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Some patients will attend clinical
practice complaining of problems with
near tasks such as reading or VDU work.
This is usually due to a comitant
deviation and, although troublesome
because of the symptoms produced, they
are never life-threatening. However,
patients may also attend with
heterotropias. Whilst these are usually
long-standing deviations, patients may
occasionally present complaining of
sudden onset diplopia with unnoticed
attendant heterotropia. The distinction
between long-standing and recent onset
heterotropia must be clearly made as
acquired deviations usually indicate
active pathology which can be lifethreatening.
If not detected and
adequately managed, these may result in
malpractice claims. In order to
differentiate between such entities, a
sound knowledge of the extraocular
musculature and innervation is essential.
BV FACTS
Ten percent of all primary care patients
exhibit a BV problem, and 2-3% of all
subjects exhibit a heterotropia.
Therefore, an optometrist that sees an
average of 15 patients per day (75 per
week), can expect approximately seven
with a BV anomaly and one or two with
a heterotropia.
Lyndon Jones, BSc, PhD, FCOptom, DCLP, DOrth, Frank Eperjesi, BSc, MCOptom and
Bruce Evans, BSc, PhD, FCOptom, DCLP
Binocular vision evaluation in practice
Some form of binocular vision (BV) evaluation should be undertaken on every
patient. The equipment required is easily obtainable, easy to use and, relative
to other optometric instrumentation, of low cost. General BV assessments can
be conducted with a cover stick, near and distance targets, an RAF rule, prism
bars, pen torch, Mallett Unit and tape measure (Figure 1).
Figure 1 BV equipment
THE BV SYSTEM
Appropriate examination of the BV
system requires an understanding of
the anatomy of the visual apparatus,
the motor system and coordinated eye
movements, as well as the sensory
system, which under normal
circumstances integrates two
monocular inputs to form a single
percept.
BV ROUTINE
As mentioned above, BV equipment is
low cost, easily available and easy to
use - the difficult part is in the
interpretation; this is easier if the
techniques are carried out correctly
and accurate information is obtained.
Particular attention needs to be paid to
working distances and the speed at
which procedures such as the cover test
are conducted. A common error is not
to allow enough time for dissociation
and not to adequately occlude the eye
under the cover. Poor technique results
in inaccurate data, which makes
diagnosis difficult, if not impossible.
OBSERVATIONS
General observations of the patient are
important and can be made on first
sight of the patient and during history
and symptoms. Head turns and tilts
often indicate some problem with
extraocular muscles and tend towards
the field of action of the underacting
muscle to help the patient avoid
binocular embarrassment. Postural
changes and excessive effort during
visual tasks may indicate that all is not
well. Patients will often experience
more difficulty during the test, as they
become tired. This is especially the case
if they are only just coping and then are
forced to operate beyond their comfort
zone.
HISTORY & SYMPTOMS
OCULAR HISTORY
Since amblyopia and heterotropia present
at specific ages, it is important to elicit
details of onset and the course of the
condition. This is useful in determining
the likely prognosis and if referral is
necessary. Take note of the presenting
problems or concerns, especially double
vision and if this is present at distance or
near, or when looking in a particular
direction. Is it worse at a certain time of
the day? Is there any history of lazy eye, eye
turns and, if so, what was the age at onset?
Ask about history of spectacle wear and
from what age. Also ask about previous
HES visits, previous trauma, patching or
orthoptic exercises, and any prisms in
glasses. Birth history, pre-term deliveries,
forceps or Caesarian delivery and the
health of the baby after parturition are also
important areas to look at.
FAMILY OCULAR HISTORY
Ask about any family history of
heterotropia, amblyopia or spectacle
wear. There is a 10x increased risk if a
parent has a heterotropia (incidence of
anomaly 25% vs. 2.5%); 86% incidence
of heterotropia if family history of
heterotropia and child is >+2.00DS.
MEDICAL HISTORY
Ask about general health, current
medication and previous surgery. Is there
any hypertension, diabetes, heart disease
or thyroid dysfunction?
FAMILY MEDICAL HISTORY
Ask about family history of hypertension,
heart disease, thyroid problems and
diabetes.
VISUAL ACUITY
Be aware of visual acuity at various ages,
and use appropriate tests for the age of
the patient. Also be aware of the
crowding phenomenon and its effects on
acuity measured with single and linear
optotypes. Problems often arise for
sustained near vision with IVN palsy
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FEBRUARY 26 • 1999 OPTOMETRY TODAY 33

Binocular vision evaluation in practice
although distance vision is normal, so
always carry out motility on those who
complain of near vision problems (see
later). With very young patients
alternate occlusion can be used along
with preferential looking, e.g. Keeler or
Cardiff Acuity Cards. For older children,
you can try matching pictures or shapes,
e.g. Kay, Ffookes symbols, and for school
age children Sheridan Gardiner,
Sonksen-Silver or Glasgow Acuity Cards.
