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136 Theory and Practice of Squint and Orthoptics
the fusional response. In general terms, the field
of binocular fixation is more or less circular with
a radius of about 45–50° from the fixation point
in the primary position, except below when it is
restricted on either side by the nose (Fig. 6.33C).
In a patient with paresis of an extraocular
muscle, it may be helpful to record a patient's
successive fields of binocular fixation on the same
chart, thus making it easy to observe the clinical
cause of the condition. As an example, Fig. 6.29
shows field of binocular fixation before and after
operation (on the same chart) in a patient having
3rd nerve palsy of 2 years duration.
V. BIELSCHOWSKY'S PHENOMENON TEST
This test is performed for confirming the
diagnosis of alternating sursumduction
suspected on alternating cover test. The eye
under cover deviates upwards and extorts. When
the cover is removed, the eye slowly rotates
downward to return to its previous position.
To perform the Bielschowsky phenomenon
test, patient is asked to fixate a spotlight with one
eye and the other eye is covered by an occluder.
It is observed that the eye under cover moves up
and extorts. Then, a filter is held before the
fixating eye, keeping a watch on the eye under
cover, which moves downward and intorts. This
influence of changing the light stimulus in the
fixating eye on the deviation of the covered eye
is known as the Bielschowsky phenomenon. Its
presence confirms the diagnosis of alternating
sursumduction made on alternate cover test.
VI. BIELSCHOWSKY'S HEAD TILT TEST
This test was originally recommended by
Bielschowsky to differentiate between superior
oblique palsy in one eye and superior rectus palsy
in the contralateral side. The modified three-step
technique of Bielschowsky's head tilt test is used
for the diagnosis of paretic vertical recti and
oblique muscles. For details, see pages 307–311.
SENSORY EVALUATION: ASSESSMENT FOR
BINOCULAR CO-OPERATION AND
SENSORY ANOMALIES
Normal binocular single vision consists of three
grades: Simultaneous perception, fusion and
stereopsis. It is maintained with central fixation
and normal retinal correspondence.
There are variety of sensory adoptations that
occur in response to clinical situations that
disrupt binocular vision. The development of a
specific type of sensory adaptation depends on
when (age of the patients) the sensory anomaly
occurred and the severity and type of binocular
disruption.
Visually mature patients may develop
following sensory adaptations:
• Diplopia,
• Confusion, or
• Rivalry
Visually immature patients may develop
following sensory adaptations:
• Monofixation syndrome,
• Anomalous retinal correspondence (ARC), or
• Large regional suppression
Amblyopia, not actually a sensory adaptation
may occur as a consequence of suppression.
Sensory adaptations and amblyopia are
described in details on pages 179–213.
Tests for binocular co-operation and sensory
anomalies are as given below.
Fig. 6.29 Field of binocular fixation before and after ex-
traocular muscles surgery in a patient with third nerve palsy.
A. TESTS FOR FIXATION BEHAVIOUR
Fixation behaviour should be tested in each
patient with strabismus having vision less than
Fig. 6.30 Types of fixation.
6/6 Snellen's. It can be tested with the help of a
visuscope (see page 141) or fixation star of the
ophthalmoscope. Patient is asked to cover one
eye and fix the star with the other eye. Fixation
may be centric (normal on the fovea) or eccentric
(which may be unsteady, parafoveal, paramacular, centrocaecal, paracaecal or temporal;
Fig. 6.30). The preliminary checking of fixation
should be done without dilating the pupil, since
this would be an obstacle to the pursuit of the
rest of the diagnostic tests. However, in the end,
pupils should be dilated and fixation test
repeated along with the detailed fundus
examination.
A steady central foveal fixation is a good
prognostic sign. An unsteady but central foveal
fixation indicates a possibility of good vision
with conventional occlusion while a steady
paramacular or peripheral eccentric fixation
indicates a poor prognosis.
B. TESTS FOR THE STATE OF
RETINAL CORRESPONDENCE
Assessment for the state of retinal correspondence is necessary only in the presence of a
constant manifest deviation. It is absolutely
essential to know the state of monocular fixation,
whether it is eccentric or central, so that this can
be taken into account, when evaluating the
results of the various tests.
In the absence of normal retinal correspondence (NRC), a patient with strabismus may
develop anomalous retinal correspondence
(ARC). ARC is an unstable secondary adaptation
of sensory interaction between the two eyes that
has developed under conditions of everyday
stimulation and exists under these conditions.
137Evaluation of a Case of Strabismus and Orthoptic Instruments
The tests employed to evaluate state of
retinal correspondence are described here in
decreasing order of their similarity to normal
circumstances.
