[4] Evaluating the Deviation

What is strabismus, and what components of binocular vision can it affect?

Strabismus is a misalignment of the eyes that represents a breakdown in one or more components of normal binocular vision. Associated abnormalities may be:

• Sensory

• Motor

• Both sensory and motor

What are the three major types of ocular deviation (ocular misalignment)?

• Heterophoria: Latent deviation controlled by fusion and revealed when fusion is disrupted.

• Heterotropia: Manifest deviation present despite binocular viewing (may be constant or intermittent).

• Fixation disparity: Small residual misalignment despite maintained fusion.

What are the key requirements for normal binocular vision?

Normal binocular vision requires:

1. Healthy eyes and visual pathways

2. Accurate monocular fixation by each eye

3. Proper neuromuscular coordination of intra- and extraocular systems

4. Accurate bifoveal fixation

5. Normal retinal correspondence (foveae serving as corresponding points)

6. Similarity of the two retinal images

7. Sensory fusion (unification of the two ocular images)

Which sensory and motor factors are essential for maintaining binocular single vision?

Motor factors:

• Accurate monocular fixation

• Proper neuromuscular coordination

• Bifoveal alignment/fixation

Sensory factors:

• Normal retinal correspondence

• Similar retinal image size/clarity

• Sensory fusion of the two images

What is fixation disparity, and how does it differ from a tropia or phoria?

Fixation disparity is a small ocular misalignment present under fused (associated) viewing conditions where the retinal images still fall within Panum's fusional area, so diplopia does not occur.

• Fusion is maintained

• Usually < 30 minutes of arc

• Smaller than a phoria or tropia

• Considered a residual alignment error during binocular viewing

Under what viewing conditions is fixation disparity measured, and what tests are used?

Fixation disparity is measured under associated (fused) viewing conditions, meaning fusion is maintained during testing.

Common tests:

• Borish Card

• Saladin Card

• Wesson Card

• Disparometer

• Bernell Box

What is a heterophoria (phoria), and when is it measured?

A heterophoria is a latent ocular deviation that is normally controlled by fusion.

It is measured only when fusion is disrupted (dissociated viewing conditions).

Key distinction:

• Phoria = latent deviation + fusion broken

• Fixation disparity = tiny residual deviation + fusion maintained

How is a heterophoria quantified, and what are the common testing methods?

Heterophorias are measured in prism diopters (Δ).

Common tests:

• Alternating Cover Test

• Von Graefe

• Maddox Rod

• Modified Thorington

Compare fixation disparity and heterophoria.

Feature	Fixation Disparity	Heterophoria
Fusion present?	Yes	No (disrupted)
Viewing condition	Associated	Dissociated
Size	Very small (<30 min arc)	Larger latent deviation
Units	Minutes of arc	Prism diopters (Δ)
Symptoms	Can contribute to asthenopia	May decompensate and cause symptoms
Example tests	Borish, Saladin, Wesson	Cover test, Von Graefe, Maddox Rod

What is strabismus, and what are its common synonyms?

Strabismus is a manifest ocular deviation in which binocular fixation is not present under normal viewing conditions. One eye's visual axis fails to intersect the object of fixation.

AKA:

• Heterotropia

• Tropia

• Squint

• Manifest deviation

• Strab

How is a tropia different from a phoria?

Tropia (heterotropia):

• Manifest deviation

• Present during normal binocular viewing (associated conditions)

• Fusion cannot fully maintain alignment

• Detected with unilateral cover test

Phoria (heterophoria):

• Latent deviation

• Revealed only when fusion is broken (dissociated conditions)

• Detected with alternating cover test and other dissociating techniques

High-yield:

• Tropia = manifest

• Phoria = latent

How is strabismus measured and what tests are commonly used?

Strabismus is measured in prism diopters (Δ).

Common tests:

• Unilateral (cover-uncover) Cover Test

• Hirschberg Test (corneal light reflex assessment)

Exam pearl: A unilateral cover test detects a manifest deviation (tropia), whereas an alternating cover test reveals a latent deviation (phoria).

What are the major risk factors for developing strabismus?

