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6
Sensory Aspects of
Strabismus
Kenneth W. Wright
SENSORY ADAPTATIONS
Visual neurodevelopment changes in response to abnormal
stimulation from a blurred retinal image or strabismus. These
changes are referred to as sensory adaptations. The specific type
of sensory adaptation depends on when the abnormal visual
stimulation occurred, the severity of the abnormal stimulation,
and type of binocular disruption. In Chapter 4, we discussed cortical suppression and amblyopia, which are basic sensory adaptations to a blurred image or strabismus. This chapter provides
a list of more specific sensory adaptations that are encountered
clinically. These adaptations are divided into two sections based
on the onset of the sensory insult: (1) visually mature and (2)
visually immature. A discussion of important sensory tests is
provided at the end of this chapter.
MATURE VISUAL SYSTEM
The following sensory adaptations occur after the development
of bifoveal fusion, when the visual system is mature. Visual
development continues until approximately 7 to 8 years of age.
After that, there is minimal visual-neurological plasticity. There
are some exceptions, however, and prolonged visual plasticity
into adulthood has been reported (see discussion at the end of
this section: Prolonged Visual Plasticity).
Diplopia
Acquired strabismus in patients over 7 or 8 years of age usually
results in double vision (i.e., diplopia). Diplopia is also reported
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175
in younger children with acquired strabismus, but it is usually
transient and lasts only 2 to 4 weeks before the diplopia is cortically suppressed. The patient with diplopia will fixate on an
object with one fovea, and see a diplopic image of that object
that comes from the perifoveal retina of the deviated eye (Figs.
6-1, 6-2). The fovea of the deviated eye is suppressed to avoid
simultaneously seeing two different objects, one from each fovea
(see below: Confusion). Thus, the patient with one eye fixing on
Red
Filter
Patient's Perception
Uncrossed Diplopia
FIGURE 6-1. Esotropia with uncrossed diplopia. The image of the skier
falls on the fovea of the left eye and on the nasal retina of the deviated
right eye. A red filter over the right eye causes the diplopic image from
the right eye to be red. Note at the bottom of the figure: the patient perceives the red image from the right eye to be located to the right of the
clear image, resulting in uncrossed diplopia.
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Red
Filter
Patient's Perception
Crossed Diplopia
FIGURE 6-2. Exotropia with image falling on the fovea of the left eye and
on temporal retina of the right eye, causing crossed diplopia. A red filter
over the deviated right eye causes the diplopic image from the right eye
to be red. Note at the bottom of the figure: the patient perceives the red
image from the right eye to be located to the left of the clear image, resulting in crossed diplopia.
a painting and the deviated eye pointed to a lamp will see two
paintings, not a painting superimposed on a lamp. The image
from the fixing eye will be in clear focus located directly in front
of the patient, while the diplopic image from the deviated eye
will appear blurred and off center because it comes from the
peripheral retina.
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OD fixing
RE
LE
FIGURE 6-3. Patient with a left hypertropia. A red filter over the deviated left eye causes the diplopic image from the left eye to be red. The X
projects to the fovea of the fixing right eye and to the superior retina of
the deviated left eye. Because the superior retina views the inferior visual
field, the red diplopic image in the left eye is seen below the clear image
from the right eye.
Esotropia causes the image to fall on the nasal retina of the
esotropic eye, which projects temporally and causes uncrossed
diplopia because diplopic image is on the same side as the
deviated eye (see Fig. 6-1). Exotropia causes the image to fall
temporal to the fovea of the exotropic eye, which projects to
the nasal field, producing crossed diplopia (see Fig. 6-2). We
can remember the s in esotropia means same side diplopia
(uncrossed), and the x in exotropia means a cross for crossed
diplopia. In cases of vertical strabismus, the hypertropic eye perceives the object as being below the image from the fixing eye
(Fig. 6-3).
Aniseikonia is a difference in image size between eyes and
is a cause of diplopia. Aniseikonia is usually caused by anisometropia and is treated with spectacles. An acquired retinal
image size disparity up to 7% is usually tolerated, but aniseikonia over 10% may result in diplopia.
Confusion
Under rare circumstances, instead of diplopia, patients with
acquired strabismus see two different images superimposed on
each other, one image from each fovea. If the right eye is looking
at a painting and the left eye is pointed at a lamp, the patient
with confusion will see the lamp superimposed on the painting.