1. Striated glass test (Bagolini test)
This test, performed with the Bagolini striated
glasses, is closest to everyday visual conditions.
The eyes are not dissociated during the test and
can be observed by the examiner.
Bagolini's striated glasses (sometimes referred
to as lenses) are in fact glass plates without
refractive power. The glass plates contain
extremely fine parallel striations on the surface.
When looking through them, a spotlight appears
as a fine streak of light perpendicular to the
striations. The principle basically is the same as
for the Maddox rod except that the patient can
actually see through Bagolini's glasses. The
glasses are mounted so that they can be inserted
into a trial frame. Marks on the glass indicate
the direction of the streak seen by the patient.
Procedure to perform the test (Fig. 6.31).
Preferably the test should be performed in a
room with subdued light. The test is performed
for distance (6 metres) as well as near (33 cm).
Patient is instructed to fixate on a spotlight. The
striated glasses are placed in a trial frame with
their axis oriented respectively at 45° and 135°,
so that a normal subject would see two streaks
of light forming a × intersecting at the spotlight
(Fig. 6.31A). In a patient with strabismus, one
of the following observations may be made:
1. A patient with a constant tropia having
normal retinal correspondence (NRC) with no
demonstrable suppression will experience
diplopia, i.e. will see two spotlights each one
crossed by one streak of light (Fig. 6.31B).
According to the deviation, they will be seen
either in crossed or in uncrossed diplopia.
2. In the presence of suppression of one eye, the
patient will see the spotlight crossed by the line
in front of the non-suppressing eye only
(Fig. 6.31C).
3. A patient with harmonious anomalous retinal
correspondence will see a perfect cross, as seen
by a normal person (Fig. 6.31A), but the cover
test will show the presence of a tropia.
4. Two streaks, but only one crossing through
the centre of the light (the other one being
138 Theory and Practice of Squint and Orthoptics
displaced away from the light with a portion of
it missing), indicate a suppression area with
either normal retinal correspondence or
unharmonious ARC (Fig. 6.31D).
5. In the presence of a small angle tropia, if the
patient sees a perfect cross as seen by a normal
person (Fig. 6.31A), and there is no movement
on cover test, NRC is indicated (although there
Fig. 6.31 Bagolini's striated glass test (for explanation see text).
139Evaluation of a Case of Strabismus and Orthoptic Instruments
may be lack of bifoveal fixation). Parks records
such cases as having 'unknown retinal
correspondence'.
Advantages of Bagolini's test
• This test is closest to the everyday visual
conditions, i.e. there is minimal interference
with normal visual condition since the patient
can see with both eyes.
• It is a simple and easy test both for the patient
and the examiner. Even a child can describe
exactly what he sees.
• The test can be performed for any fixation
distance.
• Since the eyes are not dissociated during the
test, these can be observed by the examiner.
Disadvantages
• The test is only qualitative since the angle of
anomaly cannot be measured.
• Small angles of anomaly may be over-
looked.
2. Diplopia test
To perform this test, patient's deviation is first
determined objectively and the diplopia test is
then performed under the same conditions (i.e.
same fixation distance and refractive correction)
to permit comparison. In the diplopia test, the
patient fixates a spotlight on the centre of a
tangent scale through a red filter and the
deviating eye is uncovered. To begin with, each
eye is covered alternately, so as to show him, that
the fixation light and the tangent scale or screen
is seen with one eye and red spot of light with
the other eye. When both eyes are uncovered, the
patient may see one or two lights as follows:
• When the patient sees one red light and one
white light, it indicates either normal retinal
correspondence or unharmonious ARC (if the
separation of the images is not compatible
with the angle of deviation).
• When the patient sees only one red light, it
indicates suppression of the deviating eye.
• When the patient sees a mixture of red and
white or a light red light, it indicates
probability of harmonious ARC.
Advantages
The test is very simple and can be performed in
children of average intelligence who are as
young as 4 years of age.
Disadvantages
It is difficult to differentiate between fusion of
the images and suppression of the deviating eye,
since even in binocular vision, the fixating eye
will be dominant and the image will tend to
appear red.
3. Prism bar and red filter test
To perform this test, patient is asked to fixate a
spotlight at 6 metres distance and a prism bar
cover test is carried out with prism bar in front
of the fixating eye, till the deviation is neutralized. The prism bar reading at this point equals
the objective angle of squint. A red filter is then
placed in front of the deviating eye and the
patient is asked to describe what he/she sees.
The various possibilities are as below:
1. Patient may suppress one eye, i.e. he/she does
not see red light or a mixture of red and white
light. This makes the test useless.
