Eye Position
Measurement
Walter Huang, OD
Yuanpei University
Department of Optometry
Eye Position
In the ideal world…

All six extraocular muscles have the same
muscle tone so that eye position at rest is
slightly deviated outward
Eye Position
In the real world…
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Eye position at rest varies due to different
muscle tones across individuals
Heterophoria
Latent deviation of the eye
If fusion is disrupted by dissociating the two eyes,
the eye that is prevented from the act of seeing
will return to its natural position of rest
This deviation of the eye to its natural position of
rest is called heterophoria
Heterophoria is also commonly known as phoria
or eye position at rest
Most phoria positions are parallel to slightly
divergent
Heterotropia
Manifest deviation of the eye
There is no fusion between the two eyes
As a result, binocular vision is absent
Heterotropia is also commonly known as tropia,
strabismus, or eye turn
Heterotropia may be accompanied by
suppression, amblyopia, eccentric fixation,
and/or anomalous retinal correspondence
Direction of Deviation
Ortho
Horizontal deviation
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Eso
Exo
Vertical deviation
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Hyper
Hypo
Cyclo deviation
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Incyclo
Excyclo
Deviation versus Visual Axis
Esophoria or esotropia
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One visual axis converges
Exophoria or exotropia
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One visual axis diverges
Hyperphoria or hypertropia
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One visual axis is higher than the other
Two Methods of Dissociation
Cover one eye
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Cover test
Present two different images to two eyes
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Red and green filters
Prism dissociation
Maddox rod
By dissociating the two eyes, we can
determine a person’s binocular vision
status
Cover Test
Purpose
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To objectively establish the presence or
absence of heterophoria or heterotropia
To measure the objective angle of deviation of
the visual axis
To serve as an important diagnostic test for
strabismus, especially for pediatric patients
Cover Test
Set-up

Done at distance (6M) and near (40cm)
because the amount of deviation may be
different at different test distances
Cover Test
Procedure

Consists of two parts
Unilateral cover test (cover-uncover test)
Alternating cover test
*Note: The unilateral cover test should always be
performed before the alternating cover test
Unilateral Cover Test
Purpose

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To determine whether a patient has a
heterophoria or heterotropia
To determine the frequency, laterality, and
direction of the heterotropia
Theory
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An eye that deviates under binocular viewing
conditions will move to foveally fixate the
target under monocular viewing conditions
Unilateral Cover Test
Movement of the deviating eye on the
unilateral cover test indicates a tropia
No movement indicates a phoria
Unilateral Cover Test
Procedure
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Start with both eyes open
Intermittently cover OS while observing for
any movement of OD
If OD fixates target with fovea, when covering
OS, OD will not move
If OD has tropia, when covering OS, OD will
move to fixate target
Repeat the procedure on OS
Unilateral Cover Test – Right
Exotropia
Unilateral Cover Test – Right
Exotropia
a) The two eyes under normal binocular
viewing conditions
b) The right eye makes no movement
upon being covered
c) The right eye makes no movement
upon being uncovered
d) When the left eye is covered, the right
eye turns inward to take up fixation, and
the left eye turns outward
e) When the left eye is uncovered, the
right eye turns outward, and the left eye
turns inward to take up fixation
Unilateral Cover Test - Esotropia
Unilateral Cover Test - Esophoria
Alternating Cover Test
Purpose

To determine the direction and magnitude of a
deviation, whether it be a heterotropia or
heterophoria
Theory
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Binocular vision is disrupted by occluding one
eye
When the occluder is moved to the opposite
eye, the uncovered eye will move to fixate the
target the eye
Alternating Cover Test
Movement on the alternating cover test
indicates a deviation, though it does not
differentiate a tropia from a phoria
Alternating Cover Test - Esophoria
Measurement
Eye movement can be measured by prism
Magnitude
After a deviation of the eye is observed, its
magnitude can be measured
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Eso deviation is neutralized by base out (BO)
prism
Exo deviation is neutralized by base in (BI)
prism
Hyper deviation is neutralized by base down
(BD) prism
Hypo deviation is neutralized by base up (BU)
prism
Magnitude
One prism diopter movement is hard to
see
Movement must be at least 3 to 4 prism
diopters to be detected
Recording
As a rule, only a hyper deviation is
recorded
A phoria has neither laterality (except for
vertical phoria which has right or left) nor
frequency
Recording
Test distance
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Distance (D) or Near (N)
Deviation
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Heterotropia (T) or Heterophoria (P)
Laterality*
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Right (R) or Left (L), Unilateral (Unilat) or
Alternating (Alt)
Frequency*
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Constant (Const) or Intermittent (Int)
Recording
Direction
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Ortho ( ), Eso (E), Exo (X), Hyper (H) versus
Hypo (R or L)
Magnitude
In prism diopters (△)
*Only applicable for tropia, except for vertical
phoria which has laterality right or left (R or L)
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Expected Findings
Phorias are more common than tropias
70% of people have phorias
Distance: 0 to 2 △ XP
Near: 0 to 6 △ XP
Things to Know
A patient having a large phoria must have
good motor reserve (fusional range);
otherwise, the patient will have binocular
vision symptoms (i.e., headache,
asthenopia, eye fixation problem)
A phoria can change with different states
of accommodation
Through near Add, most presbyopia
patients will have a large exo deviation
due to lack of accommodation and is not
likely to show symptoms
Things to Know
A patient should wear his habitual
prescription during the Cover test
An uncorrected hyperope will need to
accommodate so that the expected finding
is esophoria; however, this may not be the
case in reality
Drugs may have an effect on the deviation
of the eye (e.g., alcohol can make the
ocular deviation more exo)