Chapter 15: Examination of the Patient

CHAPTER
15
Examination of the
Patient—V
DEPTH PERCEPTION
tereopsis is an epiphenomenon of normal binocular vision (see Chapter 2). Its presence or
absence is an important indicator of the state of
binocularity in patients with ocular motility disorders. Barring a few notable exceptions (see Chapter 16), patients with essential infantile esotropia
are stereoblind or, at best, have markedly reduced
stereopsis, and the potential for regaining it is
practically nil. In childhood strabismus with a later
onset or in adults with acquired strabismus it is
an important therapeutic goal to reestablish stereopsis. Whether this can be accomplished depends
on many variables, among them the age of onset
and the duration of the strabismus and the completeness of ocular realignment.
S
Development of Stereopsis
Depth perception on the basis of binocular disparity is not fully developed at birth. Several studies
using different paradigms such as line stereograms
and a preferential looking procedure, random dots
with a forced-choice preferential looking technique, and random dots with visually evoked responses have shown remarkably consistent findings: stereopsis is absent in almost all infants less
than 3 months old, after which it rapidly develops
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to normal levels which are reached by the sixth
month of life. Interestingly, this rapid rate of maturation far exceeds that of visual acuity.11 The duration of the plasticity period of stereopsis in humans still needs to be established. For a review of
the literature, see Teller31 and Birch.5
Stereopsis and Strabismus
Patients with a large manifest deviation do not
have useful stereopsis in casual seeing. Nevertheless, they can function quite well in space, making
use of nonstereoscopic clues to depth perception,
especially if the strabismus is of early origin. They
may have trouble with fast-moving objects, such
as flying balls, and this experience may be frustrating to young children. However, when the strabismus is acquired later in life the loss of stereopsis
is felt acutely and may present a real handicap. It
appears as if stereopsis is useful in the comprehension of complex visual presentations and those
requiring good hand-eye coordination. Although
the importance of stereopsis is often stressed, studies addressing the functional effects of stereoscopic deficits are sparse.8
It is always interesting and useful to determine
whether a patient with strabismus has stereopsis
Examination of the Patient—V
or the potential for such. Some patients may respond to disparate stimulations with a degree of
stereopsis if the targets are placed at the objective
angle, as in a major amblyoscope. Some patients
(e.g., intermittent exotropes) may respond with
good stereoscopic acuity even when a stereoscope
is used, although they seemingly may be unable
to superimpose dissimilar targets. Such patients
require strong fusional stimuli to keep their eyes
aligned and to fuse. When they do, they gain
motor and sensory fusion, often with a high degree
of stereopsis.
Some ophthalmologists use stereoscopic tests
to determine whether patients with small or intermittent deviations have foveal suppression. If the
stereoscopic threshold is low enough, they conclude that there is no foveal suppression.27 A positive result is certainly conclusive, but a negative
result does not necessarily mean that foveal images are completely suppressed. There are patients
who fuse all but disparate retinal stimuli, which
are selectively suppressed.
A positive stereoscopic response of a patient
with a neuromuscular anomaly of the eyes at any
fixation distance and in any part of the binocular
field is of paramount importance prognostically
and in directing treatment. This finding makes it
mandatory that every effort be made, both nonsurgically and surgically, to restore to the patient full
binocular cooperation with stereopsis at all fixation distances and in every part of the field.
Testing for stereopsis should always be done
after operations have properly aligned the eyes.
The findings may give indications whether and
how to follow up the operation by nonsurgical
treatment.
Testing for Stereopsis
Equipment for testing stereopsis ranges from simple equipment to complex laboratory apparatus.
Only tests that the ophthalmologist can conveniently apply in the office are discussed in this
section. A test for stereopsis must incorporate two
essential features. The two eyes must be dissociated; that is, each eye must be presented with a
separate field of view, and each of the two fields
or targets must contain elements imaged on corresponding retinal areas. Thus a frame of reference
is provided, and disparately imaged elements can
be fused and seen stereoscopically. In addition,
299
there should be fiducial marks that permit the
examiner to check whether both eyes are used
simultaneously.
Major Amblyoscope or Stereoscope
The targets may be opaque or transparent and may
be used in a major amblyoscope or stereoscope.
Both devices have mechanically separated fields
of view, are set optically at infinity, and use exchangeable targets. The advantage of the major
amblyoscope is that its arms can be set at the
patient’s angle of deviation, thus allowing control
of the retinal area being stimulated. Similarly the
stereoscope may be used with prisms, but this
procedure may not be accurate, and the distortions
induced by prisms may become bothersome.