2.In the presence of normal retinal correspondence (i.e. when the foveae have a
common visual direction), the patient may see
the light as a blend of red and white.
3. In the presence of ARC, i.e. when the foveae
have different visual directions, the patient
will see two lights, a white and a red one. In a
patient with esodeviation, the diplopia will be
crossed and with exodeviation uncrossed
(paradoxical diplopia).
4.To measure the angle of anomaly, in the
presence of ARC, the prism bar is now moved
slowly (decreasing the base-out strength for
esodeviations or the base-in strength in
exodeviations) until the diplopia disappears
or until the type of diplopia is reversed. The
prism bar value at this point equals the
subjective angle of squint. The difference
between the objective and subjective angles
represents the angle of anomaly.
4. Synoptophore test
To detect ARC by synoptophore method,
objective and subjective angles of the squint are
measured using dissimilar slides (e.g. lion and
the cage) as described on page 118, respectively,
and the results are interpreted as below:
1. If the objective and subjective angles of the
squint coincide, normal retinal correspondence
(NRC) is present.
140 Theory and Practice of Squint and Orthoptics
2. If the objective angle is greater than subjective
angle, the anomalous retinal correspondence
(ARC) is present; and the difference between
these angles is called the angle of anomaly, when
the angle of anomaly is equal to the objective
angle, i.e. when subjective angle is zero, the ARC
is harmonious. In unharmonious ARC, angle of
anomaly is smaller than the objective angle.
5. Worth's four-dot test
For this test, patient wears red-green goggles
with red lens in front of the right eye and green
lens in front of the left eye and views a box with
four lights—one red, two green and one white
(Fig. 6.32A). Since the lights are of the colours
complementary to those of the filters before the
patient's eye, he/she can see the red light only
through the red filter and the two green lights
only through the green filter. The white light
can be seen with both eyes.
Depending upon the patient's observation, the
results are interpreted as below:
1. If the patient sees all the four lights (one red,
two green and one white or red or green or
mixture of red and green) in the absence of
manifest squint, he/she has normal binocular
single vision (Fig. 6.32A).
2. With abnormal retinal correspondence (ARC),
patient sees all the four lights as above even
in the presence of a manifest squint
(Fig. 6.32B).
3. If the patient sees only two red lights, he/she
has left suppression (Fig. 6.32C).
4. If the patient sees only three green lights, he/
she has right suppression (Fig. 6.32D).
5. When the patient sees three green lights and
two red lights alternately, it indicates presence
of alternating suppression.
6.If the patient sees five lights (2 red and 3
green), he has diplopia (Fig. 6.32E).
6. Bielschowsky’s after image test
In this test, patient's right fovea is stimulated
with a vertical bright light and left fovea with a
horizontal bright light (Fig. 6.33A) for 15 seconds
each and the patient is asked to draw the
position of after images. Perception of the after
images is easiest, when the patient closes his/
her eyes or when he/she looks at a blank screen.
Fig. 6.32 Worth's four-dot test.
The results are interpreted as below:
1. A patient with normal retinal correspondence will draw a cross (Fig. 6.33B).
2. A right esotropic patient with ARC will draw
vertical image to the left of horizontal image
(Fig. 6.33C).
3. A right exotropic patient with ARC will draw
vertical image to the right of horizontal
(Fig. 6.33D).
4. A response showing the images in a crossed
position in exotropia (Fig. 6.33E) and in an
uncrossed position in esotropia (Fig. 6.33F)
indicates the presence of paradoxic diplopia
in the presence of ARC with eccentric fixation.
5. The patient may draw only vertical image (in
left suppression) or only horizontal image (in
right suppression). In alternate suppression,
patient sees vertical and horizontal lines
alternately.
Disadvantages
1. The after image test is the most unphysiologic
of all the tests for ARC, since an after image
and a normal visual stimulation are so
different that they cannot even be compared.
141Evaluation of a Case of Strabismus and Orthoptic Instruments
Fig. 6.33 Bielschowsky's after image test (for explanation, see text).
2. Small children do not understand what they
should observe.
7. Cupper's binocular visuscope test
In this test, the patient sits 5 metres away from
a Maddox scale and is asked to fixate the light
on the centre of scale with fixing eye and the
examiner looks the images of the visuscope on
the retina of patient's deviated eye. Since it may
be difficult for the examiner to look through the
visuscope without blocking the patient's view
of the fixation object, i.e. the patient is asked to
fixate through a plane mirror or prism (which
changes the direction of fixation) (Fig. 6.34A).