Major risk factors include:

• Prematurity / low birth weight

• Maternal smoking during pregnancy

• Family history

• Neurologic or CNS disorders

Examples of neurologic associations:

• Cerebral palsy

• Down syndrome

Which refractive errors are commonly associated with esotropia and exotropia?

• Hyperopia → Esotropia

  • Excessive accommodative effort can drive accommodative convergence.

• Anisometropia and astigmatism → Exotropia

  • Reduced binocular sensory input can predispose to exodeviation.

High-yield association:
Hyperopia = ET, whereas anisometropia/astigmatism = XT.

What are the major etiologic categories of strabismus?

Strabismus can result from four broad categories:

1. Optical abnormalities

2. Sensory (ocular disease) abnormalities

3. Innervational abnormalities

4. Anatomical abnormalities

How can optical abnormalities lead to strabismus? Give examples.

Optical abnormalities can disrupt binocular vision and sensory fusion, leading to strabismus.

Examples:

• Anisometropia

• High refractive error

Mechanism: Unequal or blurred retinal images weaken fusion and may result in ocular misalignment.

How does sensory deprivation from ocular disease influence the type of strabismus that develops?

Sensory ocular disease can cause loss of fusion and subsequent strabismus.

Age-dependent pattern:

• Younger children → Esotropia (ET) more likely

• Older children and adults → Exotropia (XT) more likely

Exam pearl: Sensory deprivation tends to produce ET in young children but XT in older patients.

What are innervational causes of strabismus?

Innervational strabismus occurs from abnormalities in muscle tone or neural control of eye movements.

Causes include:

• Intoxication

• Medications

• Trauma

• Neurologic disease

• Congenital innervational abnormalities

• Abnormal AC/A ratios (high or low)

High-yield: Think of a defect in the neural command to the extraocular muscles, rather than the muscles themselves.

What anatomical abnormalities can cause strabismus?

Anatomical causes involve structural abnormalities affecting ocular alignment.

Examples:

• Extraocular muscle (EOM) abnormalities

• Abnormal orbital contents

• Abnormal ligaments or tendons

Mechanism: Physical restriction or altered muscle anatomy prevents normal ocular alignment.

What are the key characteristics used to describe a strabismic deviation?

• A deviation is characterized by:

  1. Direction (eso, exo, hyper, hypo)

  2. Frequency (constant or intermittent)

  3. Magnitude (size of deviation)

  4. Laterality (unilateral vs alternating)

  5. AC/A ratio

  6. Cosmesis (appearance/significance of the deviation)

  7. Comitancy (whether deviation remains the same in all gazes)

What does "comitancy" mean when evaluating a strabismus?

Comitancy refers to whether the angle of deviation remains constant in all directions of gaze.

• Comitant deviation: Same magnitude in all gaze positions.

• Incomitant deviation: Magnitude varies with gaze direction, often suggesting neurologic, muscular, or mechanical pathology.

Why are laterality, frequency, and magnitude important when describing a strabismus?

• Laterality: Determines whether one eye consistently deviates (unilateral) or fixation alternates between eyes.

• Frequency: Indicates whether the deviation is constant or intermittent.

• Magnitude: Quantifies the size of the deviation, usually in prism diopters (Δ).

These characteristics help guide diagnosis, prognosis, and treatment planning.

How is strabismus classified by frequency?

Strabismus can be classified as:

• Heterophoric (latent deviation, controlled by fusion)

• Intermittent strabismus (manifest only part of the time)

• Constant strabismus (manifest all of the time)

How is control of intermittent strabismus quantified?

Common methods include:

• POTS = Percentage of Time Strabismic

• POTT = Percentage of Time Troped

• Office Control Scale (commonly used for exotropia)

High-yield: The more frequently the eye manifests a tropia, the poorer the patient's binocular control.

How is the magnitude of a strabismic deviation measured and classified?

Magnitude is measured in prism diopters (Δ) at both distance and near.

Classification:

• Micro: 0-5Δ

• Small: 6-20Δ

• Moderate: 21-40Δ

• Large: >40Δ

What are the major directional classifications of strabismus?

