Chapter 72 – Sensory Status in Strabismus
GARY R. DIAMOND
• Fusion: cortical integration of slightly dissimilar images perceived by the two eyes into a unified percept.
• Horopter: the locus in space representing the intersection of all points that stimulate corresponding retinal points.
• Panum fusional area: area in space surrounding the horopter in which objects can be fused.
• Stereopsis: a form of depth perception that demands binocular vision and usually sensory fusion.
• Monofixation syndrome: a form of binocular vision found in many patients who have small amounts of strabismus that permits peripheral fusion, stereopsis, and alignment stability.
• Subjective tests for binocular vision and retinal correspondence are an important part of every patient’s examination.
• Monofixation syndrome can be diagnosed reliably only by sensory testing.
Binocular patients who develop strabismus before the age of 7–9 years usually develop the sensory adaptations of suppression and anomalous retinal correspondence to obviate diplopia and visual confusion (see Chapter 71 ). Older patients who develop strabismus for the first time suffer from diplopia and visual confusion as long as vision remains in both eyes, until the eyes are aligned or the patient learns to ignore one image. Nonbinocular patients (or those who perceive with one eye at a time) are not troubled by symptoms of double vision if their eyes become strabismic.
Clinical testing of sensory status in strabismic patients is easier to understand after the basic physiology of sensory fusion and stereopsis has been mastered.
Sensory fusion is the cerebral cortical integration of the slightly dissimilar images perceived by the two eyes into a unified percept. If images are sufficiently dissimilar, they cannot be fused; examples are red and green variants of the same object or lines seen vertically by one eye and horizontally by the other. Binocular rivalry usually occurs under these conditions, and a varying percept is obtained. Motor fusion with vergence amplitudes, and even stereopsis, may be produced by rivalrous stimuli in the absence of sensory fusion. Such stimuli are fortunately uncommon in daily life.
A retinal element is a small retinal patch that has an associated directional value. The fovea’s directional value is defined subjectively as straight ahead; peripheral retinal elements possess directional values in other orientations. Corresponding retinal points are a pair of retinal elements, one in each eye, that have the same directional value. Comfortable single binocular vision occurs when objects in the binocular field stimulate corresponding retinal points and the higher cortical function—termed sensory fusion—occurs.
The locus in space that represents the intersection of all points in space that stimulate corresponding retinal points is termed the horopter. Interestingly, sensory fusion still occurs if the object that projects upon a retinal element in one eye projects upon a range of elements that surrounds the corresponding retinal element in the second eye. The area in space that projects from this range of elements in the second eye that intersects with the projection from the retinal element in the first eye is termed the Panum fusional area ( Fig. 72-1 ). This Panum fusional area surrounds the horopter anteriorly and posteriorly; it permits fusion to take place when exact retinal correspondence does not occur. The binocularly perceived object imaged on noncorresponding retinal loci, but fused within the Panum fusional area, is perceived to have one subjective visual direction. The foveal Panum area is circular, of diameter about 14?min of arc; thus, an object projected upon the fovea of one eye may be displaced by this amount and the patient still maintains bifoveal vision. The size of the Panum fusional area increases toward the retinal periphery (see Fig. 71-5 ), but the ultimate size and shape depend upon the temporal and spatial frequency of the patient’s alignment drift when fixing upon a stationary target.
Objects in front of or behind the Panum fusional area stimulate physiologic diplopia, which is not usually noted but may in turn stimulate fusional vergence eye movements. The horopter shape may be defined in a pair of perfectly spherical eyes that have refractive seats at the nodal points of each eye as the locus of points of zero vertical disparity relative to the fixation point. In a horizontal plane, the horopter, which includes the fovea, is the Vieth-Müller circle ( Fig. 72-2 ).  In a living animal visual system the horopter is flatter (the Hering-Hellebrand horopter deviation). The vertical horopter tilts away from the observer, who stands on the horopter; the inclination is a function of fixation distance.
