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116 Theory and Practice of Squint and Orthoptics
Fig. 6.8 Simultaneous cover test: Hirschberg test depict-
ing small esotropia (A). Simultaneous placement of prism
on the esotropic eye and occluder on the fixing eye will
show fixing movement in the esotropic eye when the power
of prism is less (B) and no movement when the power of
prism is equal to the degree of tropia (C).
moved along its arc till the corneal reflexion is
centred in the pupil of the squinting eye. This
point on perimeter gives the angle of manifest
squint in degrees.
This method, used in the past, is not popular
nowadays.
Maddox rod test
It is a subjective test, based on the principle of
diplopia, which can be employed to measure
both heterophoria as well as heterotropia. The
Maddox rod consists of a series of parallel glass
cylinders of higher power (usually of red
colour) set together in a metallic disc
(Fig. 6.9A). The Maddox rod produces a linear
image of a point light. When viewed through
the rod, the line image is formed perpendicular
to the axis of the cylinders.
Measurement of heterotropia
The patient is asked to fix on a point light in the
centre of a Maddox tangent scale (Fig. 6.9B) or
any point light at a distance of 6 metres. The
Maddox rod is placed before one eye with axis
of the rod parallel to the axis of deviation
(Fig. 6.10). Thus, for measuring a horizontal
deviation, the rod is placed in such a way that
the patient sees a vertical line of light
(Fig. 6.10A). Depending upon the type of
deviation, the red vertical line will be seen either
to the right or to the left of fixation light. The
number on Maddox tangent scale where the red
line falls will be the amount of deviation in
degrees. Alternatively, prisms of successively
increasing power (with apex towards the
deviation) are placed in front of the rod until
the patient sees the line passing through the
fixation light. This gives the amount of
heterotropia in prism dioptres. The test should
always be repeated with the Maddox rod in
front of the other eye, so that deviation during
right fixation and left fixation can be compared,
and any discrepancy, if there, can be noted. Such
an endeavour, specially gives information about:
Fig. 6.9 (A) Maddox rod; (B) Maddox tangent scale.
Fig. 6.10 Maddox rod test for horizontal (A) and vertical (B) heterotropia.
117Evaluation of a Case of Strabismus and Orthoptic Instruments
• Primary and secondary deviations in the
presence of paralytic element.
• Any change in retinal correspondence with
the change in fixation.
• Presence of dissociated vertical deviations
may be discovered.
Maddox rod test in conjunction with the
Maddox tangent scale can be performed
successfully in co-operative children as young
as 3 to 4 years of age. Such children should be
asked to go to the scale and put their finger on
the place where they saw the line rather than
asked to tell the number from a distance.
In case of vertical deviation, the Maddox rod
is rotated so that the line is seen horizontally
(Fig. 6.10B) and the deviation is measured
directly from the tangent scale or by using prism
base-up or base-down depending upon the
direction of deviation (apex of prism is kept
towards deviation). To measure a cyclotropia,
the patient is asked to turn the Maddox rod
around the anteroposterior axis, until he/she
has the impression that the line is horizontal.
The amount of cyclotropia in degrees can be
read from the trial frame.
Measurement of heterophoria
To measure heterophoria, the Maddox rod test
is performed exactly in a similar manner as
performed to measure the heterotropia with the
following one exception:
In heterophoria, an occluder is placed before
the Maddox rod and while the patient fixates
the light source with his/her other eye, the
occluder is removed only for a second and the
necessary enquiries are made. After making
necessary adjustments of the prism, the occluder
is again removed for a second. The procedure is
repeated till the patient sees red line and point
light as superimposed.
The use of a cover is necessary in a phoria,
because, if both the fixation light and red line
are seen continuously, there will be a constant
change in the degree of deviation and the red
line will never achieve a steady position relative
to the fixation light.
118 Theory and Practice of Squint and Orthoptics
Limitations of the maddox rod test
1. It can be performed, only if there is no
suppression under the test conditions.
