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5.6.3 Analysis andEvaluation
By interpreting the patterns observed through
the polariscope, optical professionals can assess
the internal stress level in different materials. For
example, if this is present toward the edges of
the lens, it indicates that the lenses are t too
tight, and if not modied, the lenses may have
warpage and may chip off. If it is in the center, it
indicates that the lens is polycarbonate. Patients
may not face any visual disturbances, but these
affect the mechanics and, in severe cases, inuence the quality of vision. Hence, this information is crucial for lens selection, frame tting,
and ensuring the quality and durability of optical
products.
5.7 Corneal Reex
Pupillometer—Measuring
Pupillary Distance
S. Maseedupalli
The corneal reex pupillometer is a specialized
tool to accurately measure pupillary distance
(PD) or interpupillary distance (IPD) (Fig. 5.7)
[5]. The corneal reex pupillometer utilizes the
reection of light from the cornea to determine
the PD. It comprises a handheld device with a
light source and a viewing lens. This measurement is essential for the precise tting of spectacles and for ensuring optimal visual comfort for
patients. Ill-tting spectacles can result in asthenopia, blurry or distorted vision, and visual discomfort and can inuence binocular vision. The
eyes must work harder to adjust to the incorrect
optical alignment.
PD is the distance between the center of the
pupil and the median plane.
IPD is the distance between the two pupil
centers.
5.7.1 Measuring Pupillary Distance
(PD)
Step 1: Preparation: ensure that the patient is
comfortable and relaxed and looking straight
Fig. 5.7 A corneal reex pupillometer or PD meter
ahead. Set the pupillometer to measure the PD
for an innite distance.
Step 2: Positioning: hold the pupillometer
with the headrest on the patient’s face. Align the
patient’s eyes with the viewing lens of the
pupillometer.
Step 3: Activating the light source: activate the
light source of the pupillometer, which will create reections on the patient’s corneas.
Step 4: Identifying the pupil centers: direct the
patient to focus on a distant target or a specic
point. Observe the reections of the corneal light
on each eye through the viewing lens. Identify
the centers of the pupils based on the position of
the corneal reections.
Step 5: Measuring PD: move the pupillometer
to align the measurement markers with the identied pupil centers. Read the displayed value on
the pupillometer, which represents the PD.
The PD measures can also be done quickly in
the clinic without a pupillometer.
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Step 1: The examiner and the patient should
sit at the same eye level.
Step 2: The examiner must be equipped with a
simple ruler and a torch for measurement.
Step 3: The examiner closes one of their eyes.
For example, let’s say the examiner closes right
eye.
Step 4: The patient is made to look at the eye
that is kept open, which in this case is the examiner’s left eye.
Step 5: The light from the torch is directed
into the patient’s eye for measurement.
Step 6: The zero mark of the ruler is positioned to the patient’s right eye corneal reex
(First Purkinje’s image).
Step 7: The value on the ruler that aligns with
the median plane of the patient’s face is read and
noted [6]. This reading provides the right eye PD
(RE PD) of the patient.
Step 8: Now, the examiner closes left eye and
opens right eye.
Step 9: The patient should look into the examiner’s right eye.
Step 10: The ruler is read while the zero mark
is aligned to the median plane of the patient’s
face, this time at the corneal reex of the patient’s
left eye.
Step 11: This reading provides the left eye PD
(LE PD). The torchlight must be shifted from the
patient’s right eye to their left eye.
Step 12: The measurements for both RE PD
and LE PD have been obtained.
Step 13: To nd the Interpupillary Distance
(IPD), the RE PD and LE PD are added.
Step 14: Note that providing individual eye
PD measurements (RE PD and LE PD) is more
accurate than providing the IPD.
Step 15: The logic behind this method is that
the patient’s right eye and left eye align parallel
to the examiner’s open (opposite) eye, simulating
distance viewing.
Step 16: Generally, subtracting approximately
3 mm from the distance IPD approximates the
near IPD [6].
5.8 Conclusion
Each of the devices mentioned in this chapter is
important for accurate optical dispensing. By
being meticulous and using these devices correctly, one could ensure spectacles t well and
provide clear vision.
Funding Hyderabad Eye Research Foundation,
Hyderabad, India.
Disclosure None.
References
1. Brooks CW. System for ophthalmic dispensing. 3rd
ed. Butterworth-Heinemann; 2007.
2. Fannin TE, Grosvenor TP. Clinical optics.
Butterworths; 1987.