DEPTH PERCEPTION AND STEREOPSIS
Depth perception may occur without binocular vision and depends on both monocular ( Box 72-1 ) and binocular clues. Stereopsis is a form of depth perception that demands binocular vision and usually sensory fusion but under certain conditions may be stimulated by rivalrous objects whose images cannot be fused. Stereopsis is the perception of depth stimulated by objects that possess horizontal disparity, with one object also usually located before or behind the fixation point. Horizontal disparity alone is sufficient to stimulate the stereoptic percept. Visual contours are not necessary, and disparity may be stimulated
Figure 72-1 Panum fusional area. The left eye fixates a square target, and a search object visible only to the right eye is moved before and behind this target. The ellipse of retinal area, for which typical dimensions are given for the parafoveal area, is the projection of the Panum fusional area. Diplopia is not perceived for two targets within this area.
by random dots.  Stereoacuity, the disparity threshold at which a depth difference may just be appreciated, is best at the fovea and depends on the level of visual acuity in each eye. Stereoacuity dissipates rapidly into the peripheral visual field and with increasing object distance  and is proportional to interpupillary distance. Under ideal conditions, foveal stereoacuity is 10?sec of arc.
The clinician must determine the sensory status of each patient, specifically whether the patient is binocular and if so whether the patient has normal retinal correspondence (NRC) or abnormal retinal correspondence (ARC) and suppression (see Chapter 71 ). Binocular patients who have constant tropias measurable on cover testing may exhibit NRC with diplopia and visual confusion, ARC and suppression, or monofixation syndrome. The last possesses features of both NRC and ARC but is considered closer to the former.
Asymptomatic patients who have tropias >8? horizontally or 4? vertically usually have ARC and suppression, although these may be difficult to demonstrate. Asymptomatic binocular patients who have smaller tropias, or a smaller tropia with superimposed phoria, usually have monofixation syndrome.
Many sensory tests are available to the busy clinician, but access to and understanding of just a few enable evaluation of the patient’s sensory status. It is important to perform sensory testing at the beginning of the examination; prolonged monocular occlusion to evaluate visual acuity may dissociate the eyes and confound determination of the patient’s ambient sensory status.
Testing for Binocularity (Simultaneous Perception)
Many tests require simple tools to demonstrate binocularity. Holding a red lens before one eye and presenting a white light detects perception of two lights, red and white, in patients who have NRC and diplopia. Prisms may be used to project one light beyond the bounds of a suppression scotoma in patients who have ARC and suppression or NRC-monofixation syndrome. Commercially available Polaroid projection slides, when viewed through polarized lenses, present one half of an optotype line to each eye; binocular patients view the entire line, whereas nonbinocular patients
Figure 72-2 Vieth–Müller circle. If the eyes are assumed to be spherical with rotational centers at the nodal points, all points in space that have a zero disparity fall on this circle. Angle a1 = angle a2 ; thus, equal retinal distances map into equal angles in space in this idealized system.
Monocular Clues to Depth Perception*
Apparent size of objects of known size
Superimposition of near object on more distant object
Loss of contrast of distant object
Movement parallax (shift in relative position of two objects as subject moves head)
Light and shape effects
Linear perspective (such as convergent railroad tracks)
Fading of texture with distance
* Binocular vision is not always necessary to determine the relative position of objects in space.
view the half perceived by the foveating eye only. Prismatically, overcorrection of a strabismic patient elicits diplopic symptoms, which proves the presence of binocular vision.
The Worth four-dot test uses a fixed wall target for distance fixation ( Fig. 72-3 ) and a handheld wand for testing at variable near-fixation distances ( Fig. 72-4 ). The stimulus is an array of four round targets (“dots”), usually presented with the red dot above two green dots that in turn are above one white dot. The diameter of the target array subtends 6° at 20?ft (6?m) and 1.25° at 1?ft (33?cm). The targets are viewed through red-green (anaglyph) glasses, and the patient describes the percept to the examiner or simply counts the lights viewed. Binocular patients perceive red and green lights simultaneously, but the near wand must be held very close to a patient with a large strabismic deviation to project the target beyond the bounds of a suppression scotoma ( Fig. 72-5 ). Nonbinocular patients see two red or three green lights at all testing distances ( Fig. 72-6 ).