2. The true angle of deviation is measured, only
if the patient has normal retinal correspondence.
3. It is useful only to measure small deviations,
since when large prisms are used, it is
difficult for the patient to see both the red
line and the fixation light, simultaneously.
Maddox wing test
Maddox wing is an instrument (Fig. 6.11) by
which the amount of heterophoria for near (at a
distance of 33 cm) can be measured subjectively.
Like the Maddox rod test, the Maddox wing test
is also based on the basic principle of
dissociation of fusion by dissimilar objects.
The instrument is designed in such a way that
when the patient looks through the eyepiece of
the instrument, the right eye sees a vertical white
arrow and a horizontal red arrow, while the left
eye sees a vertical and horizontal line of
numbers. After a few seconds have elapsed to
allow the eyes to assume the fusion free position,
the patient is asked to tell the number on the
horizontal line to which the vertical white arrow
is pointing (this will give the amount of
horizontal phoria) and the number on the
vertical line at which the red arrow is pointing
(this will measure the vertical phoria). The
cyclophoria is measured by asking the patient
to align the red arrow with the horizontal line
of number (Fig. 6.12).
Fig. 6.12 Maddox wing test.
Advantages
1. Since the numbers can be read, only when
the patient accommodates sufficiently, this
makes the test much more reliable than one
in which the patient fixates a light.
2. Horizontal, vertical and cyclophorias can be
measured simultaneously.
Disadvantages
1. The interpupillary distance is not adjustable.
2. The test is purely subjective, so the examiner
has no objective check.
Synoptophore method
All types of heterophorias and heterotropias can
be measured accurately both objectively and
subjectively with the help of synoptophore. The
synoptophore has been described on page 144.
Techniques of measuring heterophorias and
heterotropias using this instrument are as
follows.
Fig. 6.11 Maddox wing.
Measurement of objective angle of deviation
with synoptophore
The synoptophore is set for the patient's height
and interpupillary distance. Simultaneous
perception slides (e.g. lion and cage) are used.
The patient is asked to look at the pictures, and
the arm controlling the picture in front of the
deviating eye is moved by the examiner until
there is no movement of either eye on a cover
test performed by alternately turning off the
light (Fig. 6.13). The reading on the horizontal
scale in front of the deviating eye as well as the
one of the vertical scale, represents the objective
Fig. 6.13 Measurement of objective angle of deviation with
synoptophore.
angle of deviation. For example, if the arm of
synoptophore in front of the deviating eye is at
20 base out and has to be raised 2, the objective
angle is recorded as 20 esotropia and 2
hypertropia.
Measurement of the objective angle for near fixation.
It can also be made as follows:
To measure the objective angle of deviation
for near, a –3.0D lens is inserted in the lens
holder situated in front of the eyepiece lenses.
In this way, the patient has to exert 3D
accommodation in order to get a clear image of
the slides. In doing so, each eye exerts 3 of
convergence for each dioptre of
accommodation. In other words, 9 of
convergence in one eye or 18 of convergence
in both eyes—considering the interpupillary
distance (IPD) as being 60 mm. For a smaller
IPD, the convergence requirement is less and
for a bigger IPD, it is more (provided the AC/A
ratio is normal). Thus, when recording the angle
of deviation, one must keep this in mind and
either subtract 18 (for esodeviations) from or
add 18 (in case of exodeviations) to the
synoptophore readings. In other words, a
synoptophore reading of 22 base-out should be
recorded as 4 esotropia and a reading of 22
base-in should be recorded as 40 exotropia.
3. Deviation of any size can be measured since
prisms can be used in the lens holder, when
necessary.
4. Measurement of objective angle for near
fixation can also be made.
Disadvantages
The disadvantages of measuring deviation
objectively with synoptophore are as follows.
1. Small children may not co-operate and might
be frightened.