3. Alonso J, Gómez-Pedrero JA, Quiroga JA. Modern
ophthalmic optics. Cambridge University Press; 2019.
4. Manchikanti N, Maseedupalli S. Birefringence in
ophthalmic lenses. Indian J Ophthalmol Case Rep.
2021;1:455.
5. Elliott DB. Clinical procedures in primary eye care.
Butterworth Heinemann Elsevier; 2007.
6. Gantz L, Shneor E, Doron R. Agreement and inter-
session repeatability of manual and automatic
interpupillary distance measurements. J Optom.
2021;14:299–314.
Extraocular Muscle Tests
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andCover Tests
6
AparnaRaghuram andBenjaminJastrzembski
6.1 Introduction
Extraocular muscle (EOM) and cover tests are
integral to ophthalmic examinations. These tests
allow the clinician to detect the presence or
absence of strabismus (ocular misalignment).
Additionally, these tests give a wealth of information on both the afferent and efferent function of
the eye and may diagnose cranial nerve palsies and
even systemic diseases such as myasthenia gravis.
6.2 History
Extraocular muscle movement is described by
two eponymous laws: Hering’s law of equal
innervation and Sherrington’s law of reciprocal
innervation. Sherrington’s law states that a muscle’s contraction is accompanied by the relaxation of its antagonist muscle. For example, the
contraction of the medial rectus muscle is accompanied by the relaxation of the lateral rectus muscle in the same eye. Charles Sherrington won the
1932 Nobel Prize in Physiology and Medicine
A. Raghuram
Boston Children’s Hospital/Harvard Medical School,
Boston, MA, USA
e-mail: aparna.raghuam@childrens.harvard.edu
B. Jastrzembski (*)
University of California, Davis,
Sacramento, CA, USA
e-mail: benjast@ucdavis.edu
for his work on this concept, as well as his work
on synapses (a term he coined) and neuronal circuits [1]. Hering’s law of equal innervation states
that neural impulses to an EOM accompany an
equal impulse to its synergistic EOM in the other
eye. In other words, the neural impulse to one
muscle is yoked to its partner muscle in the other
eye. For example, the contraction of the medial
rectus in the right eye is yoked to the contraction
of the lateral rectus in the left eye, causing the
eyes to have a conjugate movement to the left.
The concept was developed in 1868 by the
German scientist Ewald Hering, who also contributed to understanding color vision and respiratory reexes [2]. Although these concepts are
described as “laws” they have exceptions, such as
in Duane’s retraction syndrome.
6.3 Technology
The equipment to perform most EOM and cover
tests in the clinical setting are simple: a xation
target, loose prisms and/or prism bars, and a simple occluder. Additional equipment, including
Maddox rod lenses, red and green color lter
lenses, a synoptophore, and a prism ipper, may
sometimes be used. Maddox rods in trial frames
are used to quantify cyclotorsion. A synoptophore is a mechanical instrument that allows for
the subjective and objective measurement of
strabismus and assessment of retinal correspon-
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_6
67
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A. Raghuram and B. Jastrzembski
dence and stereoacuity [3]. In addition to these
mechanical instruments, software is also available to measure strabismus.
6.4 Technique
6.4.1 Motility andVersions
In general, the rst EOM test to be performed is
the motility test to check for the movements of
both eyes. The patient is asked to follow a target,
such as the examiner’s index nger, in all directions of gaze. Although the task is easily per-
formed by a conscious, cooperative, and
neurotypical patient, cooperation may be aided in
children and those with cognitive impairments by
using a toy to encourage attention. The head may
be gently held to prompt eye movement rather
than head movement in these cases. Normal, full
versions (conjugate movements) of the eyes consist of unimpeded movement in all directions of
gaze, with both eyes moving in the same direction (Fig. 6.1). Under Hering’s Law, a pair of
partner muscles, one from each eye, primarily
drive the movement of the eyes in each of the six
diagnostic positions of gaze (Fig. 6.2). Normal
vergences refer to coordinated eye movement in
Fig. 6.1 The cardinal positions of gaze
Fig. 6.2 The six diagnostic positions of gaze. RSR right
superior rectus, RLR right lateral rectus, RIR right inferior
rectus, LIO left inferior oblique, LMR left medial rectus,
LSO left superior oblique, RIO right inferior oblique,
RMR right medial rectus, RSO right superior oblique, LSR
left superior rectus, LLR left lateral rectus, LIR left inferior rectus
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different directions (disconjugate movements) to
achieve xation on a near or distant target.