This simultaneous perception from the fixing fovea and the devi-
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ated fovea is termed confusion. Most patients with acquired
strabismus do not experience confusion because they suppress
the foveal area of the deviated eye and see the diplopic image
from the peripheral retina. Confusion is exceedingly rare;
however, this author has reported a patient with tunnel vision
secondary to glaucoma and acquired strabismus who had confusion rather than diplopia.10 The peripheral visual field loss
associated with the glaucoma probably forced foveal fixation of
the deviated eye. It is likely that suppression of the fovea of the
deviated eye is dependent on peripheral retinal stimulation by
the diplopic image and, therefore, foveal suppression is not possible when the peripheral field is eliminated.
IMMATURE VISUAL SYSTEM
Sensory adaptations occur when the binocularity is disrupted by
strabismus or a blurred retinal image during the first few years
of life, usually before 6 years of age. The specific type of sensory
adaptation depends on many factors, including the size of the
strabismus, whether it is intermittent or constant, the age of
onset of the strabismus, and the age when the strabismus is corrected. Once childhood sensory adaptations are acquired, they
are usually present throughout the patient’s life. Cortical suppression is a basic mechanism present in virtually all sensory
adaptations to strabismus and a unilateral blurred retinal image.
Cortical suppression and amblyopia are discussed in Chapter 4.
Herein is a discussion of specific patterns of suppression and
abnormal binocular vision.
The following discussion of sensory abnormalities presumes
that strabismus is the primary event and that the brain develops sensory adaptations in response to the abnormal visual
stimulation. In this author’s view, this is probably true for the
majority of strabismus cases; however, strabismus can also
occur as a secondary consequence of poor binocular fusion.
Examples of a primary fusion deficit and secondary strabismus
include sensory strabismus (i.e., unilateral congenital cataract)
and central fusion loss associated with closed head trauma. It
should be pointed out that some would argue that most types of
childhood strabismus are a consequence of congenitally abnormal fusion centers within the brain, not motor misalignment
degrading binocular fusion. The answer to this controversy—
which came first, the strabismus or the sensory fusion abnor-
chapter 6: sensory aspects of strabismus
179
mality?—remains unanswered. The fact that one can recover
excellent binocular fusion and stereoscopic vision with early and
aggressive treatment suggests that, at least in some cases, the
sensory abnormality is secondary to the strabismus.
Monofixation Syndrome (Peripheral Fusion)
Small-angle strabismus (10 prism diopters, PD) or mild to moderate unilateral retinal image blur in young children and infants
causes a suppression of the central visual field of the deviated
or blurred eye. The small suppression scotoma allows for peripheral fusion (Fig. 6-4). This sensory adaptation, first described by
Marshall Parks, is termed the “monofixation syndrome.”3 Suppression is localized to within the central 4° to 5° because the
central retina has small receptive fields and high spatial resolution potential; therefore, relatively small differences in image
clarity or retinal image position are recognized. In the peripheral fields, however, slight interocular image differences are not
detected, as the peripheral retina has large receptive fields and
relatively low spatial resolution. Thus, small retinal image discrepancies between the eyes are not disruptive in the peripheral
fields, and peripheral fusion occurs. The size of the suppression
scotoma is directly proportional to the amount of image blur and
size of the strabismus. If the interocular image disparity is too
great, even peripheral fusion will be disrupted. Thus, strabismus
greater than 10 PD or severe unilateral image blur (e.g., unilateral dense cataract) will disrupt even peripheral fusion. These
patients will lack binocular fusion and will not have the
monofixation syndrome.
Because patients with the monofixation syndrome have
motor fusion, they often have a relatively large underlying
phoria in addition to a small tropia, giving rise to the term
phoria-tropia syndrome. Patients with monofixation syndrome
usually have stereoacuity in the range of 3000 to 70 s arc, and
the central suppression scotoma measures between 2° and 5°.
The Bagolini striated lens test is a sensory test that presents a
linear streak of light to each eye oriented 90° apart and centered
on the fixation light (Fig. 6-5). Patients with normal binocular
vision describe a cross through the center of a fixation light (Fig.
6-5A). In contrast, patients with the monofixation syndrome
will describe a cross with a gap in the center of the line presented to the deviated eye (Fig. 6-5B). The gap represents a
central suppression scotoma of the nonfixing eye. It is impor-
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Small Angle
Esotropia
Patient's Perception
Suppression
Scotoma
A
Anisometropic
Amblyopia
Suppression
B
Scotoma
FIGURE 6-4A,B. (A) Diagram of monofixation syndrome secondary to a
small-angle esotropia. (B) Hypermetropic anisometropia with amblyopia.