COVER TEST
This is a very important test as it can
easily differentiate between a
heterotropia and a heterophoria. It can
also be used to estimate or measure the
direction and size of the deviation, and
to give some indication whether it is
compensated or not.
Choose a suitable target in order to
induce accommodation, do so slowly
with good lighting. Cover, uncover,
remove cover vertically and ensure the
eye is covered to avoid peripheral fusion
locks. If amblyopia is suspected, take a
little longer before removing the cover as
the amblyopic eye requires longer to take
up fixation. Decide if movements
indicate a heterotropia, heterophoria or
orthophoria, eso or exo, hypo or hyper.
Estimate the size and speed of recovery
and measure with a prism bar if you wish.
Then do alternate cover and look for an
increase size of deviation which is useful
for small vertical deviations. Watch the
upper lids as any movement may indicate
a vertical problem. Do not assume there
is no heterotropia if you see no
movement; there may be a microtropia
with abnormal retinal correspondence so
consider a four base-out prism test to
determine if suppression is present.
Figure 2 NPC assessment
CONVERGENCE AMPLITUDE (NEAR
POINT OF CONVERGENCE - NPC)
This is only applicable for binocular
patients and is particularly important for
symptomatic exophores. It is important
to choose an appropriate target, either
the dot and line on the RAF rule, or a
near 6/9 letter, which is considered to be
the clinical standard. It is absolutely
inappropriate to use a pen-top. NPC is
most accurately measured with the RAF
rule, in the depressed position - 45
degrees, and a slow speed (Figure 2).
In order to check for fatigue it should
be measured three times near the
beginning of the assessment and twice at
the end. The value can be noted in terms
of break (either when the subject reports
diplopia [subjective result] or when the
observer notices one or both eyes diverge
[objective result]) and in terms of
recovery (either when the subject
reports single vision or the observer
notices that both eyes are pointing to the
test target). Both break and recovery are
measured to the nearest half centimetre.
A remote NPC with a break greater
than 10cm is considered to be the most
consistent finding in subjects with
convergence insufficiency. Fatigue needs
to be assessed as a subject may be able to
produce one good result with the RAF
rule, but the NPC may increase with
further testing. Many subjects report the
occurrence of symptoms only after
several minutes of near point task
performance.
FUSIONAL RESERVES
These (also called horizontal vergence
reserves and prism vergences) can be
measured in several ways. Orthoptists
tend to use a prism bar (step vergence).
Figure 3 Fusional reserves assessment
Those optometrists with an interest in
this field tend to use a Risley rotating
prism (smooth vergence) either
monocularly in a trial frame, or
binocularly in a phoropter. The results
differ according to which procedure is
used so, when making records, it is
important to note the instrument used
(Figure 3).
Base-in and base-out values for near
are usually the most useful although
distance values can also be obtained.
The blur, break and recovery points for
both base directions need to be noted.
This is a direct measure of the fusional
vergence available to compensate for a
phoria. The blur point is reached when
the subject has used all their fusional
reserves and has to use accommodative
convergence to keep the test target
single. As accommodative convergence
is brought into play, the subject
accommodates and the test target
becomes blurred. The break point
corresponds to the point when the
subject no longer has any fusional or
accommodative vergence remaining and
the test target becomes double.
Norms are age-dependent. It is
possible to have normal convergence
amplitude as measured with the RAF
rule push-up test and still have a
vergence problem. This is known as
fusional vergence dysfunction.
AMPLITUDE OF ACCOMMODATION
This is a measure of the maximum
amount of accommodation an individual
can exert and is usually measured using
the RAF rule with the smallest text
readable. It should be conducted
monocularly and binocularly and
repeated at least three times for each
situation in order to assess for fatigue.
This is important in esophores and
symptomatic patients with NV problems.
Ask the patient to read out loud the
smallest line they can see, then move the
target in and watch for saccades to make
sure they are accommodating on the
target; measure monocularly and
binocularly and repeat to check for
fatigue.
ACCOMMODATIVE FACILITY
This determines the speed of
accommodative change. The dioptric
accommodative stimulus is alternated
between two different levels and the
subject reports when a letter target is
seen clearly after each alternation in
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FEBRUARY 26 • 1999 OPTOMETRY TODAY

Binocular vision evaluation in practice
Figure 4 Accommodative facility assessment
accommodative stimulus. The examiner
counts the number of cycles completed
in one minute (one cycle being the
change from one stimulus level to the
other and back again). Accommodative
stimulus can be varied either by lens
power changes or by viewing distance
changes. The first is referred to as lens
rock and the second as distance rock,
indicating that the accommodative
stimulus is ‘rocked’ back and forth.