If the versions and vergences are full, there is
no indication for further extraocular muscle testing during most eye examinations. Normal versions and vergences typically demonstrate that
the cranial nerves serving the EOMs are functioning as expected, that the brain stem nuclei of
the cranial nerves and the higher level central
nervous system visual processing centers are
intact, that each eye has at least some vision, that
there are no gross abnormalities of the orbit (e.g.,
a large mass or a fracture) restricting movement,
and the EOMs themselves are functioning. Given
the number of processes required for normal
EOM motility, the EOM motility test may be
considered a “vital sign” of the eye examination,
analogous to a heart rate or blood pressure measurement in the general medical examination.
However, on an initial EOM motility test, it
can be easy to miss subtle underaction or overaction in motility. Notably, Sherrington’s law of
reciprocal innervation dictates that not only is
EOM underaction abnormal, but overaction of an
EOM is abnormal as well. Therefore, in the setting of the strabismus or neuro-ophthalmology
clinic, or when there is concern about an abnormality of the EOMs, additional testing beyond
the fundamental EOM motility test is indicated.
6.4.2 Ductions
A reasonable second test to perform after the testing of EOM versions is the test of EOM ductions
with one eye open and the other eye covered,
especially when abnormalities are detected in the
versions testing. Ductions are dened as the
movement of a single, isolated eye, in contrast to
the versions, which refer to movements with both
eyes open. The terms used to describe the duction
movements of a single eye are supraduction,
infraduction, abduction (away from the nose in
the horizontal), adduction (towards the nose),
incyclotorsion (incycloduction), and excyclotorsion (excycloduction). Each of the six extraocular muscles has a primary action described with
this terminology, and the superior and inferior
muscles have secondary and tertiary actions
(Table6.1).
Duction testing may be useful to highlight
subtle underaction that may be difcult to observe
with version testing, especially when an overaction is the most obvious nding on version testing. For example, a patient with a partial VI
(abducens) cranial nerve palsy of the right eye,
when observed with both eyes open, may show
obvious overaction of adduction in the left eye
due to Hering’s Law and little to no limitation in
abduction of the right eye. With the right eye
closed, ductions of the left eye will be normal,
and with the left eye closed, ductions of the right
eye may demonstrate a subtle abduction decit.
6.4.3 Ocular Alignment
Abnormalities in ocular alignment are categorized as heterotropia (tropia) or heterophoria
(phoria). A tropia is a misalignment of the eye
that occurs without disassociation and may be
observed with both eyes open. It may be constant
or intermittent. A phoria is a latent misalignment
observed only after breaking fusion between the
eyes. Observation of a phoria is most commonly
observed by covering an eye to break fusion. In
Table 6.1 Actions of the extraocular muscles
Muscle Primary action Secondary action Tertiary action
Medial rectus Adduction – –
Lateral rectus Abduction – –
Superior rectus Elevation Intorsion Adduction
Inferior rectus Depression Extorsion Adduction
Superior oblique Intorsion Depression Abduction
Inferior oblique Extorsion Elevation Abduction
Adapted from Table1.1 of Rowe (2012) [3]
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other words, the phoria is observed when the
brain does not have normal afferent input from
the eyes to maintain ocular alignment.
Clinically, the terms heterotropia or heterophoria are infrequently used. Rather, the strabismus is named based on the direction of
misalignment. An esodeviation, either an esophoria or esotropia, is dened as a deviation toward
the nose; an exodeviation, either an exophoria or
exotropia, is dened as a deviation away from the
nose; a hypertropia or hyperphoria as a deviation
upwards or cranial; and a hypotropia or hypophoria as a deviation downwards or caudal.
Given that xation depends on vision, all tests
that measure ocular alignment should be performed with the patient’s best possible correction
in place.
6.4.3.1 Tests Based ontheLight Reex
The next tests to perform are the Hirschberg and
Krimsky tests, which estimate the patient’s strabismus, if present. In the Hirschberg test, a penlight is directed at the patient’s open eyes from a
distance of about 1/3 of a meter, and the clinician
characterizes any deviation of the corneal light
reex to estimate the strabismus. Alternatively, a
ash photograph may provide analogous information. Many eyes with normal alignment may
appear slightly exotropic on Hirschberg testing
due to a small nasal displacement of the corneal
light reex. This normal, slightly exotropic
appearance is due to the temporal location of the
fovea in relation to the center of the pupil; it is
referred to as a positive angle kappa. In these
cases, cover testing will demonstrate normal
alignment—also called orthophoria. In the
Krimsky test, a penlight is shone at the patient’s
open eyes from a distance of about 1/3 of a meter.