Bagolini lenses are finely ruled plano lenses that give a streak appearance to a point light source perpendicular to the ruled direction. The lenses are placed in orthogonal orientation (traditionally at 135° right eye and 45° left eye; Fig. 72-7 ) in a trial frame and the patient views a light at distance fixation. Binocular patients perceive an “X” figure or, if a suppression scotoma exists, one complete line and the peripheral elements of the second. Nonbinocular patients see only one entire line.
Haploscopes, for example, the major amblyoscope, may present slightly different but fusible images to each eye; if portions
Figure 72-3 Distant Worth four-dot target. This is fixed to a wall, traditionally with the red dot placed at the top.
Figure 72-4 Near Worth four-dot target and anaglyph glasses. The near target is brought to the face to elicit a binocular response in patients who have strabismus and large scotomas.
of each image are perceived, the patient is binocular. The viewing tubes may be displaced to the strabismic angle if such exists, or the tubes may be kept in the straight position and targets used that are large enough to project beyond the suppression scotoma.
Tests of Retinal Correspondence
Many of the preceding tests may be used in binocular patients to diagnose ARC and suppression, NRC bifoveality, or NRC-monofixation syndrome at a given testing distance at a given moment. As ARC exists only under binocular testing conditions, some tests may yield an ARC response at a given moment whereas other tests yield an NRC response, depending on the room illumination and the length of time ARC has been present. Tests that confound correctable single binocular vision and that poorly reproduce ordinary binocular viewing conditions demonstrate ARC later than tests that closely simulate typical binocular viewing conditions. Retinal correspondence tests are listed by depth of abnormal correspondence in Box 72-2 .
The Worth four-dot test demonstrates suppression of one eye when presented with a distant viewing target and fusion of lights of the near viewing target in patients who have ARC and suppression and NRC-monofixation syndrome ( Fig. 72-5 ); thus, it cannot be used to differentiate ARC from NRC easily.
The Bagolini lens test most closely simulates ordinary viewing and is the least dissociating of all retinal correspondence tests. Central (foveal) fixation must be assumed and the alignment of the eyes known; possible outcomes are given in Figure 72-8 .
Figure 72-5 Possible Worth four-dot percepts in binocular patients. Note the similar distant responses in patients who have esotropia with abnormal retinal correspondence (ARC) and suppression and in those who have monofixation syndrome. Patients who have exotropia with ARC and suppression give the same responses, but the suppression scotoma is larger and shaped somewhat differently (see Fig. 71-2 ). The red lens is over the right eye and the green lens over the left eye.
Figure 72-6 Possible Worth four-dot responses in patients who do not have binocularity. The red lens is over the right eye and the green lens over the left eye.
The afterimage test is most removed from ordinary binocular viewing and the most dissociating of all commonly performed retinal correspondence tests; an ARC response on this test declares the ARC to be deep seated. An afterimage (positive in dim illumination, negative in bright) is imprinted on each retina in turn with a photographic flash device. The fellow eye is covered during the flash. Usually a vertical flash is presented to one eye and a horizontal flash to the other. The fovea is protected by a central mask with fixation target and thus “labeled” as the center of an afterimage line. An NRC response yields a cross pattern (see Fig. 72-9 ) as the fovea in each eye retains the straight-ahead directional value. A crossed heteronymous localization occurs in ARC with esotropia, as the straight-ahead directional value lies in
Figure 72-7 Bagolini lenses. Placed at 135° orientation in the trial frame before the patient’s right eye and at 45° before the patient’s left eye.
Figure 72-8 Possible Bagolini lens percepts, central fixation.
the nasal retina of the strabismic eye; the fovea has a temporal directional value.  In patients who have ARC and exotropia, the afterimage percept is an uncrossed homonymous localization.