2. Though the instrument is optically arranged
for distance, there is tendency for the patient
to converge as he/she thinks the pictures are
close to him/her. Consequently, esotropias
usually increase and exotropias decrease in
size. Therefore, synoptophore is not being
considered a very reliable instrument to
measure horizontal deviations.
Measurement of subjective angle of deviation
with synoptophore
After measuring the deviation objectively (as
above), the patient is asked to comment on the
position of the pictures used. If the patient claims
superimposition (i.e. the lion seen in the cage)
at his objective angle, this angle is also his
subjective one. If this is not the case, the arms
are moved back to zero and the patient is asked
to move the handle controlling the picture in
front of the non-fixating eye until he/she sees
the two pictures superimposed (Fig. 6.14).
Adjustments can be made for vertical or
torsional separation, if necessary. This is the
subjective angle. At this point, one should by
119Evaluation of a Case of Strabismus and Orthoptic Instruments
Advantages
Advantages of measuring objective angle of the
deviation with synoptophore are as follows:
1. The objective angle can be measured with
either eye fixating and in all cardinal
directions of gaze.
2. It is possible to measure horizontal, vertical,
and torsional deviations fairly accurately.
Fig. 6.14 Measurement of subjective angle of deviation
with synoptophore.
120 Theory and Practice of Squint and Orthoptics
means of rapid alternate flashing, check whether
or not the eyes move, when the patient is asked
to fixate on each picture in turn. This is done
mainly to make sure that an actual change in
the angle between the visual axis has not
occurred, as happens frequently through
relaxing or increasing the accommodative effort
or in cases of a variable angle of deviation.
Problems which may come across while
performing this test are as follows:
1.Suppression may prevent the patient from
superimposing the pictures. In such cases,
simultaneous macular perception or simultaneous paramacular perception slides can be
used. The larger the image formed on the
retina, the less likely it is to be suppressed.
2. The patient may never succeed in putting the
lion in the cage, and it may suddenly be seen
on the other side of the cage (in an uncrossed
or homonymous position in divergent
deviations and in a crossed or heteronymous
position in convergent deviations). In such
cases, the crossing point is considered to be
the subjective angle.
3.It must be realized that the measurement
obtained by the subjective method is only the
true angle of deviation, if normal retinal
correspondence is present.
Measurement of cyclodeviation with
synoptophore
There is no way to carry out an objective
measurement of a cyclodeviation. The subjective
measurement can be performed as follows.
Simultaneous perception slides are used. The
slide with lion is kept in front of the right eye
and that with cage is kept in front of the left eye.
The patient is asked to look at each one in turn
and is asked whether the cage appears level. In
the presence of cyclodeviation, the cage appears
tilted. In incyclotropia, the cage's left-hand side
is seen lower than the right-hand side. This is
corrected by wheel rotating the slide towards
the patient. In the presence of excyclotropia, the
cage's right-hand side appears lower than the
left-hand side. This can be corrected by wheelrotating the slide away from the patient
(towards the examiner). The amount of deviation
is read in degrees from the scale located on the
slide holder of the instrument. It should be
remembered that the tilt of the image is in the
direction opposite to the tilt of the eye.
Double prism test
Double prism test consists of two prisms which
are mounted base to base. It is used to elicit
cyclophorias. To perform this test, the double
prism is placed before one eye in such a manner
that the junction of the two bases intersects the
pupil and is horizontal. Then the patient is asked
to look at a horizontal line against an empty
background which does not offer any fusional
stimuli and inferences drawn are as follows:
• Patient will see two parallel lines with the eye
having double prism in front of it, i.e. one line
displaced above and the other displaced
below with respect to the single line seen by
the other eye.
• In the absence of any cyclophoria, all three
lines will be parallel.
• If a cyclophoria is present, the single line will
have an angle relative to the other two lines
as follows:
– In incyclophoria, the line or lines seen by
the right eye will be tilted towards right and
those seen by left eye will be tilted towards
left.