A loose prism or prism bar is placed over the
deviated eye to center the corneal reex. If the
prism is placed over the xating eye to center the
deviating eye’s corneal reex, it is called the
Modied Krimsky test. Using the dimmest light
possible is advisable as a bright penlight may be
dissociative for some people. Since the
Hirschberg and Krimsky tests do not interfere
with xation, they only measure tropia, not
phoria.
6.4.3.2 Cover Tests
Cover testing provides a more accurate characterization of strabismus than Hirschberg and
Krimsky testing. However, eyes with the best
corrected visual acuity worse than 20/200 will
not be able to xate on a target reliably enough to
make cover testing useful. In these situations, the
Hirschberg and Krimsky tests must be relied on.
The patient must also cooperate with cover testing by actively xating on the target of interest. It
is important for the examiner to encourage the
patient to actively xate on the object and be
aware that poor cooperation may lead to misleading results, especially in children who may nd
cooperation challenging.
The cover test begins with covering the xating eye or eye with better vision with an occluder
and observing any movement of the other, weaker
eye. The occlusion interrupts afferent input from
the stronger eye, and the brain is forced to switch
xation to the weaker eye. This is the cover test.
The rexation movement of the weaker eye
reveals the direction of strabismus: an observed
movement away from the nose is an esodeviation, and a movement toward the nose is an exodeviation. The speed of rexation reects the
vision in the weaker eye as well as the attention
of the patient. An eye with poor vision or a patient
with impaired attention or cognition will have
slower rexation. The magnitude of the tropia
may be quantied by adding a plastic prism in the
visual plane over the weaker eye as the occluder
is placed over the stronger, xating eye. The
magnitude of the prism required to eliminate all
rexation movement in the weaker eye quanties
the tropia. This test is called the simultaneous
prism and cover test (SPCT).
If the vision is equal or close to it in both eyes,
the cover test should be repeated with the
occluder placed on each eye. The tests are also
typically repeated in the primary position, with
the eyes xating on a near (1/3 m) or distant (6
m) target and in the secondary directions of gaze
at a distance. If no rexation movement is
observed on the cover test, then no tropia is present, although a phoria may exist.
To quantify a phoria, the prism and alternate
cover test (PACT) is performed. In this test, the
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occluder is alternated between the two eyes, and a
prism is placed over the weaker eye to neutralize
any rexation movement. The prism diopters measured in the PACT test are the sum of the deviation
combining both the phoria and tropia. The
expected normal deviation range for distance is 1
Δ exophoria ±2 Δ and for near 3 Δ exophoria ±3
Δ [4]. If no tropia were observed previously with
the cover test, the measurement would quantify
the phoria only. When the prism is placed over the
stronger eye during the PACT, the measurement is
called the primary deviation. If the prism is instead
placed over the weaker eye, the measurement is
referred to as the secondary deviation. The secondary deviation is always larger than the primary
deviation. The PACT is frequently repeated in
multiple directions of gaze at a distance.
Incomitance in the angle of deviation (i.e., a difference in the measured prism diopters of strabismus
in different directions of gaze) is an important clue
for paretic strabismus secondary to cranial nerve
palsy. For example, a patient with a right VI (abducens) cranial nerve palsy will show an esotropia,
greater in the right gaze than in the left gaze, and
this incomitance will be revealed by testing with
the PACT in multiple directions of gaze.
If the magnitude of the deviation is so large
that one prism is insufcient the prisms may be
stacked in the same direction on a single eyeor a
singleprism may be placedover each eye, which
is preferable. Similarly, if a vertical and horizontal strabismus exists, a horizontal prism may be
placed over one eye and a vertical prism over the
other. In these cases, the use of multiple prisms
should be noted in the documentation. As the
deviation becomes larger, the accuracy of the
measurement decreases.
6.4.4 Vergence Testing
Vergence describes two eyes moving in opposite
directions (disconjugate eye movements).
Detailed vergence testing informs us on how well
the eyes are working together [4–10].