Clinicians who have access to a major amblyoscope may set the tubes at the objective angle of strabismus; if the targets are superimposed, NRC exists. Crossed diplopia occurs in patients who have ARC and esotropia and uncrossed diplopia in patients who have ARC and exotropia. An “angle of anomaly” is defined when the patient moves the tubes until the targets are superimposed; this subjective angle equals the objective angle in “harmonious” ARC and is less or greater in “unharmonious” ARC.
Clinically useful stereopsis tests provide slightly different views of the same target to each eye; each unique view is maintained by
Figure 72-9 Afterimage test percepts, central fixation. Shown are those possible in patients who have central fixation and binocular vision.
Figure 72-10 Titmus stereotest with Polaroid glasses.
Retinal Correspondence Testing*
• Bagolini striated lenses; Aulhorn phase-difference haploscope
• Synoptophore (major amblyoscope)
• Red glass test
• Worth four-dot test; Polaroid lens and mirror test
• Afterimage test
• Dazzle test
* The lower the listing of an abnormal retinal correspondence response, the more the depth of the abnormal correspondence. After successful treatment, a normal retinal correspondence response develops with time, initially shown by the bottom tests and then through to the top. Not all listed tests are described in the text.
either Polaroid filters (Titmus, Wirt, Randot, Lang) or anaglyph glasses (TNO).
The Titmus stereotest ( Fig. 72-10 ) provides disparity in the range 3000?sec of arc at 40?cm testing distance (fly wings above background) to 40?sec of arc (ninth circle). Younger children may respond to the depth illusion of three sets of five animals, one of which appears to float above the background (400, 200, 100?sec of arc). The older Wirt stereotest provided circles with disparity as fine as 14?sec of arc. The first few circles may be identified accurately by nonstereoptic patients because the circles possess monocular clues ; the Randot test ( Fig. 72-11 ) avoids this problem because it provides similar targets as random dots with no monocular clues.
TABLE 72-1 — SYNOPSIS OF SENSORY TESTING IN STRABISMUS
Abnormal Retinal Correspondence
Worth four-dot, distance (6?m)
2 or 3
2 or 3
2 or 3
Worth four-dot, near (40?cm)
2 or 3
None to 14?sec arc
None to 67?sec arc
The Worth four-dot test and Titmus stereotest are used to define a patient’s sensory status. Appreciation of four distant lights demands normal retinal correspondence (NRC) and bifoveality, as does recognition of seven or more circles on the stereotest. Any level of stereoptic appreciation on this test signifies NRC at that moment at that testing distance. Appreciation of four lights on the Worth test at any testing distance signifies binocular vision.
Figure 72-11 Randot stereotest with Polaroid glasses.
Children who reject the Polaroid glasses may be tested using the similarly targeted Lang test, in which random-dot stereograms are presented through a cylinder grating that overlies the target. The TNO stereotest uses random-dot stereograms viewed through anaglyph glasses and contains disparities in the range 480 down to 15?sec of arc.
Test for Monofixation Syndrome
One feature of this syndrome (see Chapter 71 ) is a small, round scotoma that surrounds the fovea of one eye under binocular viewing conditions. As the patient views a distant target, a 4? prism, usually held base out, is introduced before one eye. If held before the fixing eye, it will saccade to the new target position toward the prism’s apex, as does the fellow eye. A slower fusional vergence movement in the fellow eye in the opposite direction follows. When the prism is held before a nonfixing eye there is no saccade, as the image displacement falls within a scotoma and is therefore not perceived. The test must be performed with the prism before each eye. Some patients switch fixation to the fellow eye when a prism is introduced before either and no saccadic shift is generated.
The busy clinician may determine the sensory status of most patients by using two straightforward and easily available tests—the Worth four-dot test and the Titmus stereotest. A summary of sensory testing interpretation using these commonly available testing devices is given in Table 72-1 .
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