Strabismus can be classified by direction as:

• Eso-: Eye turns inward

• Exo-: Eye turns outward

• Vertical: One eye higher or lower than the other

  • Hypertropia

  • Hypotropia

• Cyclotorsional: Rotational misalignment

• Combination: Horizontal + vertical and/or torsional components

What do the terms esotropia, exotropia, and hypertropia mean?

• Esotropia (ET): Deviating eye turns toward the nose.

• Exotropia (XT): Deviating eye turns away from the nose.

• Hypertropia: Deviating eye is higher than the fellow eye.

Naming rule: The deviation is named by the position of the deviating eye.

• Left eye inward → Left ET

• Left eye outward → Left XT

• Left eye higher → Left HT

How is the cosmetic significance (cosmesis) of strabismus graded?

Cosmesis describes how noticeable the deviation appears to an observer.

Common grading:

• Excellent

• Good

• Fair

• Poor

At what magnitude does strabismus become cosmetically noticeable?

General guidelines:

• < 10Δ → Usually not cosmetically noticeable

• 10-20Δ → May be cosmetically noticeable

• > 20Δ → Usually cosmetically noticeable

Exam pearl: Cosmetic concerns become increasingly likely once the deviation exceeds 20Δ.

What facial and ocular features can make an esotropia appear more cosmetically noticeable?

Factors associated with esotropia appearance:

• Negative angle kappa

• Wide nasal bridge

• Prominent epicanthal folds

• Small interpupillary distance (IPD)

What facial and ocular features can make an exotropia appear more cosmetically noticeable?

Factors associated with exotropia appearance:

• Positive angle kappa

• Narrow nasal bridge

• Minimal/absent epicanthal folds

• Large interpupillary distance (IPD)

These features can accentuate the appearance of an outward deviation.

What is the AC/A ratio, and what is considered normal?

The AC/A ratio (Accommodative Convergence/Accommodation ratio) describes how much convergence occurs for a given amount of accommodation.

Normal AC/A: 4:1 ± 2

Approximate normal range:

• 2:1 to 6:1

How are AC/A ratios classified?

• Low AC/A: Less convergence than expected for a given accommodative response

• Normal AC/A: Approximately 4:1 ± 2

• High AC/A: Greater convergence than expected for a given accommodative response

How do cosmesis and AC/A ratio contribute to a strabismus evaluation?

Cosmesis

• Assesses how noticeable the deviation is

• Influenced by deviation size and facial anatomy

• Guides treatment decisions and patient concerns

AC/A Ratio

• Assesses the relationship between accommodation and convergence

• Helps explain differences between near and distance deviations

• Can identify abnormal accommodative-convergence relationships

What should be evaluated during direct observation of a patient with suspected strabismus?

Key observations include:

• Eyelid position (ptosis vs pseudoptosis)

• Palpebral fissure width and whether it changes

• Presence of epicanthal folds

• Abnormal head posture

  • Head turn

  • Head tilt

  • Head tip/chin position

Why are eyelids and epicanthal folds important when evaluating strabismus?

They can create the appearance of strabismus or mask a true deviation.

Look for:

• Ptosis (true drooping eyelid)

• Pseudoptosis (appears droopy but not true ptosis)

• Changes in palpebral fissure width

• Prominent epicanthal folds

What abnormal head postures should be assessed in strabismus, and why do they occur?

Evaluate for:

• Head turn

• Head tilt

• Head tip (chin up/down posture)

These are often compensatory positions adopted to:

• Maintain binocular single vision

• Reduce diplopia

• Place the eyes in a field of gaze with less deviation

How can an abnormal head posture help localize an extraocular muscle palsy?

Patients frequently adopt a posture that minimizes the deviation and diplopia.

Rule:
The head is often turned toward the field of action of the paretic muscle.

Example:
A patient with a left lateral rectus palsy may turn the face to the left to reduce the need for left gaze and improve binocular alignment.

What information can the Hirschberg test provide about a strabismus?

The Hirschberg test can:

• Determine whether strabismus is present

• Identify the direction of deviation

• Identify laterality (which eye deviates)

• Estimate frequency

• Estimate magnitude

How does the Hirschberg test work?