The number and variety of targets are limited
only by the ingenuity of the designer and user, but
standard sets of targets and cards are commercially
available for the different major amblyoscopes
and stereoscopes. Targets of special interest in the
present context are those that contain objects with
differing amounts of disparity (e.g., the Keystone
DB6 card), so that they appear at different relative
depth distances. The object seen in depth, which
has the least disparity, denotes the patient’s stereoscopic threshold.
Stereogram
A useful clinical application can be made of the
simple stereogram consisting of eccentric circles,
one set seen with each eye (see Fig. 2–15). If the
patient reports that two fiducial marks and two
circles are seen, but not in depth, one should
inquire whether the two circles are concentric.
They cannot be seen concentrically unless they
are also seen stereoscopically. If they are seen
eccentrically, one may now ask whether the inner
circles are closer to the right or left of the outer
circle. The patient’s answer determines whether
the disparate elements are suppressed in the right
or the left eye.
Titmus Stereo Test
Vectograph cards dissociate the eyes optically. A
vectograph consists of Polaroid material on which
the two targets are imprinted so that each target is
polarized at 90⬚ with respect to the other. When
the patient is provided with properly oriented Po-
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Introduction to Neuromuscular Anomalies of the Eyes
FIGURE 15–1. The Titmus Stereo Test.
laroid spectacles, each target is seen separately
with the two eyes. This principle is used in the
Titmus Stereo Test (Fig. 15–1). In this test a gross
stereoscopic pattern representing a housefly is provided to orient the patient and to establish whether
there is gross stereopsis (threshold: 3000 seconds
of arc). In testing young children, one must ask
questions the child will understand. For example,
one may ask the child to take hold of the wings
of the fly. If the child sees them stereoscopically,
the child will reach above the plate. It is amusing
to watch the child’s startled look when he or she
does so. It is indeed an eerie feeling not to have
a tactile sensation of a seen object. Some children,
though they have stereopsis, will touch the wings
on the plate because they ‘‘know’’ they are there.
The examiner must explain to these children that
he or she does not inquire about what they know,
but what they see.
The Polaroid test also contains three rows of
animals, one animal in each row imaged disparately (thresholds: 100, 200, and 400 seconds of
arc, respectively). The child is asked which one
of the animals stands out. The animal figures contain a misleading clue. In each row one of the
animals, correspondingly imaged in two eyes, is
printed heavily black. A child without stereopsis
will name this animal as the one that stands out.
Last, the Titmus test contains nine sets of four
circles arranged in the form of a lozenge. In this
sequence the upper, lower, left, or right circle
is disparately imaged at random with thresholds
ranging from 800 to 40 seconds of arc. If the child
has passed the other tests, he or she is now asked
to ‘‘push down’’ the circle that stands out, beginning with the first set. When the child makes
mistakes or finds no circle to push down, the
limits of stereopsis are presumably reached.
If there is doubt whether the patient actually
does see stereoscopically, one may occlude one
eye and inquire whether there is a difference in
appearance, say, of the housefly, with one or both
eyes open. And since only horizontal disparity
produces stereopsis, one can also turn the plate
90⬚, which should block out the stereoscopic effect.
Because of its simplicity, the Titmus Stereo
Test is widely used. On the basis of this test alone,
however, one is not always justified in stating
simply that ‘‘the patient has no stereopsis,’’ that
is, that there is no sensitivity for disparate stimuli.
One must keep in mind that the vectograph test is
used for testing near vision. Some patients suppress disparate stimuli at near but respond to them
in distance fixation, or vice versa, usually when
the deviation is intermittent at one fixation dis-
Examination of the Patient—V
tance and constant at the other. If such a pattern
is suspected, it is always wise to supplement the
vectograph test with a projected vectograph test at
distance fixation (Polaroid Vectographic ProjectO-Chart, American Optical Reichert) or with the
B-VAT (Mentor) projection device.
In recent years much emphasis has been placed
on the use of stereoacuity testing as a screening
method to detect anomalies of binocular function.9,
29, 30
Normal stereoacuity is said to preclude suppression, amblyopia, or heterotropia, and a subnormal test result may indicate the presence of
such anomalies. In applying the Titmus test as a
screening device, Simons and Reinecke29 found
that, with the exception of the fine stereoacuity
circles 5 to 9, this test often is unreliable in differentiating patients with amblyopia and heterotropia
from those with normal vision. Moreover, the Titmus test is capable of indicating an artifactual
stereocapability when none actually exists (see
also Köhler and Stigmar14). Some of the circles of
the Titmus test may be selected even by stereoblind
observers because they look ‘‘different’’ and not
because they are seen stereoscopically. Some patients notice an image jump in the disparate portions of the test target (e.g., the wings of the fly)
301
when fixating alternately and utilize this clue to
pass the Titmus test despite the fact that they may
be stereoblind on any of the tests using random
dots.29 Archer1 described a test based on dynamic
circles designed to mimic the Titmus circles as
closely as possible, while eliminating lateral displacement cues as well as the possibility of passing the test by alternation.