– In excyclophoria, the line or lines seen by the
right eye will be tilted towards left and those
seen by left eye will be tilted towards right.
Haploscopic tests
Tests based on the haploscopic principle to
measure the deviation include Lancaster redgreen test, Hess and Lees screen tests. These tests
are very useful for measuring incomitant
strabismus in patients with diplopia (see pages
128–133).
ASSESSMENT OF OCULAR MOVEMENTS
ASSESSMENT OF DUCTIONS
1. Duction test. Ductions are monocular
movements and are measured at near distance.
When examining ductions, one eye is covered
and the fellow eye fixates a spotlight which is
moved to bring the fixating eye to the farthest
possible position, in all the cardinal directions
of gaze. For interpretation of the observations,
following methods are in vogue:
i. In most frequent practice, the examiner
observes whether movement lags or is excessive
in any direction. If no lags are noticed, the
ductions are recorded as full; if lags are
noticed, the muscle and the eye involved are
indicated. Usually, a subjective assessment
is made on scale of 7 points (+3 to –3) or
9 points (+4 to –4). Further, a note is also
made of the occurrence of any nystagmoid
movements in the presence of full ductions.
ii. Judging the normalcy of adduction and
abduction in relation to fixed points. Following
useful guidelines have been suggested:
• In maximal adduction, an imaginary
vertical line through the lower lacrimal
punctum should coincide with a boundary line
between the inner one-third and the outer twothirds of the cornea (Fig. 6.15A).
– In excessive adduction, more cornea is
hidden (Fig. 6.15B).
– In defective adduction, more cornea is
visible. Some of the sclera may also be
visible (Fig. 6.15C).
• In maximal abduction, the lateral limbus
touches the outer canthus (Fig. 6.15D).
– In excessive abduction, some of the cornea is
hidden under the outer canthus (Fig. 6.15E).
– In defective abduction, some of the sclera is
visible between the outer canthus and the
limbus (Fig. 6.15F).
2. Kestenbaum's limbus test of motility. The
duction movements are measured with the help
of a transparent ruler as follows:
• Adduction is measured by noting a difference
between the position of the temporal limbus
in primary position and maximum adduction.
• Abduction is measured by noting a difference
between the position of the nasal limbus in
primary position and maximum abduction.
• Similarly, elevation and depression are
measured with respect to inferior limbus and
superior limbus, respectively.
• Normal values reported are:
– Adduction : 10 mm
– Abduction : 10 mm
– Elevation : 5–7 mm
– Depression : 10 mm
121Evaluation of a Case of Strabismus and Orthoptic Instruments
Fig. 6.15 Judgement of adduction (A, B, C) in relation to
lower punctum and abduction (D, E, F) in relation to lateral
canthus. For explanation see text.
3. Subjective perimeter method of measuring
ductions. In this method, to measure the
amplitude of duction movements, the patient's
122 Theory and Practice of Squint and Orthoptics
head is placed into the chin rest of a perimeter
in such a way that the eye to be examined is in
the centre of the perimeter arc or perimeter
hemisphere. The other eye is occluded and the
patient is asked to fixate and follow the
perimeter target that is moved from the centre
of the field to periphery. He/she is instructed
to indicate, when he/she can no longer see the
target. This point indicates the limit of the
duction movement in that particular direction.
Normal values reported by this method are:
– Adduction : 50°
– Abduction : 50°
– Depression : 50°
– Elevation : 40°
4. Objective perimeter method or corneal
reflex method of measuring ductions. The
amplitude of duction movements can be
checked somewhat more objectively by using
corneal light reflex. In this method, after closing
one eye, patient is asked to turn his/her eye
maximally in a given direction. Then the
examiner moves a small flash light along the arc
of the perimeter until the reflex from the
patient's cornea appears to be centred in the
pupil. The examiner views it with one eye from
the position of flash light. This point gives the
limit of the particular duction movement.