In both cases, patient perceives a clear single image, as the suppression
scotoma eliminates the discrepancy from the esotropia and blurred image,
respectively. Because of the suppression scotoma, the patient sees one
clear image.
tant to note that as soon as the dominant fixing eye is occluded,
the suppression scotoma vanishes and the patient fixes with the
fovea (Fig. 6-5C). The suppression scotoma is often referred to
as a facultative scotoma, because its presence is dependent upon
fixation with the dominant eye. Worth 4-dot testing is another
good method to document the monofixation syndrome. Patients
with the monofixation syndrome will fuse the near Worth 4-dot
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181
A
B
C
FIGURE 6-5A–C. Monofixation with microtropia and visual perception
with Bagolini lenses. (A) Bagolini lenses over right small-angle esotropia
and suppression scotoma, right eye. (B) Retinal images from (A). Note the
patient’s perception is one continuous line LE, and one line with an interruption in the center RE. (C) Covering the fixing eye (LE) eliminates the
suppression scotoma, and the patient sees a single, continuous line from
RE.
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(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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183
Patient's Perception
A
B
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.
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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
chapter 6: sensory aspects of strabismus
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A
B
C
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.
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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. 66). 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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187
A
B
C
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-
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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
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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-
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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.
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Red Filter Test
Othotropia NRC
Penlight
RE
LE
F
F
L
Center
R
Binocular
Perception
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.
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Red Filter Test
R-Esotropia ARC
Penlight
LE
RE
F
F
P
L
Center
R
Binocular
Perception
One Pink Light
FIGURE 6-11. Red filter test in a patient with a right esotropia and ARC.
Red filter is placed in front of the right eye (RE) and the image falls on
the pseudo-fovea (P) and fovea, representing corresponding retinal points
in a patient with ARC. The patient has a single binocular perception and
sees one pink light. LE, left eye.
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193
TABLE 6-1. Types of Diplopia Tests.
Most dissociating
Maddox rod
Dark red filter
Worth 4-dot with room lights out
Worth 4-dot with room lights on
Least dissociating
Bagolini striated lenses
SPECIFIC DIPLOPIA TESTS
RED FILTER TEST
One of the simplest diplopia tests is the red filter test. Place a
red glass over one eye and direct the patient to fixate on a single
light source, or an accommodative fixation target. Patients
with straight eyes and normal retinal correspondence will see
one pinkish-red light (see Fig. 6-10). If a phoria is present, the
red filter may dissociate the eyes and then the patient will manifest their deviation and see double. The denser the red color,
the more dissociating the test. Another way to make the standard red filter test more dissociating is to turn down the room
lights. In dim illumination, the eye behind the red filter will
only see the light source, not background objects in the room,
which will eliminate peripheral fusion clues. The red filter test
is useful for identifying NRC, ARC, and suppression. Esotropia
with NRC causes uncrossed diplopia, with the red light seen on
the same side as the red filter (see Fig. 6-1). Alternately, exotropia
with NRC is associated with crossed diplopia as the red light is
opposite to the red filter (see Fig. 6-2). When the deviation is
neutralized with a prism, the diplopia disappears and the images
will be superimposed.
Patients with ARC will generally see one light, even though
they have strabismus, because they use a pseudo-fovea. In Figure
6-11, the red light falls on the pseudo-fovea of the right eye. This
image is cortically superimposed with the foveal image of the
left eye to produce the perception of one pink light. If partial or
full prism neutralization of the deviation results in diplopia,
then the patient has ARC.
Strabismus associated with suppression results in the perception of a single light, either a red or a white light, depending
on which eye is fixing. In Figure 6-12, the left eye is fixing, so
the patient sees one white light and suppresses the red light
falling on the right retina. If a dark red filter is placed over the
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Red Filter Test
(Esotropia and Suppression RE)
NRC
Penlight
Red Filter
LE
RE
F
F
Suppression
Scotoma
L
Center
R
Binocular
Perception
LE
One White Light
FIGURE 6-12. Red filter test in a patient with childhood esotropia who
developed suppression and a fixation preference for the left eye. Patient
fixes left eye with a suppression scotoma of the right eye. Note that the
retinal image of the penlight falls within the suppression scotoma, so the
patient only perceives one white light from the left eye.
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195
fixing left eye, then fixation switches to the right eye, and the
left eye is suppressed (Fig. 6-13). Patients who alternate fixation
may report seeing two lights: a red light alternating with a white
light. When a child with a manifest strabismus claims to see two
Red filter over LE
Penlight
Dark red
filter
LE
RE
F
F
Suppression
Scotoma
L
Center
R
Binocular
Perception
RE
One White Light
FIGURE 6-13. A dark red filter is placed over the left eye to shift fixation to the right eye. With the right eye fixing, patient suppresses the
image in the left eye and perceives one white light from the right eye.