Random-Dot Stereograms
To avoid any such visual clues, two tests are
available that use random-dot stereograms.2 The
physiologic principle underlying these tests has
been discussed in Chapter 2. Random-dot stereograms are devoid of any monocular clues, and the
patient has no way of guessing what the stereofigure is and where it is located on the test
plate.13 Reinecke and Simons28 introduced the random-dot E test (RDT) (Fig. 15–2), which contains
three cards and Polaroid spectacles. One card is a
bas-relief model of the stereotest figure and is
used to show the child what to look for. One of
the two other test cards contains the E stereo
figure, and the other is stereoblank with an identical random-dot background. The test is performed
FIGURE 15–2. Random-dot E test set. (From Simons K, Reinecke RD: Amblyopia screening and
stereopsis. In Symposium on strabismus: Transactions of the New Orleans Academy of Ophthalmology. St. Louis, Mosby–Year Book, 1978, p 15.)
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Introduction to Neuromuscular Anomalies of the Eyes
FIGURE 15–3. A, The TNO test. B, The random-dot stereogram offers no monocular clues as to
the presence of a large circle in the center of the upper right quadrant and a smaller circle in the
center of the lower left quadrant. (From Noorden GK von: Present status of sensory testing in
strabismus. In Symposium on Strabismus: Transactions of the New Orleans Academy of Ophthalmology. St. Louis, Mosby–Year Book, 1978, p 51.)
by holding both test cards 50 cm in front of the
patient, who is then requested to indicate which
card contains the letter E. The test is simple to
perform, and the patient will give a ‘‘pass’’ or
‘‘fail’’ response. It can be quantitated by increasing the testing distance from the patient. Many
modifications of the RDT have become available
in the meantime.6 Random-dot stereopsis can be
measured also for distance with the Mentor BVAT II-SG computerized testing system (Mentor
O & O, Norwell, MA). This is particularly useful
in intermittent exotropia.32
TNO Test
Another procedure, the TNO test, is based on a
similar principle but has the advantage of eliciting
quantitative responses without changing the testing distance. This test uses a pair of red-green
spectacles and a test booklet (Fig. 15–3). Each
test plate in the booklet consists of a stereogram
in which the half-images have been superimposed
and printed in complementary colors (anaglyphs).
The test plates, when viewed binocularly with redgreen spectacles by a normal subject, will elicit
Examination of the Patient—V
perception of an image in depth. The TNO test is
graded to provide retinal disparities ranging from
15 to 480 seconds of arc. Comparative studies
have shown that this test compares favorably with
the Titmus test when used as a screening
device.26, 33 Together with the Lang test (see below) it is the preferred test in our clinic. It must
be emphasized, however, that even random-dot
testing of stereopsis is not a fail-safe method to
assess visual acuity and binocular function in preschool and school-age children, since normal levels of stereoacuity have been observed in anisometropic and visual deprivation amblyopia.3, 7, 21 How
should stereopsis, determined with any of the
tests, be recorded? Cards and vectographs that
attempt to qualify stereopsis are graded in different ways. Some use artificial scales (such as the
Sheppard scale); many speak of percentage of
stereopsis, assuming a certain threshold to mean
100%. All this is misleading and arbitrary. The
only proper way to record stereopsis is by the
amount of disparity incorporated into the target. It
is unequivocal, and it should be generally understood when it is stated that a patient has stereopsis
with a threshold of 400 or 100 or 40 seconds of
arc or whatever the threshold may be.
Lam and coworkers15 evaluated the response of
normal subjects to various visual function tests,
including stereopsis. They found a wide range
of responses in completely normal subjects, thus
raising the question which level of stereopsis reflects normalcy (see also Fisher9). It appears as a
difficult task to identify a cutoff value separating
303
normals from abnormals. These authors further
stated that because of the fact that more than 40%
of normal children demonstrate stereoacuity of
less than 40 seconds of arc, random-dot testing is
not a real measure of a biological function.
Awaya et al.4 studied the effects of aniseikonia
on stereopsis measurements. With their aniseikonia test, they found that aniseikonia of 7% to 13%
is still compatible with binocular fusion. However,
aniseikonia of greater than 5% is incompatible
with testing higher levels of stereoacuity with the
Titmus and TNO tests.