Note: It is important to be aware of the fact
that, in practice, the measurement of ductions
is not of much value in the investigation of
strabismus, since only a small fraction of the
fibres of a muscle need to function in order to
rotate the eye to the limits of its field of duction.
A defect in the amplitude of duction occurs, only
when almost complete paresis of a muscle
occurs. Therefore, a partial paresis usually
cannot be diagnosed on testing ductions.
ASSESSMENT OF VERSIONS
In general, study of versions is more important
factor than the study of ductions, when deciding
on which muscle or muscles to operate.
Further, the investigation of versions is of
greatest importance in patients with noncomitant strabismus, because comparison of the
extent of movement of the two eyes relative to
each other during a version is the most sensitive
test to detect underfunction of a muscle.
1. Version test
It is performed at approximately 15 inches. The
patient is asked to hold his head straight and
still and to make eye movements on command
or to follow a fixation light in all the cardinal
directions of gaze. The fixation light should be
kept at such a distance that one can always
observe the corneal reflections in both eyes. The
following observations should be made on
version test.
• For excessive or defective movements in any
direction.
• To detect underaction of one muscle and
overaction of its contralateral synergist.
• To detect overaction of one muscle without
underaction of its contralateral synergist.
• To note any retraction of the globe and
narrowing of palpebral fissure in certain
direction of gaze (as seen in Duane's retraction
syndrome).
• To detect the overaction of inferior and
superior obliques.
Clinically, the overaction of oblique muscles
can be graded by following methods:
i. Depending upon the vertical deviations, the
overactions of obliques is graded as:
a. Mild overaction—when vertical deviation
(e.g. hypertropia in inferior oblique
overaction) is appreciated only in
sursumadduction.
b. Moderate overaction—when vertical
deviation is appreciable on adduction
itself.
c. Severe overaction—when hypertropia is
seen in primary position.
ii. Depending on the angle, the adducting eye
makes with the horizontal line as it elevates
and abducts (if overacting) on lateral version
to the opposite side, the overaction of inferior
oblique is graded as shown in Fig. 6.16.
Similarly, the overaction of superior oblique
also can be graded by observing the angle the
adducting eye makes with the horizontal line
as it depresses and abducts.
2. Perimeteric method of measuring versions
The amplitude of versions can be measured on
the perimeter in the same way as ductions except
Fig. 6.16 Grading of inferior oblique overaction depending
on the angle adducting eye makes with horizontal line.
that the patient fixates and follows the test object
with both eyes until he/she sees it double or
until it moves too far out for him/her to follow.
In general, such a measurement of the
absolute amplitude of versions is of little
practical value.
Quantitative methods of
assessing version eye movements
Quantitative methods for assessing version eye
movements involve measuring the range, speed,
and accuracy of eye movements in various
directions of gaze. These assessments are
important for diagnosing and monitoring eye
movement disorders, neurologic conditions, and
evaluating the function of the extraocular
muscles. Here are several quantitative methods
used to assess version eye movements:
123Evaluation of a Case of Strabismus and Orthoptic Instruments
1. Saccade testing: Saccades are rapid, voluntary
eye movements that allow us to shift our gaze
from one point to another. Quantitative
assessment of saccades involves measuring
parameters such as:
• Saccade latency: The time it takes for a saccade
to initiate after a visual stimulus.
• Saccade velocity: The speed at which the eye
moves during a saccade.
• Saccade accuracy: The ability of the eye to
accurately land on a target.
2. Smooth pursuit testing: Smooth pursuit eye
movements are slow, tracking movements that
help maintain a moving target on the fovea
(central part of the retina). Quantitative
assessment includes:
• Gain: The ability of the eyes to maintain a
stable and accurate fixation on a moving
target. It is often calculated as the ratio of eye
velocity to target velocity.
• Latency: The delay in initiating smooth pursuit
after the onset of a moving target.