The Hirschberg test is an objective assessment of ocular alignment that evaluates the position of the corneal light reflex relative to the center of the pupil.

• Symmetric reflexes → aligned eyes

• Asymmetric reflexes → ocular deviation present

Why is the Hirschberg test especially useful in young children?

The Hirschberg test:

• Requires minimal patient cooperation

• Is objective

• Does not require verbal responses

• Preserves normal binocular viewing conditions

Why is direct observation important before formal strabismus testing begins?

Direct observation helps determine:

• Direction of deviation

• Laterality

• Frequency (constant vs intermittent)

before any formal testing is performed.

Why is cosmesis an important part of strabismus evaluation?

Cosmesis is important because:

• It may influence treatment decisions

• It is often considered before surgical referral

• It commonly represents the patient's or parent's primary concern

• The perceived deviation may matter as much as the measured deviation

What is angle kappa (lambda), and under what conditions is it measured?

Angle kappa (lambda) is the relationship between the visual axis and pupillary axis, assessed by observing the position of the corneal light reflex.

• Measured under monocular conditions

• Usually symmetric between eyes

• Most commonly, the reflex is displaced ~0.5 mm nasally

• Occurs because the fovea lies temporal to the pupillary axis

Why is angle kappa important when evaluating strabismus?

Although angle kappa is not highly sensitive for measuring eye turns, it can:

• Simulate strabismus (pseudostrabismus)

• Conceal true strabismus

• Exaggerate the appearance of strabismus

High-yield: Always compare binocular Hirschberg findings with each eye's monocular angle kappa before diagnosing a deviation.

What is the normal relationship between angle kappa and the corneal light reflex?

In the typical (positive) angle kappa:

• The fovea lies temporal to the pupillary axis.

• The corneal light reflex is therefore seen slightly nasal to the pupillary center.

• Normal displacement ≈ 0.5 mm nasal.

How can corneal light-reflex displacement be converted to prism diopters?

Approximate Hirschberg conversion:

• 1 mm displacement ≈ 22Δ

• 0.5 mm displacement ≈ 11Δ

What is the Hirschberg test procedure?

• Patient fixates a penlight/transilluminator binocularly (typically ~50 cm away).

• Observe the corneal light reflex in each eye.

• Compare the reflex position relative to the center of each pupil.

• Determine whether the reflexes are:

  • Symmetric → alignment likely normal

  • Asymmetric → possible strabismus

• Compare findings with each eye's monocular angle kappa.

How are angle kappa and Hirschberg testing related?

Angle Kappa	Hirschberg Test
Measured monocularly	Measured binocularly
Normal physiologic reflex position	Screens for strabismus
Usually symmetric	Compares reflexes between eyes
Can mimic or mask strabismus	Used to detect direction and estimate magnitude

How do you interpret horizontal corneal light reflex displacement during the Hirschberg test?

Compare the displaced reflex to the normal position.

• Equal reflexes in both eyes → No strabismus

• Temporal displacement of the reflex → Esotropia (ET)

• Nasal displacement of the reflex → Exotropia (XT)

How do you interpret vertical corneal light reflex displacement during the Hirschberg test?

• Inferior displacement of the corneal reflex → Hypertropia

• Superior displacement of the corneal reflex → Hypotropia

What Hirschberg findings correspond to eso-, exo-, hyper-, and hypotropia?

Reflex Displacement	Deviation
Temporal	Esotropia (ET)
Nasal	Exotropia (XT)
Inferior	Hypertropia (HT)
Superior	Hypotropia

What is the Krimsky test, and when is it useful?

The Krimsky test is a modification of the Hirschberg test that uses prisms to quantify the magnitude of a strabismus.

Especially useful for:

• Young children

• Poorly cooperative patients

• Patients with severely reduced vision in one eye

• Patients in whom cover testing is difficult

Why is the Krimsky test often preferred over Hirschberg alone for measuring deviation size?

The Hirschberg test requires estimating corneal reflex displacement in millimeters, which can be difficult and imprecise.