The examiner projects the star of the visuscope
on the patient's fovea and asks the patient to
142 Theory and Practice of Squint and Orthoptics
tell its position on the Maddox scale in respect
to the central fixation light. The results are
interpreted as below:
1. In the presence of normal retinal correspondence, the patient sees star superimposed on
the fixation light (Fig. 6.34B).
2. In the presence of ARC, patient sees star to
the right or left of the fixation light depending
upon the deviation. The number on the
Maddox scale coinciding with the star gives
the angle of anomaly (Fig. 6.34C).
After the presence of ARC is established, the
examiner moves the visuscope until the star and
the fixation light coincide, and at this point, the
examiner notes the position of the star on the
patient's retina. This peripheral point on the
retina of the patient's deviated eye has acquired
a common visual direction with the fovea of the
dominant eye. This point is not always the same
as the one used for eccentric fixation. In other
words, the angle of anomaly is not always
identical with the distance between the fovea
and the retinal point used for fixation.
Fig. 6.34 Cupper's binocular visuscope test (for explanation, see text).
143Evaluation of a Case of Strabismus and Orthoptic Instruments
Disadvantages
The binocular visuscope test is difficult to
perform with young children.
Evaluation of tests for retinal correspondence
A great disparity between the results of various
tests performed for evaluation of state of retinal
correspondence is reported in the literature. In
general, as stated earlier, the tests that interfere
least with the ordinary conditions of seeing (e.g.
Bagolini's test) show more ARC response and the
tests which cause most dissociating conditions
(e.g. after image test) show less ARC response.
C. ASSESSMENT FOR GRADES OF BINOCULAR
SINGLE VISION
Assessment for grades of binocular single vision
(BSV) is essential, since its achievement is the
ultimate goal in the management of a case with
strabismus. As stated earlier, the three grades of
BSV include simultaneous perception (first
grade), fusion (second grade) and stereopsis
(third grade). Various tests employed to assess
the state of BSV have been described on page 61.
2. Exercises to improve the relative convergence
or relative accommodation.
3. Anti-suppression exercises.
4. Amblyopia therapy.
Working principle of orthoptic instruments
Working of most orthoptic instruments is based
on the fact that they either allow or detect the
dissociation of fusion of binocular vision.
The common modes by which an orthoptic
instrument can cause dissociation of two eyes
are as follows:
1. Use of septum so that each eye sees the
different half of the field, as in Maddox wing,
diploscope, Remy separator, cheiroscope and
pigeon-cantonnet stereoscope.
2. Use of two tubes, one in front of each eye as
in synoptophore.
3. Use of red and green complimentary glasses
one in front of each eye.
4. Use of polaroid glasses.
5. Use of striations as in Bagolini's glasses.
6. Use of cylindrical lenses as in Maddox rod.
D. TESTS TO ASSESS SUPPRESSION AND AMBLYOPIA
See pages 183 and 198.
ORTHOPTIC INSTRUMENTS
GENERAL CONSIDERATIONS
Uses
The orthoptic instruments are required for
diagnostic, therapeutic or both purposes.
A. Diagnostic uses of orthoptic instruments
1. Measurement of angle of deviation (subjective
and objective).
2. Measurement of range of fusion.
3.Measurement of accommodative convergence/accommodation (AC/A) ratio.
4. To know the sensory status of binocular vision
and to detect the sensory anomalies such as
suppression, amblyopia and ARC.
5. To evaluate for stereoacuity.
6.To evaluate the motor status of binocular
vision.
B. Therapeutic uses of orthoptic instruments
1. Exercises to improve the fusional range.
Types of orthoptic instruments
• Conventional, i.e. non-computerised orthoptic
equipment, and
• Computerised orthoptic programs see
page 159.
CONVENTIONAL (NON-COMPUTERISED)
ORTHOPTIC INSTRUMENTS
Like any other branch of science, the science of
orthoptic and strabismus is also advancing and
changing fast. With time, certain instruments
have become obsolete and some have become
less important. For example, even synoptophore
is no more considered an essential equipment
for orthoptic set-up. However, its persence do
adds grace to the orthoptic clinic. Description
of certain instruments which are used only for
diagnostic purposes has been given along with
the diagnostic tests under the evaluation of a
case of strabismus.