• Directional error: Any deviation from the ideal
tracking path.
3. Optokinetic nystagmus (OKN) testing: OKN
is a reflexive eye movement elicited by large
moving visual stimuli (e.g. stripes). Quantitative
assessment may include measuring:
• OKN slow-phase velocity: The steady tracking
movement of the eyes following a moving
stimulus.
• OKN fast-phase velocity: The rapid, resetting
movements (nystagmus) that occur when the
eyes reach the limits of their range.
MEASUREMENT OF VERGENCES
The status of motor fusion is assessed by
measuring the vergences, i.e. the fusional
amplitudes. In the presence of heterophoria or
an intermittent heterotropia, the fusional
amplitudes can be measured both by the prism
method or synoptophore method. While, in a
patient with heterotropia, only the synoptophore method is useful, since fusion in casual
gaze is necessary for testing with prism method.
Fusional divergence is measured from the
subject's phoria position, whereas relative
divergence is measured from the position of
fusional demand, i.e. the orthoposition.
124 Theory and Practice of Squint and Orthoptics
Testing of fusional amplitudes with prism method
The test can be performed using a prism bar or
a rotatory prism (Risley prism) or single prism.
A prism bar consists of a series of prisms of
increasing strength (Fig. 6.17). It is held by the
examiner in front of one of the patient's eyes and
merely needs to be moved higher or lower to
bring a stronger or weaker prism into the line
of sight. Amplitudes of divergence are measured
first and those of convergence second.
Measurement of amplitude of divergence
To perform the test for distance, patient is asked
to fixate the 6/12 symbol at 6 metres and the
prism bar is used with the prism base directed in
BI in front of the one eye (preferably the nondominant one). By progressively increasing the
amount of base in prism power, the eyes are
diverged to the limit of bifoveal single vision, i.e.
up to the point, when the patient just appreciates
diplopia. This point is the end point of the test
and is called the break point. Its reading is
recorded. At this point, the power of the prism is
decreased slowly, until he/she again fuses. This
point called as the recovery point is also noted.
To measure the amplitude of divergence for
near, the above test is repeated at 33 cm. The
end point and the recovery point are recorded.
But unlike the test for distance, the end point
for the near test is blur point, i.e. the maximum
amount of base-in (BI) prism power after which
the patient's vision is blurred. The mechanism
of blurring of vision is as follows:
The retinal disparity produced by the use of
base-in prism evokes fusional divergence that
maintains bifoveal single vision until its
amplitude is exhausted. At this point, the
patient, who is accommodating during near
vision, can produce further divergence and
A
maintain single vision longer, if he/she relaxes
his/her accommodation, because this simultaneously decreases the amount of accommodative, convergence present. But, due to
relaxation of accommodation, the near object
becomes blurred.
Measurement of amplitude of convergence
To perform the test for near, patient is asked to
fixate 6/12 symbol at 33 cm and the bar is used
with prism base directed out (BO). By
progressively increasing the amount of BO
prism power, the eyes are converged to the limit
of bifoveal single vision, i.e. up to the point,
when the patient just appreciates diplopia. This
point is the end point, of the test and is called
the break point. Its reading is recorded. At this
point, the power of the prism is decreased slowly
until he/she again fuses. This point, called the
recovery point, is also recorded. Theoretically,
before the break point, there will be a blur point
because after the exhaustion of the fusional
convergence patient starts using his/her
accommodative convergence to avoid diplopia.
This, however, can only be done by accommodating in excess of the requirements for the
given distance (pseudomyopia) and
consequently the image is blurred. Therefore, it
is important to record the blur point in order to
know what kind of fusional amplitudes are
measured.
To perform the test for distance, the same
procedure is repeated at 6 m and the blur point,
break point and recovery point are recorded.