The Krimsky test improves accuracy by:

• Using prisms

• Neutralizing the deviation

• Determining the prism power needed to align the corneal reflexes

What are the steps of the Krimsky test?

• Observe the corneal reflexes (as in Hirschberg).

• Place a prism over the fixating eye.

• Increase or decrease prism power until the corneal reflexes become symmetric.

• The prism power that equalizes the reflexes equals the approximate deviation magnitude.

Where is the prism placed during a Krimsky test?

The prism is placed over the fixating eye and adjusted until the corneal light reflexes are symmetrical.

What endpoint indicates a successful Krimsky neutralization?

The endpoint is reached when the corneal light reflexes are symmetric between the two eyes.

Interpretation:
The prism power required to achieve symmetry is the estimated magnitude of the strabismus in prism diopters (Δ).

What is the Bruckner test and how is it performed?

The Bruckner test is a screening test for ocular misalignment using the red reflex.

Procedure:

1. Patient fixates an ophthalmoscope light from approximately 1 meter away.

2. Light illuminates both eyes simultaneously.

3. Compare the brightness of the two red reflexes.

How are Bruckner test results interpreted?

Compare the red reflexes from the two eyes.

• Brighter reflex = deviating eye

• Symmetric reflexes suggest normal alignment

High-yield: A brighter red reflex generally comes from the non-fixating/deviating eye because of the altered optical alignment.

In which patients is the Bruckner test most useful?

The Bruckner test is especially helpful in:

• Infants

• Young children

• Patients with developmental or special needs

• Patients unable to cooperate with cover testing

What factors improve visualization during the Bruckner test?

• Use an ophthalmoscope from about 1 meter

• Evaluate both eyes simultaneously

• Pupil dilation may improve results

• Observe relative brightness of the red reflexes

What are the three major cover tests used in strabismus evaluation, and what does each measure?

• Unilateral Cover Test (UCT) → Detects a manifest deviation (tropia).

• Simultaneous Prism Cover Test (SPCT) → Measures the manifest component of a deviation.

• Alternating Cover Test (ACT) / Prism Alternate Cover Test (PACT) → Measures the total deviation (phoria + tropia).

Why is target selection important during cover testing?

Accurate cover testing requires an accommodative target that keeps the patient actively fixating and accommodating.

Key principle: Ensure the target remains clear throughout testing.

How can you verify that a patient is truly fixating an accommodative target during cover testing?

Ask the patient to:

• Read letters on the target

• Describe details in a picture

• Identify specific features of the fixation object

How should fixation targets be modified for patients with amblyopia?

The target should be adjusted so it falls within the patient's visual threshold.

• Use larger optotypes if needed.

• Ensure the amblyopic eye can actually see the target.

What fixation targets can be used with digital acuity systems during cover testing?

Digital systems may use:

• Cycling picture targets

• Animated fixation targets

• Movies (especially for young children)

• Age-appropriate accommodative letters

What is the purpose of the Unilateral Cover Test (UCT)?

The Unilateral Cover Test (Cover-Uncover Test) is used to detect and characterize a manifest deviation (tropia).

It helps determine:

• Direction of deviation

• Laterality (unilateral vs alternating)

• Frequency (constant vs intermittent)

• Approximate magnitude (based on amount of movement)

How is the Unilateral Cover Test performed?

• Have the patient fixate binocularly on an accommodative target.

• Start in primary gaze (midline).

• Cover one eye.

• Observe the uncovered eye.

• Look for any movement as it takes up fixation.

• Then "chase" the deviation by covering the fixating eye and observing the other eye.

How can the Unilateral Cover Test help determine frequency and laterality of strabismus?

Repeat the test multiple times (5-10 repetitions).

This helps determine:

• Constant vs intermittent deviation

• Unilateral vs alternating fixation

Findings:

• Same eye always deviates → unilateral tropia

• Either eye can fixate → alternating tropia

• Eye turn seen only some of the time → intermittent tropia

Why should the examiner allow time for refixation during a UCT?

The patient must be given enough time for the uncovered eye to:

• Detect the target

• Acquire fixation

• Complete any refixation movement

Failing to wait may cause small or slow tropias to be missed.