Lang Test
Occasionally, young children will refuse to wear
Polaroid or red-green spectacles, and observing
the position of the eyes while the patient is being
tested for stereopsis may be desirable. To overcome these difficulties, Lang17, 18 reported a new
test (the Lang test) based on panographic presentation of a random-dot pattern. Glasses are not
needed to recognize the stereoscopic images of a
star, a car, and a cat (Fig. 15–4) embedded in
random dots on the test card. A separate image is
provided to each eye through cylindrical lenses
imprinted on the surface lamination of the test
card (Fig. 15–5) When held at a testing distance
of 40 cm in the frontoparallel plane in front of the
patient (Fig. 15–6), the disparity of the car and
star is 600 seconds and of the cat 1200 seconds
of arc.17 A revised version of this test (Lang II
test)19 with smaller disparities and a less dense
FIGURE 15–4. Stereoscopic images embedded in random dots of the Lang test. (From Lang J: A
new stereotest. J Pediatr Ophthalmol Strabismus 20:72, 1983.)
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Introduction to Neuromuscular Anomalies of the Eyes
and the object is seen in depth does recognition
take place.
Two-Pencil Test
FIGURE 15–5. Cylinder gratings provide separate images for each eye. (From Lang J: A near stereotest. J
Pediatr Ophthalmol Strabismus 20:72, 1983.)
arrangement of random dots has become available.
One of the stimuli in the Lang II test is perceived
binocularly and serves as a control mark. The
subject can see it also in the absence of stereopsis.
Test results obtained with the older and newer
version of the Lang test have been reported to be
comparable.25 The advantage of the Lang I test is
that it can be performed in children as young as 6
months of age. If the baby stares for a few seconds
at the card one can infer the presence of stereopsis,
following the same reasoning underlying the preferential looking testing technique.
Stereo tests that use random dots are an accurate and established method to measure stereoacuity; however, the results obtained with different
tests will vary widely.21 As stated in Chapter 2,
testing based on random dots exposes the child to
visual demands that are different from and more
difficult than those prevailing under more casual
conditions of seeing. For instance, random-dot
tests contain no information about the shape or
nature of the object hidden in the visual noise of
random dots. Only when the images from the right
and left eye are combined at the neural level
The two-pencil test, though somewhat crude, indicates how well a child is able to cope with a
simple visual-motor task that is at least partially
based on intact stereopsis. The two-pencil test was
popularized by Lang but must have been known
at least 388 years ago (1613) as shown by a sketch
by Peter Paul Rubens to illustrate Aguilonius’
textbook on optics12 (Fig. 15–7). In this illustration, perhaps the oldest one available that shows
the superiority of binocular over monocular vision, the cherub teasingly holds a vertical rod in
front of the scholar who tries to touch the rod
with his index finger from the side while keeping
his left eye closed. He will not accomplish this
task easily, of course, because his stereopsis cannot function with one eye closed, and the three
cherubs anticipate the scholar’s apparent lack of
skill with great merriment.
We agree with Lang16 that the test is better
performed by approaching the rod from above,
since this makes better use of horizontal disparity
detectors and approximates daily manual tasks that
require good stereopsis, such as pouring milk into
a glass or hitting a nail with a hammer. There is
no question that monocular clues to depth perception (see p. 25) also are involved in completing
this test. However, the drastic change of performance when one eye is covered or, for instance,
when a child is fusing through bifocals but has a
manifest deviation when looking through the upper segments suggests that stereopsis must be involved to a large extent in this visual task. The
test is performed as shown in Figure 15–8. Its
threshold values have been estimated to be between 3000 and 5000 seconds of arc, depending
on the subject’s interpupillary distance and arm
length.20
Finally, we must mention recent developments
aimed at testing stereopsis objectively in infants.
With the current emphasis on early diagnosis and
treatment of strabismus, such efforts are of more
than theoretical interest. The principle of such
tests is based on the ability to elicit optokinetic
nystagmus2, 10 or saccadic eye movements22 by
electronically generated stereograms moving back
and forth on a television screen. Although such
methods are still largely confined to the laboratory,
Examination of the Patient—V
FIGURE 15–6. The Lang test. The child points to the stereoscopic image. (From Lang. J: A new
stereotest. J Pediatr Ophthalmol Strabismus 20-72, 1983.)
FIGURE 15–7. Illustration by Peter Paul Rubens in Aguilonius’ textbook on optics. (From Jaeger W:
Die Illustrationen von Peter Paul Rubens zum Lehrbuch der Optik des Franciscus Aguilonius/1613.
Heidelberg, Verlag Brausdruck, 1976, p 36.)
305
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Introduction to Neuromuscular Anomalies of the Eyes
one would hope that simplified equipment will
eventually become available for use in a clinical
environment.
FIGURE 15–8. The two-pencil test. A, Examiner holds
pencil vertically in front of the patient. The patient’s
task is to touch the upper tip of the examiner’s pencil
with one swift movement from above. B, Patient passes the test with both eyes open. C, Patient fails the
test with one eye closed (or when both eyes are open
but stereopsis is absent). (From Noorden GK von:
Atlas of Strabismus, ed 4. St Louis, Mosby–Year
Book, 1983.)
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