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lights, be sure to distinguish between diplopia, where the red
and white lights are seen simultaneously, and alternating suppression, where one light is seen at a time. Partial or full prism
neutralization of the strabismus will not result in diplopia. The
patient with suppression will continue to see just one light.
VERTICAL PRISM RED FILTER TEST/SUPPRESSION
VERSUS ARC
Another way ARC can be distinguished from NRC in patients
with suppression is by placing a red vertical prism (usually 15
PD base-down) over the deviated eye. A vertical prism causes
patients with ARC to see two vertically displaced images, with
the red light directly over the white light (Fig. 6-14). The lights
are vertically aligned because the light in the deviated eye is over
the pseudo-fovea that corresponds to the true fovea of the fixing
eye.
When a vertical prism is introduced to the deviated eye of
a patient with central suppression and NRC, the patient reports
seeing two lights that are horizontally and vertically displaced
because there is no pseudo-fovea and the center of reference is
the true fovea of each eye (Fig. 6-15).
WORTH 4-DOT
The Worth 4-dot test consists of two green lights, one red light,
and one white light (Fig. 6-16). The patient wears red/green
glasses, usually with the red lens over the right eye, and views
a Worth 4-dot flashlight at one-third of a meter, or a Worth 4dot light box at 6 m (20 ft). The near Worth 4-dots are separated
by 6° at near (flashlight at 1/3 m) and by 1.25° for the distance
(light box at 6 m). When the test is performed with the room
lights out, the white dot is the only binocular fusion target, as
it is the only light seen by both eyes. Green lights are seen
through the eye behind the green filter, and the red light is seen
with the eye with the red filter. If the room lights are turned on,
however, the patient can see the room environment with both
eyes, including the Worth flashlight and examiner, thus providing strong fusion clues; this is why Worth 4-dot testing in the
dark is much more dissociating than testing with the room
lights on.
The normal fusion response is seeing four lights, two red
and two green. Another normal response is one red light, two
green lights and one light that flickers between red and green.
The light that flickers is the white light that is seen by both
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Esotropia ARC
Penlight
RE
L-ET
F
P
F
F
Red prism base down
L
Center
R
Binocular
Perception
Vertical Diplopia with
two lights in horizontal alignment
FIGURE 6-14. Patient with esotropia and ARC is presented with a basedown vertical prism and a red filter over the left eye. The prism deflects
the retinal image below the pseudo-fovea (P) and the patient perceives
two images: vertically, one on top of the other. Remember, the pseudofovea (P) is the center of vision during binocular viewing.
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Esotropia NRC Suppression
Penlight
L-ET
RE
ET
F
F
F
Red prism base down
L
Center
R
Binocular
Perception
Vertical and Uncrossed Diplopia
FIGURE 6-15. Patient with esotropia and suppression of left eye. A basedown prism is placed in front of the left eye, which displaces the retinal
image inferiorly and out of the central scotoma. The patient perceives
two images: vertically and horizontally displaced. Note that there is no
pseudo-fovea (F) and the true foveas are at the center of vision.
chapter 6: sensory aspects of strabismus
199
FIGURE 6-16. Worth 4-dot test in a normal patient with straight eyes.
Three lights are projected to the left eye and two lights to the right eye.
Patient fuses the two images and perceives four lights.
eyes, the flicker being color rivalry. Patients with acquired strabismus and diplopia will see five lights: three green and two red.
Patients with cortical suppression report seeing either three
green lights or two red lights, depending on which eye is fixing.
In Figure 6-9, the left eye is fixing and the right eye is suppressed
so the patient sees three green lights. If the right eye was the
preferred eye and the left eye was suppressed, then the patient
would see two red lights. Patients who alternate fixation usually
describe seeing two red lights, alternating with three green
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lights. A few patients, however, will report the sum total of the
alternating lights, that is, five lights. Thus, alternating suppression can be confused with diplopia, because patients with
diplopia also report seeing five lights. Patients with large scotomas (scotomas greater than 6°) will suppress both the distance
(central field) and near (peripheral field) Worth 4-dot.
Patients with the monofixation syndrome have a small
central suppression scotoma (5°) and peripheral fusion. They
fuse, or see, four lights for the near Worth 4-dot (which subtends
6°) because the dots fall outside the scotoma (Fig. 6-17), but sup-
FIGURE 6-17. Near Worth 4-dot test in a patient with monofixation
syndrome and 8 PD (4°) esotropia. The near Worth 4-dot subtends 6° and
the dots fall outside the scotoma. Patient perceives four dots.
