
Binocular overlap
área
FIGURE 5-2—Horizontal visual field limits.
a. Orthophoria. b. Esotropía of the left eye.
c. Exotropia of the left eye. (f = fovea.)
remembered that the limits of the suppression zone depend on the testing conditions and the size of thetargets used.
Intensity of suppression varíes on a continuous scale from shallowto deep (Table 5-2). This is nec-essarily a qualitative determination. It is made by finding the ease with which suppression can be broken by using various testing procedures. The more unnatural the environment (laboratory type of testing conditions), the less likely is suppression.
For example, the Worth dot test using red-green fil-ters in a dark room is relatively unnatural and serves as a strong stimulus to break through sup-pression. Conversely, in more natural seeing con-
ditions (e.g., Pola-Mirror), the patient will more likely suppress an eye. Illuminated targets, such as a penlight or Worth lights, become less natural by lowering room illumination.
In effect, intensity is described in terms of the testing procedure that is required to break (elimí-nate) the suppression response. Some of the meth-ods commonly used to test the intensity of suppression are Usted in Table 5-2. The more natu-ral tests appear at the top of the list, with the less natural following in descending order. Using this as a guide, it is reasonable to assume, for example, that a strabismic patient who notices pathologic diplopia when viewing a penlight in an illumi-
Chapter 5 139
TABLE 5-1. Size of Suppression Zone in Either Superímposition or Fused Targets
QassJficatlon
Separation from Target Center to Suppression Clue
Central Foveal Parafoveal Paramacular Peripheral
<5 degrees <1 degree <3 degrees (but >1 degree} <5 degrees (but >1 degree} >5 degrees
nated room has shallow suppression. In contrast, if the room must be darkened and the patient must wear red-green filters to perceive diplopia, then the suppression would be deep.
Several attributes of the strabismic deviation affect the suppression response. Magnitude of the deviation is one: Generally, the larger the devia-tion, the larger is the suppression zone. The inten-sity of suppression, however, ¡s not necessarily correlated with the magnitude. It may be that a patient with a constant, small-angle esotropía will have a small suppression zone but one that ¡s sup-pressed very deeply. Another factor ¡s eye laterality.
If the strabismus ¡s alternating, the suppression is also likely to altérnate from eye to eye. If the stra-bismus is unilateral, suppression is confined to the deviating eye. Frequency of the strabismus is another important variable. The more frequent the strabismus, the more likely is it that deep suppres-
sion will be found. If anomalous retinal correspon-dence (ARC) ¡s present, these relations do not necessarily apply, because ARC ¡s also an antidi-plopia mechanism that partially obviates the need for suppression.
Suppression is usually shallow in noncomitant strabismic patients. Intensity is less because the magnitude of the deviation is continuously chang-ing as fixation shifts from one field of gaze to another. This means that point zero (the target point) is not at a fixed site on the retina; thus, diplopia is more likely to be perceived. Fortu-nately, the accompanying diplopia with noncomi-tant deviations can warn individuáis of possible neurologic problems that require immediate health care attention.