Convergence is dened as the vergence when
both eyes move toward the nose, and divergence
is away from the nose. Supravergence describes
when one eye moves upwards relative to the
other, and infravergence is when one eye moves
downwards relative to the other. Appropriate
refractive correction should be worn before vergence testing begins.
6.4.4.1 Near Point ofConvergence
To assess vergence, start by measuring the maximum ability of the eyes to converge by determining the near point of convergence (NPC). In this
test, an accommodative target size two steps
larger than the maximum resolvable size is recommended (typically, a 20/30 single letter or vertically aligned letters is/are used) (Fig.6.3a). The
target is placed approximately 60 cm from the
eyes and moved at about 1cm per second closer
toward the eyes. The patient is asked to maintain
focus on the target, to keep the target single, and
to report when it splits into two. The distance
from the middle of the forehead to the point when
the target can no longer be perceived as single is
recorded as the NPC “break” in cms. Next, the
target is moved back along the midline to where
fusion is recovered; that is, the target becomes
single. This distance is recorded as “recovery.” A
“break” measure of 6cm or less is normal in children, adolescents, and young adults [7, 8]. It is
important to note that the reference point of measure is from the middle of the forehead (Figs.6.3a,
b and 6.4a). If NPC is measured from the lateral
canthus or tip of the nose as a reference, it cannot
be compared to the 6cm cut-off as it will lead to
an overestimation or underestimation. When
measured from the lateral canthus, the NPC can
be, on average, 1–2 cm more than when measured from the forehead [5]. Variability in human
anatomy and morphology across gender, race,
and ethnicity can also affect the normal range.
Measuring from the tip of the nose should be
avoided due to signicantly higher variability in
nose sizes across the population.
Sometimes, the patient may not report the target doubling due to suppression. In that instance,
the examiner notes when fusion is lost by keenly
observing one eye drifting out. This objectively
measures the “break” (Fig. 6.4b). When the
patient is unable to report a doubling of the target
due to strong suppression, the test can be repeated
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e
Fig. 6.3 Tools for measuring the near point of convergence. (a) A near xation stick with a vertical row of letters; (b) a near point rule to place between forehead and
Fig. 6.4 Measuring the near point of convergence. (a) An example of how the test is performed; (b) an objective mea-
sure (red arrow) of when fusion is broken is noted as a break on the “near point” of convergence testing
with a red lens (Fig.6.3c) placed in front of one
eye and room lights dimmed or switched off. A
penlight (Fig. 6.3d) is moved toward the eyes,
measure distance; (c) a red lens; (d) a transillumination
light; (e) a pair of red and green glasses
the test results improve with the +1.00 DS lenses,
this suggests that the accommodative system was
the driver of the convergence decit.
and the patient is asked to report when they see
two separate lights, one red and one yellow. A
red/green glass (Fig. 6.3e) can also be used
instead of just a red lens, and the patient would be
asked to report when they see the light split into
red and green lights [7]. If convergence is limited
due to poor accommodation, the test can be
repeated with a +1.00 DS lens on a trial frame or
held over the patient’s glasses, and the NPC test
can be repeated with an accommodative target. If
6.4.4.2 Fusional Vergence Ranges
Next, we measure fusional vergence ranges,
which estimate how much the eyes can move
horizontally (converge and diverge) and vertically (move up or down) while maintaining single vision. Prism bars are introduced while the
patient looks at a 20/40 or 20/30 letter size target
for distance and near vision testing, respectively.
Vertically aligned letters of the same letter sizes
ab c
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Fig. 6.5 Fusional
ranges testing: (a) the
xation target used for
testing; (b) the
horizontal and vertical
prism bars used for
testing; (c) a
demonstration of the
fusional divegence range
limit; (d) a
demonstration of the
fusional convergence
range limit
73
d
(Fig.6.5a) for horizontal ranges and horizontally
aligned letters for vertical ranges or a single letter
are ideal targets.
6.4.4.2.1 Horizontal Vergence
For horizontal fusional ranges, a prism bar with a
base-in conguration (Fig.6.5c; the wide edge of
the prism is closer to the nose) is placed before
one eye, and its strength is increased at 2 prism
diopters per second. At the 20-prism diopter level,
the rate of diopter change increases to 5 prism
diopters per second. This is due to a preset dioptric change of a prism bar. The patient is rst
asked to report when the target gets blurry when it
splits into two (double, horizontally separated),
and when it returns to a single target. These ndings are recorded as blur, break, and recovery in
prism diopters. Some patients may not report blur
or report blur and break at the same time. The
divergence amplitude is the prism diopter recorded
at blur or break (when blur is not reported).