The standard method of testing
accommodative facility is a lens rock
procedure using a pair of +2.00D lenses
on one side of a flipper bar, and -2.00D
lenses on the other side, although also
available as ±1.00D and ±3.00D. The
test is begun with the +2.00D lenses
over the subject’s refractive correction.
A test distance of 40cm is usually used
with the reduced Snellen letters at a 6/6
to 6/12 acuity demand for monocular
testing (Figure 4). This type of target
has no suppression control and it is more
appropriate to use either the Bernell
vectogram SOV9 or the vertical fixation
disparity bars on the near Mallett unit
for binocular testing. Both need to be
used with polarising filters.
Some clinicians suggest that it may
be more appropriate to train the
monocular accommodative facility prior
to the binocular facility, especially if the
binocular lens rock performance is
limited by fusional vergence dysfunction.
Cut-offs for test failure using
+2.00D/-2.00D flippers and a 40cm
viewing distance for children and adults
up to 30 years of age, are less than 11
cycles per minute for monocular testing
and less than 8 cycles per minute for
binocular testing. Norms are agedependent.
During binocular lens rock
testing, adjustments in fusional vergence
must occur to compensate for the
changes in accommodative vergence.
Therefore, subjects may pass the
monocular lens rock but fail the
binocular lens rock facility if a vergence
disorder is present.
ACCOMMODATIVE LAG
During accommodation for near-point
viewing, the retina usually is conjugate
with a point slightly behind the object of
regard. For near-point targets,
accommodative response is usually
slightly less than the accommodative
stimulus. The amount by which the
dioptric accommodative response is less
than the dioptric accommodative
stimulus is the lag of accommodation.
This category of accommodation tests
can be further divided into: tests that
measure the lag of accommodation; and
tests in which lens power is changed to
alter accommodative stimulus to the
point at which dioptric accommodative
stimulus and dioptric accommodative
response are equal.
Monocular estimate method (MEM)
dynamic retinoscopy is the procedure
currently preferred by the authors. A test
card with an aperture in the centre is
used for dynamic retinoscopy so that the
examiner can observe the retinoscopic
reflex close to the subject’s visual axis
through the aperture. In MEM dynamic
retinoscopy, the amount of the lag of
accommodation is estimated by judging
the width, speed and brightness of the
retinoscopic reflex. The test card and the
retinoscope are placed at the same
distance from the subject’s spectacle
plane, usually 40cm (Figure 5).
With the retinoscope in the plane
mirror mode, ‘with motion’ indicates a
Figure 5 Accommodative lag assessment
lag of accommodation and ‘against
motion’ indicates a lead of
accommodation. Neutrality indicates
that the accommodative stimulus and
accommodative response are equal. The
examiner’s estimate of the amount of
plus power that would be required to
neutralise the ‘with motion’ is the
estimate of the lag of accommodation.
The estimate of the lag can be confirmed
by very briefly placing a plus lens equal in
power to the estimated lag over one eye
and quickly checking to see whether
neutrality is observed. The lens should
only be in place a half-second or less so
that a change in accommodative
response is not induced. School age
children are reported to have a mean lag
of +0.34D. Most non-presbyopic
subjects have lags of 0 to +0.75D with
MEM retinoscopy.
MOTILITY
To examine the action of a muscle and
that of its yoke muscle, a motility test can
be performed. In this test a pen-torch is
moved in front of the subject in a star
pattern, whilst the subject keeps their
head and neck still. By moving the eyes
in this manner, the maximal actions of all
the extraocular muscles can be checked;
the corneal reflexes should be observed.
For example, the action of the
superior rectus (SR) muscle is best
evaluated by placing the eye in an
abducted position and checking the
degree of elevation, which can be
achieved. Poor function of this muscle
will be indicated by a reduced ability to
elevate the eye in this abducted position
and diplopia in the field of action of the
weakened muscle. The patient should be
asked - “Is the double vision horizontal,
vertical or oblique?” A cover test
conducted in ‘up’ and ‘right’ gaze (in a
case of a right SR weakness) will also
show a hypotropia which is maximal in
this position when compared with all
other peripheral positions.
Patients may present with either a
partial loss (paresis) or complete loss
(paralysis) of muscle function. These
terms are often used synonymously.
Maximum and minimum fields of
diplopia can be detected and an attempt
made to try and isolate muscle or at least
a pair of muscles. Alternatively, the eye
with the outer most image has the
underacting muscle; images can be
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FEBRUARY 26 • 1999 OPTOMETRY TODAY 35

Binocular vision evaluation in practice
enhanced by using red and green goggles.