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201
FIGURE 6-18. Distance Worth 4-dot test in a patient with monofixation
syndrome and esotropia of 8 prism diopters. The distance Worth 4-dot
subtends 1.25° and two dots fall within the central suppression scotoma.
Therefore, patient perceives three dots from the left eye and no dots from
the right eye.
press the distance Worth 4-dot (which subtends only 1.25°) as
the dots fall within the scotoma (Fig. 6-18). One of the best uses
of the Worth 4-dot test is to identify the monofixation syndrome
(i.e., central suppression and peripheral fusion) in a patient with
a small-angle strabismus. The results of this test will tell the
examiner if there is peripheral fusion that can be present even
if there is no discernible stereoscopic vision.
Remember, it is important to leave the room lights on when
performing the Worth 4-dot test if the goal is to promote fusion.
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With the room lights on, the patient can see background objects
in the room with both eyes, providing binocular peripheral
fusion clues. If the lights are dimmed or turned off, however,
only the Worth lights can be seen and the only target seen by
both eyes is the single white dot. Because of the lack of peripheral fusion clues, the Worth 4-dot test becomes extremely dissociating in the dark. Once one realizes the dissociating power
of the dark, one can use this phenomenon to estimate how well
a patient fuses. If a patient can maintain fusion of the Worth 4dot test with lights out, then this indicates strong motor fusion.
On the other hand, if dimming the lights changes the response
from fusion to suppression or diplopia, this reveals relatively
weak motor fusion. Patients with intermittent exotropia who
have weak motor fusion manifest their deviation when the
lights are dimmed.
The Worth 4-dot flashlight can be used to plot the size of
suppression scotomas. By moving the flashlight closer to the
patient, the lights subtend a larger angle (i.e., stimulate more
peripheral retina) and by moving the flashlight farther away, the
lights subtend a smaller angle (i.e., stimulate more central
retina). Table 6-2 describes the stimulus angle for the Worth 4dot flashlight at various distances from the patient.
BAGOLINI LENSES
Bagolini striated lenses are clear with a linear scratch through
the center of each lens that provides a streak of light on the
retina when viewing a bright light (see Fig. 6-5). One lens is
placed over each eye, and the lenses are oriented obliquely at
45° and 135°. Because the lenses are otherwise clear, they are
not dissociating. Bagolini lenses, therefore, have the advantage
of providing a free binocular view without dissociation. Patients
with straight eyes and NRC, and those with harmonious ARC,
will report seeing a cross (Fig. 6-19A). Remember, with ARC,
one line is on the true fovea and the other line falls on the
TABLE 6-2. Stimulus Angle for Worth 4-Dot Flashlight.
Flashlight distance from patient
1/6 m
1/3 m (14 in.)
1/2 m
1m
a
Standard near Worth 4-dot.
Worth 4-dot angle
12°
6°a
4°
2°
chapter 6: sensory aspects of strabismus
203
FIGURE 6-19A–E. Patient perception of Bagolini testing. (A) A cross is
perceived in orthotropia with normal retinal correspondence or strabismus with ARC. (B) Patient with strabismus and large suppression
scotoma sees one line. (C) Patient with monofixation syndrome and small
central scotoma will see one continuous line and one line broken in the
center that corresponds to the eye with the suppression scotoma. (D)
Patient with esotropia and uncrossed diplopia reports a “V” configuration. (E) Patient with exotropia and crossed diplopia reports an “A”
configuration.
pseudo-fovea (see Fig. 6-7). Patients who have large regional
suppression will report seeing only one line (Fig. 6-19B). The
monofixation syndrome, on the other hand, is associated with a
cross, but one line will have a central gap (Figs. 6-19C, 6-5).
Patients with NRC, heterophoria, and diplopia will show the
response of either an “A” or a “V.” Because esotropia is associated with uncrossed diplopia, esotropia will cause the right
line to move to the right and the left line to move to the left,
creating a “V” (Fig. 6-19D). Exotropia produces an “A” because
exotropia is associated with crossed diplopia, with the right
line moving to the left and the left line moving to the right
(Fig. 6-19E).
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MADDOX ROD TEST
The Maddox rod can be used for identifying horizontal, vertical,
and, especially, torsional deviations. The Maddox rod has a
washboard appearance as it is made up of multiple cylindrical
high plus lenses stacked on top of each other. When the patient
views a light through the Maddox rod, a linear streak of light
oriented 90° to the cylindrical ribs of the Maddox rod is seen.