What clinical information can be obtained from the Unilateral Cover Test?

Information	Can UCT Determine?
Tropia present?	✅ Yes
Direction?	✅ Yes
Laterality?	✅ Yes
Constant vs intermittent?	✅ Yes
Estimate magnitude?	✅ Rough estimate
Phoria?	❌ No

What is the Simultaneous Prism Cover Test (SPCT), and what does it measure?

The SPCT neutralizes the movement seen during the Unilateral Cover Test (UCT) and measures the size of the manifest deviation (tropia) only.

How is the Simultaneous Prism Cover Test performed?

• Identify the deviating eye during UCT.

• Place a prism over the deviating eye.

• Simultaneously cover the fixating eye.

• Observe the deviating eye's movement.

• Increase prism power until there is no movement.

• Confirm with reversal (bracketing).

What is the endpoint of the Simultaneous Prism Cover Test?

The endpoint is reached when the deviating eye shows no movement when the fixating eye is covered.

Bracketing (reversal):

• Too little prism → eye still moves in original direction.

• Neutralizing prism → no movement.

• Too much prism → eye moves in the opposite direction.

Example:

• 5Δ → eye moves out

• 8Δ → no movement (neutralized)

• 10Δ → eye moves in (reversal)

Thus the deviation is approximately 8Δ.

How does SPCT differ from UCT and PACT?

Test	Measures
UCT	Detects presence of a tropia
SPCT	Measures the manifest deviation (tropia)
PACT/ACT	Measures the total deviation (tropia + phoria)

In which patients should SPCT be performed?

SPCT should be performed only when a manifest strabismus (tropia) has already been identified.

Why?

• The test is designed to neutralize a visible deviation.

• No manifest deviation = nothing to neutralize.

What is the major advantage of the Simultaneous Prism Cover Test?

Because fusion is disrupted only minimally, SPCT measures the size of the manifest deviation alone, without allowing the latent component (phoria) to appear.

Clinical significance: It isolates the true tropic component of the deviation.

What is the primary objective of the Alternate Cover Test (ACT/PACT)?

The Alternate Cover Test measures the total angle of deviation () under fully dissociated (fusion-free) conditions.

This represents:

• Tropia + Phoria

• The patient's full deviation

How is the Alternate Cover Test performed?

• Patient fixates an accommodative target with best correction.

• Place prism over the deviating eye.

• Alternate the cover rapidly between eyes, ensuring full dissociation.

• Observe movement of the eye behind the prism.

• Increase prism power until neutralization.

• Continue until the first reversal is observed.

What is meant by “full dissociation” during the Alternate Cover Test?

Full dissociation means fusion is completely disrupted by alternately covering the eyes before binocular fixation can be re-established.

Why?

• Reveals the full latent deviation.

• Prevents fusional vergence from masking the true angle.

How is prism neutralization determined during ACT/PACT?

Use bracketing (reversal):

• Increase prism until no movement is seen.

• Continue increasing until the first reversal occurs.

• Record the highest neutral prism value.

Why should larger deviations be given additional time to refixate during ACT?

Larger angles require larger eye movements to regain fixation.

Allowing sufficient time:

• Improves measurement accuracy

• Prevents underestimation of deviation

• Makes reversal easier to identify

Clinical pearl: The bigger the deviation, the longer you should wait for complete refixation.

Why should ACT be repeated with the strabismic eye fixing?

Repeating the test with the strabismic eye fixing allows assessment for:

• Primary vs secondary deviation

• Incomitance

• Paretic deviations

High-yield: Differences between measurements with each eye fixing can suggest neurologic or muscle palsy rather than a simple comitant strabismus.

What test conditions are necessary for obtaining an accurate ACT/PACT measurement?

• Best-corrected visual acuity/refractive correction

• Accommodative fixation target

• Complete dissociation

• Adequate refixation time

• Appropriate starting prism estimate (often from UCT/SPCT)

What is the difference between the primary and secondary angle of deviation?

• Primary deviation (1°): Angle of deviation when the normal (non-paretic) eye is fixing.