It is important to determine whether the
underaction is recent or long-standing
and if the diplopia is sudden or of gradual
onset. It should be noted that the vertical
meridian is not a diagnostic direction of
gaze, however, it is still useful to test in
this direction, since it will help to identify
A and V patterns more easily.
EVALUATION OF BINOCULARITY
(NON-HETEROTROPIC CASES)
Fixation disparity is a technique familiar
to most optometrists and will not be
discussed in detail. The patient should be
instructed to compare the position of the
green markers with and without the
Polaroid. Allow the patient to settle with
the Polaroid and check that they can see
both markers, i.e. they are not
suppressing. Allow the patient to adapt
by reading a line of text and then
neutralise any displacement with the
minimum prism - this is termed the
aligning prism, or small spheres. The
displacement of the marker is referred to
as fixation disparity. The markers may
move back and forth across the neutral
position indicating binocular instability;
if the markers flicker this may be due to
alternate suppression or retinal rivalry.
ANTI-DIPLOPIA MECHANISMS
Suppression is a negative adaptation to
diplopia; it can vary in its intensity on a
continuous scale from shallow to deep as
well as in its size and position (either
central or peripheral). Central
suppression can extend ten degrees from
the fovea. Worth four-dots, stereoscope
tests, Bagolini lenses and the Nonius bars
on the Mallett unit can be used to detect
suppression. The Mallett unit also has
specific test words; some of the letters are
seen by both eyes and some by only the
left or the right. They are of increasing
size and allow a measure of the degree
of suppression.
ARC (abnormal retinal
correspondence) which is almost
always HARC and can be termed a
positive adaptation to the heterotropia
and is described as where
correspondence exists between areas
of the retina on the fixating and
deviating eye which receive the same
image. The correspondence prevents
the occurrence of diplopia and allows
for a limited degree of stereopsis. ARC
is examined by the use of a Bagolini
lens placed before the non-fixing eye
while the patient is fixating a spot light
target; if the patient reports the streak
of the Bagolini lens as being centred
on the spot, ARC may be diagnosed. A
distance Mallett unit can be used; if
the patient sees that the Nonius lines
are aligned when a heterotropia is
present, there is ARC.
ECCENTRIC FIXATION (MONOCULAR
SENSORY ADAPTATION)
This condition exists when a nonfoveal
point is used for fixation; since
retinal sensitivity is reduced
parafoveally, there is always a reduced
acuity measured in these eyes which
depends upon the degree of
eccentricity of fixation. Assessment
can be made with the eccentric
fixation graticule of the direct
ophthalmoscope while occluding the
other eye; the target is usually red-free
to ensure the patient remains
comfortable during the test. The
fixation can be classified according to
its state (steady or unsteady), position
(superior, inferior, nasal or temporal)
and by its size in degrees from the
centre of the fixation target.
Management of BV anomalies
• Diagnosis of anomaly
• Recent or long-standing?
• Aetiology
• Management
• Referral or next review
FURTHER READING
1. Cashell, G.T.W. and Durran, I.M. (1980)
‘Handbook of Orthoptic Principles 4th
edition’. Churchill Livingstone, London.
2. Stidwill, D. (1990) ‘Orthoptic Assessment
and Management. Blackwell Science,
London.
3. Evans, B.J.W. (1997) ‘Binocular Vision
Anomalies: Investigation and Treatment
3rd edition’. Butterworth-Heinemann,
Oxford.
4. Scheimann, M. and Wick, B. (1994)
‘Clinical Management of Binocular Vision,
Heterophoric, Accommodative and Eye
Movement Disorders’. Lippincott Raven,
Philadelphia.
5. Goss, D.A. (1995) ‘Ocular Accommodative,
Convergence and Fixation Disparity. A
Manual of Clinical Analysis 2nd edition’.
Butterworth-Heinemann, Oxford.
6. Birnbaum, M.H. (1993) ‘Optometric
Management of Near-point Vision
Disorders’. Butterworth-Heinemann,
Oxford.
7. Rowe, F. (1997) ‘Clinical Orthoptics’.
Blackwell Science, London.
USEFUL ADDRESSES
For accommodative facility
flipper bars without lenses, contact
Paul Adler,
50 High Street, Stotfold, Hitchin,
Herts, SG5 4LL. Tel: 01462-732393
Optometric Educators,
PO Box 172, Bromley, Kent,
BR2 OWZ. Tel: 0181-466 6535.
Email: admin@optometrist.co.uk
This is the second article in a
series of five to be published, based on last
September’s Optometric Educators
lecture series entitled
“Essential clinical skills for tomorrow’s patients”.
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FEBRUARY 26 • 1999 OPTOMETRY TODAY