The single Maddox rod test is performed by placing the Maddox
rod over one eye and having the patient view a penlight. The
Maddox rod is aligned so the streak is vertical to detect horizontal deviations and then horizontal for vertical deviations. If
the streak of light passes through the penlight, the patient is
orthophoric, or has harmonious ARC. This is one of the most
dissociating tests, because the images to each eye are totally different and there are essentially no binocular fusion clues. The
Maddox rod test is so dissociating that it will cause patients with
normal bifoveal fusion to manifest their phoria. Because of this,
the Maddox rod test, and dissociating tests in general, do not
distinguish between phorias and tropias. To make the diagnosis
of phoria versus tropia, one must assess the eye alignment
objectively before administering the dissociating diplopia test.
The Maddox rod test can also be used to measure torsion (as
described in Chapter 5).
Haploscopic Tests
In contrast to diplopia tests where there is one stationary fixation target that is viewed by both eyes, haploscopic tests have
two fixation targets, one for each eye, and the targets can be
moved separately to align with each fovea. A haploscopic presentation means each eye receives its own visual stimulus. There
are various ways to separately stimulate each eye. One way to
create haploscopic vision is to place a mirror in front of each
eye, with the mirrors angled so the right eye sees the right temporal side and the left eye sees the left temporal side. Mirror separation of vision is the principle of the amblyoscope. Another
commonly used method is to give the patient color-tinted
glasses with one eye receiving a red filter and the fellow eye a
green filter. Two movable targets are presented on a white
screen: one red and one green. The eye with the red filter sees
only the red target and the eye with the green filter sees only
the green target; thus, separate visual stimuli are presented to
each eye; this is the principle of the Lancaster red/green test. If
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205
strabismus is present, either the mirrors can be angled or the
red/green targets moved so the fixation target is aligned with
each fovea. Haploscopic tests include the Lancaster Red/Green
Test and the amblyoscope. The Lancaster Red/Green test is used
to measure the angle of strabismus (see Chapter 5; Fig. 5-14).
Note that the Worth 4-dot test is partially haploscopic because
some of the objects in the visual field are seen by both eyes. The
Worth 4-dot test is not a true haploscopic test, as targets are not
independently movable to each eye and cannot be aligned with
each fovea.
AMBLYOSCOPE
The amblyoscope provides a haploscopic view, allowing presentation of images to each eye independently. Two mirrors at the
elbow of the amblyoscope arms reflect images from transparent
picture slides into each eye (Fig. 6-20). The arms can be moved
to measure either subjective or objective angle. The subjective
angle is the amount in degrees the examiner must move the
amblyoscope arms to allow the patient to see the two pictures
FIGURE 6-20. Amblyoscope testing a patient with normal retinal correspondence (NRC) and orthotropia. A dot is a target for the left eye and a
ring is the target for the right eye. Patient sees the dot inside the ring
without moving the arms of the amblyoscope.
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as being superimposed. The objective angle is measured by alternating the target presentation from right eye to left eye, moving
the arms of the amblyoscope until there is no refixation eye
movement. The objective angle equals the deviation as measured by the alternate prism cover test. The subjective angle is
determined under binocular viewing conditions whereas the
objective angle is measured during monocular viewing.
NORMAL RETINAL CORRESPONDENCE
In a strabismic patient with NRC and diplopia, the subjective
and objective angles are the same (Fig. 6-21) because patients
with NRC always use the fovea as the center of reference.
Patients with NRC and dense large regional suppression will not
have a measurable subjective angle because they suppress one
eye, making subjective superimposition of the images impossible. The subjective angle can be measured in patients with
the monofixation syndrome and a small central suppression
scotoma by using targets that stimulate the peripheral retina.
FIGURE 6-21. Patient with NRC and esotropia. The arms of the
amblyoscope are angled so the image falls on each fovea and the patient
perceives the dot inside the circle. Each arm is moved 20 (10°) for a
total of 40 .
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207
FIGURE 6-22. Patient with harmonious ARC and right esotropia. The
arms of the amblyoscope do not have to be angled for the patient to see
the dot inside the ring, as the pseudo-fovea (P) is directly aligned with the
ring target. The patient perceives the dot in the center of the circle with
the arms of the amblyoscope parallel aligned to zero.