A few other important orthoptic instruments
which have not been described elsewhere will
be described in this section. Orthoptic instruments can be grouped as follows:
144 Theory and Practice of Squint and Orthoptics
I. Essential orthoptic instruments
The bare minimum equipment required for the
clinical work-up of a patient with strabismus
are:
1. A refraction trial set with prism of 1–8 D
2. Snellen's vision chart and single letter E-chart.
3. Prism bars, horizontal and vertical (see page
124)
4. Loose prism set
5. Fixation targets, for near and distance
6. Occluders
7. Bagolini's striated lenses (see page 137)
8. Red and green goggles
9. Maddox rods (see page 116)
10. Direct ophthalmoscope
11. Transparent foot ruler
II. Desirable orthoptic instruments
These instruments, when present, add grace and
completeness to the orthoptic clinic. These
include:
1. Synoptophore
2. Random dot stereo test (see page 91)
3. Hess screen (see page 130)
4. RAF rule (see page 125)
5. Worth four dot test (see page 140)
6. Indirect ophthalmoscope
7. Spielman’s occluder
III. Additional orthoptic instruments
There is no limit to additional orthoptic
instruments. Additional orthoptic instruments
can be grouped as below.
Priority additional orthoptic instruments
1. Haidinger brushes and after images
attachment for synoptophore.
2. Teller acuity cards with screen
3. Optokinetic nystagmus drum
4. VER and electronystagmography
5. System perimeter
6. Camera for documentation
Non-priority additional orthoptic instruments
1. Livingston binocular gauge
2. Remy separator
3. Reading bars
4. Cheiroscope
5. Neutral density filters and graded density
bar
6. Maddox wing (see page 118)
IV. Orthoptic instruments not used presently
1. Bishop-Harman diaphragm
2. Stereoscope (Holmes, Keystone)
3. Projectoscope
4. Visuscope
5. Euthyscope
6. Co-ordinator
7. CAM vision stimulator
8. Pigeon-Cantonnet stereoscope
9. Tibbs binocular trainer
10. Diploscope
SYNOPTOPHORE
Synoptophore (major amblyoscope) is a
haploscopic device. Though not an essential
instrument but its presence is most desirable in
an orthoptic clinic. It essentially consists of two
tubes, having a right-angled bend, mounted on
a base having a chin rest and a forehead rest
(Fig. 6.35). Each tube contains a light source for
illumination of slides and a slide carrier at the
outer end, a reflecting mirror at the right-angled
bend and an eyepiece of +6.5D at the inner end
(Fig. 6.36). The two tubes can be converged,
diverged and moved vertically separately or
together by means of knobs. The tubes can also
be adjusted to the patient's interpupillary
distance. Each slide carrier can be rotated to
adjust for any torsion. The horizontal, vertical
and torsional positions of each tube with regard
to normal zero position can be read on scales in
either degrees or prism dioptres.
The graduations from the zero mark inward
represent base-out prisms or degrees of
convergence (+), while those from the zero mark
Fig. 6.35 Synoptophore.
Fig. 6.36 Optical principle of synoptophore.
outward represent base-in prisms or degrees of
divergence (–).
Light switches permit the simultaneous or
alternate illumination of the tubes, useful for
performing the cover tests.
Synoptophore slides
The pair of slides used to perform various
diagnostic and therapeutic purposes include the
following.
1. Simultaneous perception slides. Two dissimilar
slides, such as one having picture of a bird and
the other of the cage, constitute a pair of
simultaneous perception slides (Fig. 6.37A).
Each slide is presented separately to each eye.
Ideally, the pictures should not have
overlapping contour since this will induce
suppression. These slides are graded by their
size into three groups:
145Evaluation of a Case of Strabismus and Orthoptic Instruments
a. Simultaneous foveal perception (SFP) slides. This
pair consists of small sized pictures, the
images of which do not exceed the size of
the fovea.
b. Simultaneous macular perception (SMP)
slides. The pictures in this pair of slides are
slightly larger than those on the SFP slides.
c. Simultaneous paramacular perception (SPP) slides.
These slides have the largest pictures and form
images that extend into paramacular areas.
(Note: As a routine, if possible, the smallest
slides should be used. However, the larger slides
may be required in the presence of suppression
or amblyopia).
2. Fusion slides. Fusion slides consist of two
similar pictures, each of which is incomplete in
one small detail. For example, there are two
rabbits each lacking either a tail or a bunch of
flowers. If fusion is present, one complete rabbit
with tail and holding a bunch of flowers will be
seen (Fig. 6.37B). In the presence of suppression,
either tail or bunch of flowers will be missing in
the respective eye.
Grading.The fusion slides are also graded
according to the size in the same way as the
simultaneous perception slides.
3. Stereoscopic slides. Stereoscopic slides consist
of two pictures of the same object which have
been taken from slightly different angles, i.e. the
picture for one eye is in part dissimilar from that
for the other eye. These dissimilar parts are
imaged on disparate retinal areas in the two eyes
and, when the entire picture is fused, the
Fig. 6.37 Synoptophore slides for simultaneous perception (A), fusion (B) and stereopsis (C).