• Secondary deviation (2°): Angle of deviation when the paretic eye is fixing.

Why is the secondary deviation larger than the primary deviation in a paretic muscle palsy?

According to Hering's Law of Equal Innervation, equal neural input is sent to yoked muscles in both eyes.

When the paretic eye must fixate, extra innervation is required to move the weak muscle. That increased innervation is also sent to the fellow eye's yoke muscle, causing a larger deviation.

How are primary and secondary deviations measured clinically?

Primary deviation

• Normal eye fixes

• Prism placed over the paretic/deviating eye

• Measure the angle of deviation

Secondary deviation

• Paretic eye fixes

• Prism placed over the non-paretic eye

• Measure the angle of deviation

What common errors can lead to inaccurate Alternate Cover Test (ACT/PACT) measurements?

• Inappropriate target or poor instructions

• Poor dissociation (cover moved too quickly)

• Inappropriate fixation (e.g., deep amblyopia)

• Poor fixation stability

• Insufficient reversal/bracketing

• Examiner observation errors

What is Hering's Law, and why is it important in strabismus evaluation?

Hering's Law of Equal Innervation states that yoked muscles in both eyes receive equal neural input.

Clinical significance:

• Explains why secondary deviation is larger than primary deviation in paretic strabismus.

• Helps identify cranial nerve palsies and incomitant deviations.

What special situations can make the Alternate Cover Test (ACT) more difficult to interpret?

Common challenges include:

• Latent nystagmus

• Rebound saccades

• Diplopia during cover testing

• Eccentric fixation

• Poor fixation stability

How should ACT be modified in a patient with latent nystagmus?

Covering an eye may induce or worsen latent nystagmus, making fixation unstable.

Strategies:

• Use a frosted occluder

• Use a high-plus lens instead of complete occlusion

What is a rebound saccade, and how can it affect ACT measurements?

A rebound saccade is an additional corrective eye movement that may occur after refixation.

Problem: It can make the examiner misjudge the true direction or size of the deviation.

Solution: Repeat the ACT several times and look for a consistent endpoint.

How does eccentric fixation (EF) affect cover test results?

Eccentric fixation can contaminate both UCT and ACT findings.

Effects on UCT:

• Alters the apparent direction of movement

• Alters the apparent size of movement

Effects on ACT:

• Produces inaccurate measurements of the angle of deviation

Why can eccentric fixation lead to inaccurate estimates of strabismus magnitude?

Because the patient fixates with an eccentric retinal locus rather than the fovea.

As a result:

• Refixation movements may appear larger or smaller than the actual deviation.

• The measured angle may not represent the true ocular misalignment.

What level of deviation can most trained examiners detect during ACT?

Most examiners can reliably detect approximately 2Δ of deviation.

What are the major causes of inaccurate ACT/PACT measurements?

• Poor fixation target

• Inadequate dissociation

• Covering too quickly

• Deep amblyopia

• Eccentric fixation

• Nystagmus

• Rebound saccades

• Insufficient bracketing/reversals

• Examiner sensitivity limitations

How does eccentric fixation affect the angle measured by ACT?

Eccentric fixation contaminates ACT measurements because fixation occurs with a non-foveal retinal locus.

∠Dt=∠Dm+EF

• Nasal EF = positive (+)

• Temporal EF = negative (−)

Therefore:

• Nasal EF → ACT underestimates the true angle

• Temporal EF → ACT overestimates the true angle

Why should you avoid stacking multiple prisms of the same orientation during strabismus measurements?

The effective prism power of stacked prisms is greater than the simple arithmetic sum of the individual prism powers.

• Stacking introduces measurement error.

• Error increases as prism power increases.

• Large deviations may be overestimated if prism stacking is not accounted for.

How should large-angle deviations be measured when high prism powers are required?

Instead of stacking multiple loose prisms in front of one eye, split the prism between both eyes.

When is prism stacking acceptable during strabismus testing?

Stacking prisms in different meridians is acceptable.

Examples:

• Horizontal prism + Vertical prism

• Base-out prism + Vertical prism