ANOMALOUS RETINAL CORRESPONDENCE
(HARMONIOUS)
Patients with strabismus and harmonious ARC have a significant objective angle, but the subjective angle is zero. The subjective angle is zero (or close to zero) because the subjective
angle is measured under binocular conditions and reflects the
alignment based on the relationship between the true fovea of
the fixing eye and the pseudo-fovea of the deviated eye. Because
patients with harmonious ARC have the pseudo-fovea positioned to compensate for the angle of deviation, there is no subjective misalignment. Patients with harmonious ARC will see
the targets from each eye as superimposed with the amblyoscope
arms set to zero (parallel) even though there is a large objective
angle (Fig. 6-22). The objective angle is measured by alternate
cover testing, blocking the vision of each eye (monocular
viewing) so the objective angle reflects the misalignment based
on the true fovea. The displacement of the pseudo-fovea off the
true fovea is called the angle of anomaly. Because the location
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of the pseudo-fovea completely compensates for the objective
deviation in harmonious ARC, the subjective angle is zero, and
the objective angle equals the angle of anomaly. For example, in
Figure 6-22, the objective angle is ET 20 PD and the subjective
angle is zero. The angle of anomaly (i.e., distance of the pseudofovea from the true fovea) is 20 PD (200).
In patients with unharmonious ARC, the pseudo-fovea is
located in a position that does not fully compensate for the
objective deviation. These patients will see double or will suppress the image that does not fall on the pseudo-fovea (Fig. 6-23:
“I” image in right eye). The subjective angle is measured by
moving the arms of the amblyoscope until the two images are
superimposed. When the images are superimposed, the image of
FIGURE 6-23. Patient with unharmonious ARC and 30 PD of esotropia.
The arms of the amblyoscope are set at zero and are not angled. As the
image (I) is falling nasal to the pseudo-fovea (P), the patient perceives
uncrossed diplopia (as diagrammed in the rectangle at the bottom of the
figure). If the arm of the amblyoscope in front of the right eye was moved
10° in to place the image on the pseudo-fovea (P), the patient would perceive the ring around the dot.
chapter 6: sensory aspects of strabismus
209
the fixing eye is on the true fovea, and the image in the nonfixing eye is on the pseudo-fovea. The subjective angle is the
number of degrees from the zero position the amblyoscope arm
needs to move to place the image on the pseudo-fovea. For
example, in Figure 6-23, the subjective angle (I-P, right eye) is 10
PD, and the objective angle (I-F, right eye) is 30 PD. Because the
angle of anomaly (P-F) is equal to the objective angle minus the
subjective angle, the angle of anomaly is 20 PD (3010).
The amblyoscope is a useful tool as it can measure fusional
vergence amplitudes, angle of deviation, area of suppression,
retinal correspondence, and even torsion. Some degree of instrument convergence, however, is usually present when using the
amblyoscope.
AFTERIMAGE TEST
The afterimage test is a fovea-to-fovea sensory test that does not
use a haploscopic apparatus, but each eye is stimulated separately. Each fovea is marked individually during monocular
viewing with a linear strobe light that bleaches the retina; this
causes a linear afterimage shadow through the true fovea that
lasts approximately 10 s. The center of the linear strobe light
is masked to spare the fovea; thus, the afterimage line has a
break in the middle. Testing is performed by having the patient
occlude one eye while the other eye fixates on the central
masked part of the strobe light held vertically in front of the
patient (Fig. 6-24). The fixing eye is stimulated to produce a vertical afterimage. Next, the fellow eye is stimulated with a horizontally oriented strobe light while the first eye is covered (Fig.
6-24B). The occluder is quickly removed, and the patient is asked
where they see the afterimage lines while they are binocularly
viewing (Fig. 6-24C). Because the stimulus is presented under
monocular conditions, the stimulus always marks the true fovea
of each eye unless there is eccentric fixation from dense amblyopia. Patients with NRC will, therefore, always see a cross
whether they are orthophoric, esotropic, exotropic, or hypertropic because their center of reference is the fovea under
monocular or binocular conditions (Fig. 6-25A,B). Patients with
ARC however, use their true fovea during monocular viewing
but, during binocular viewing, the deviated eye switches to the
pseudo-fovea. Consequently, patients with ARC have each fovea
marked by the monocular afterimage, but when binocular vision
is reestablished, the pseudo-fovea takes over as the center of
A
B
C
A
B
C
D
FIGURE 6-25A–D. Perception of afterimage test in patients with (A)
NRC orthotropia, (B) NRC and strabismus, (C) ARC esotropia, and (D)
ARC exotropia. Note that the stimulation for the afterimage test occurs
under monocular conditions and that the light always tags the fovea, even
in patients with ARC. After the stimulation, the patient is again given
binocular vision, so the patient switches back to the pseudo-fovea and
the image tagged on the fovea appears to be in an eccentric location (C
and D).