Note: When testing divergence amplitude for
distance with adequately corrected refractive
error, we do not expect “blur” to be reported as
accommodation is fully relaxed. The steps are
repeated with a prism bar in the base-out
(Fig. 6.5d; the wide edge of the prism is away
from the nose) direction to record the convergence
amplitude. The results are noted as convergence
ranges/divergence ranges, distance/near, and blur/
break/recovery in prism diopters [4–10].
Mechanism of action: When the prism bar is
oriented in the base in a direction in front of
either the right or left eye, the light rays bend
toward the base of the prism, shifting the image
viewed, nasally, away from the fovea. For the
visual system to see the target as a single image,
the eye must diverge to land the fovea to where
the image is displaced. When the eye reaches its
threshold, and the patient cannot diverge his/her
eyes any further, the patient will perceive the target doubling or being seen as two.
When the prism bar is oriented in the base-out
direction before the right or left eye, the light rays
bend toward the base of the prism, shifting the
image viewed temporally away from the fovea.
For the visual system to see the viewed target as
single, the eye must converge to land the fovea
where the image is displaced. When the eye
reaches its threshold, and the patient cannot converge his/her eyes any further, the patient will
perceive the target doubling or being seen as two.
When measured at a distance, the inuence of
accommodation is minimal, but the interplay of
accommodation with vergence cannot be discounted at near-testing distances. Retinal disparity, that is, dissimilar images on the fovea, drives
fusional vergence. When testing near the xed
distance, using letters as targets and making the
patient call out the letters helps to keep accommodation under control. However, accommodation can be driven along with fusional vergence
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to keep the target clear and single. On horizontal
fusional vergence testing, if the patient reports a
“blur,” that is the point till which fusional vergence has helped drive control; following this,
accommodative vergence primarily drives control until the target splits into two and fusion can
no longer be maintained.
Expected values for adolescent and young
adults for distance: divergence range (prism
diopters Δ): break = 5–9 and recovery = 3–5;
convergence range (Δ): blur = 7–11,
break = 15–23, and recovery = 8–12. For near
divergence range (Δ): blur = 11–15,
break = 19–23, and recovery = 10–16; convergence range (Δ): blur=14–20, break=18–24,
and recovery=7–15 [4, 11–14].
6.4.4.2.2 Vertical Ranges
A prism bar with the base up or down is placed
before one eye and moved 1 prism diopter per
second to measure vertical ranges. The patient is
asked to report when the target splits into two
(double, vertically separated), and the prism
power is lowered in magnitude to record when
the target returns to being single. The break is
recorded as right infravergence or left supravergence when the prism is placed base up before
the right eye and right supravergence or left
infravergence when the prism is placed base
down before the right eye.
Note: supravergence in one eye is equivalent
to infravergence in the other eye.
Mechanism of action: When a vertical prism
is placed before one eye, for example, base down
before the right eye, the light rays bend toward
the base of the prism, shifting the image below
the fovea in the right eye relative to the left eye.
To maintain single vision, the right eye must
move upwards (toward the apex of the prism) to
bring the image back to the fovea (supravergence) and maintain single vision. When the
base-up prism is placed in front of the right eye,
the light rays bend toward the base of the prism,
shifting the image above the fovea in the right eye
relative to the left eye. To maintain single vision,
the right eye must move downwards (toward the
apex of the prism) to bring the image back to the
fovea (infravergence) and maintain single vision.
The inuence of accommodation is not
expected for vertical fusional ranges; hence,
patients do not report blur and only report double
when fusion is not maintained. The expected values for both supravergence and infravergence for
distance and near vision are approximately 2–4 Δ
for the break and 1–3 Δ for the recovery [4].
6.4.4.3 Eciency oftheVergence
System
The vergence facility is a nuance test to assess the
efciency or exibility of the vergence system. A
12-prism base-out and a 3-prism base-in ippers
are used for testing. Testing is done for both distance and near vision using a single or vertical
column of letters (20/40–20/30 size) (Fig.6.6a,
Fig. 6.6 Vergence
facility testing. (a) A
xation stick with a
vertical column of letters
used for testing; (b)
12-prism base-out and
3-prism base-in wedge;
(c) the eyes’ response to
a convergence demand
(12-prism base-out); and
(d) the eyes’ response to
a divergence demand
(3-prism base-in)
ab c
d
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