An example of a recording of fusional
amplitudes as tested with the prism bar:
Distance :Diverged to 12 BI/recovered at 9 BI
Converged to 32 BO/recovered at 21
BO
Blurred at 12 BO
Near : Diverged to 14 BI/recovered at 9 BI
Converged to 36 BO/recovered at 24
BO
Blurred at 18 BO
Fig. 6.17 Prism bars, horizontal (A) and vertical (B).
B
Synoptophore method of measuring
fusional amplitudes
To begin with, the objective angle of deviation
is determined using simultaneous macular
perception slides. Then, the second-grade fusion
slides (similar targets with control marks for
each eye) are introduced and if the patient fuses
these targets and sees them as one with both
control marks, the examiner blocks the arms at
the objective angle. Then, first the amplitude of
divergence and second the amplitude of
convergence are measured as below.
To measure the divergence, the arms of the
synoptophore are slowly diverged and the
patient is instructed to report occurrence of
diplopia or the disappearance of one or the other
control mark of the picture (suppression). This
point—the break point—is recorded and the arms
of the synoptophore are slowly converged (i.e.
brought to less divergent position) and the
recovery point, where fusion occurs, is noted.
To measure the convergence, the arms of the
synoptophore are further converged slowly till
the fusion breaks and the break point is noted.
Then, the arms are moved back into a less
convergent position until fusion is regained and
the recovery point is noted.
To measure the amplitude of vergences for near with
synoptophore, a –3.0 DS lens is placed before each
eye. In order to see clearly with –3.0DS lens, the
subject has to overcome these lenses by
accommodating as if he/she was fixating an
object at a distance of 33 cm. To simulate the
orthoposition for near fixation, the synoptophore
tubes have to be set according to the convergence
requirement for a point 33 cm distant which, in
prism dioptres, is three times the patient's
interpupillary distance in centimetres. The
procedure of testing for near is the same as for
distance.
Normal values of vergences are as follows:
Vergence Distance (6 m) Near (33 cm)
Convergence 14–20
Divergence 5–8
Vertical vergence 2–4
Incyclovergence 10–12° 10–12°
Excyclovergence 10–12° 10–12°
35–40
15–20
2–4
An example of recording of fusional amplitudes as
tested with the synoptophore:
Distance:
• 30
ET, objectively and subjectively.
• First- and second-grade fusion at angle.
125Evaluation of a Case of Strabismus and Orthoptic Instruments
• Convergence to 42
• Divergence to 12
BO/recovery at 32
BO/recovery at 20
BO.
BO.
Near (with –3.0 D):
• 44 ET objectively and subjectively
• First- and second-grade fusion at angle.
• Convergence to 56
BO/recovery at 44
BO.
• No divergence past angle, suppression OD.
Measurement of near point of convergence
The near point of convergence (NPC) is the
closest point at which an object can be seen
single during bifoveal vision. In other words, it
is the point at which the two foveal lines of sight
intersect, when maximum convergence is
exerted.
The NPC practically measures all types of
convergence; since an object actually approaches
the eyes during testing. That is, the test for NPC
simultaneously stimulates fusional, accommodative and proximal convergence and during
the last phase, if the patient is co-operative, there
will be a strong voluntary effort to converge.
Instruments. Near point of convergence can be
measured simply with the help of a graded
plastic rule placed at the outer canthus and a
fixation target (e.g. tip of a sharp pencil) moved
towards the eye; or by use of specially designed
rule such as RAF rule (Fig. 6.18), Livingstone
binocular gauge (described on page 147) and
Prince rule. These specially designed
instruments basically consist of a bar or rule
made from plastic, metal or wood on which a
rider with the test chart can be moved back and
forth (fixation target). At one end of the bar is a
wing-like support that fits over the nose and
rests against the lower orbital margins during
the measurement. In Prince rule, the bar is 24
inches long and 1/2 inch square that has
different markings on each of its four sides. One
side is divided into centimetres (to be used for
measurement of NPC and NPA), the second one
Fig. 6.18 RAF rule.