FIGURE 6-24A–C. Afterimage test of a patient with NRC. If the patient
has NRC, the results of the afterimage test are the same whether the
patient has straight eyes, esotropia (ET), exotropia (XT), or a hyperdeviation. (A) Right eye is stimulated with a vertical strobe while the left eye
is covered. (B) Left eye is stimulated with a horizontal strobe light while
the right eye is covered. (C) The cover is removed and the patient reports
seeing a cross.
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handbook of pediatric strabismus and amblyopia
reference for the deviated eye. As the pseudo-fovea is the center
of reference, the afterimage marked on the true fovea is
perceived as coming from the peripheral visual field. With
esotropia, the fovea is temporal to the pseudo-fovea and temporal retina projects to the opposite hemifield, so the right
afterimage is seen on the left (Fig. 6-25C). Exotropia is just the
opposite, with the fovea nasal to the pseudo-fovea and nasal
retina projecting to the ipsilateral hemifield, so the right
afterimage is seen on the right (Fig. 6-25D).
Other Tests for Suppression and Fusion
VECTOGRAPHIC TEST
The vectographic test is an excellent test for suppression. The
test consists of two superimposed polarized slides of letters that
are projected onto an aluminized screen which reflects the
images while preserving polarization. The patient is asked to
read the letters on the screen while wearing polarized glasses.
The polarization of the glasses and the projected slides are oriented so some of the letters are only seen by the right eye, some
are only seen by the left eye, and some are seen by both eyes
(Fig. 6-26). Patients with normal bifoveal fusion will see all the
letters. If suppression is present, the letters projected only to the
suppressed eye will not be seen (Fig. 6-27). Some patients with
suppression will alternate fixation and will see all the letters,
although viewing them separately.
FOUR BASE-OUT TEST
This test is performed by first placing a 4 PD base-out prism over
one eye. In normal subjects, the 4 base-out test induces fusional
convergence. Remember, there are two movements to prism
convergence: first, a version movement of both eyes in the direction of the apex of the prism, and second, a fusional vergence
movement of the eye without the prism in toward the nose.
With the 4 base-out test, the examiner must look carefully for
the second convergence movement, as it is the sign of fusion.
Patients without motor fusion and large regional suppression
show no movement of either eye when the prism is placed over
the nondominant eye (Fig. 6-28A) and a version (not vergence)
movement of both eyes in the direction of the apex of the prism
when the prism is placed over the fixing eye (Fig. 6-28B).
chapter 6: sensory aspects of strabismus
213
FIGURE 6-26. Diagrammatic representation of vectograph with polarized
glasses in place and lenses oriented 90° to each other. Letters are projected
to a screen through two polarized lenses, which are also oriented 90° to
each other and match the orientation of the glasses. In this patient with
normal binocular vision, the left eye sees AC, right eye sees AB, and the
perception is ABC, which is noted in the rectangle at the bottom of the
figure.
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handbook of pediatric strabismus and amblyopia
FIGURE 6-27. Diagram of vectograph examination of a patient with a
suppression scotoma, left eye. In this case, the patient only sees images
from the right eye and reports seeing an AB.
Patients with the monofixation syndrome and a small
central scotoma usually show no movement when the 4 PD
prism is placed over the nondominant eye. Because these
patients have peripheral fusion, monofixators occasionally show
a normal fusional convergence movement. A prism over the
fixing eye always results in a version movement in monofixa-
chapter 6: sensory aspects of strabismus
215
tors, and some will show fusional convergence movement as
well. Normal patients with bifoveal fusion often show atypical
responses to the 4 base-out test.5 Some normals fail to fuse the
4 PD base-out prism, showing an initial version movement but
no secondary fusional convergence movement. These patients
often alternate fixation and report alternating diplopia. Other
normals seem to ignore the induced phoria and show no move-
A
B
FIGURE 6-28A,B. (A) Esotropia, left eye fixing, right eye deviated with
large suppression scotoma. Placing the 4 base-out prism in front of the
deviated right eye produces no movement of either eye, because the image
falls within the suppression scotoma. (B) Placing the 4 base-out prism in
front of the fixing eye results in a version movement, with both eyes
moving in the direction of the apex of the prism, because there is no suppression scotoma and the movement of the image is perceived.
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handbook of pediatric strabismus and amblyopia
ment when the 4 base-out prism is placed over one eye. Thus,
a secondary fusional convergence movement on 4 base-out
prism testing indicates fusion (central fusion or even peripheral
fusion), but because of frequent atypical responses in normals,
absence of a convergence movement does not necessarily mean
an absence of fusion.
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