10. Assessment of amblyopia

What symptoms should you expect in a patient with amblyopia?

Most patients have no symptoms because the better eye compensates. Possible complaints include:

• Blurred vision

• Eye rubbing or squinting

• Headaches

• Asthenopia

Why is amblyopia clinically significant beyond reduced visual acuity?

Amblyopia can cause:

• Nearly double the lifetime risk of bilateral visual impairment

• Limitations in occupations with visual requirements

• Visual-motor deficits

• Slow or impaired reading with increased saccades

• Reduced quality of life

• Slower completion of visually demanding tasks

What visual-function abnormalities may be associated with amblyopia?

Major associated deficits include:

• Reduced visual acuity

• Accommodative and oculomotor dysfunction

• Reduced contrast sensitivity

• Impaired spatial judgment

• Unsteady or inaccurate monocular fixation

• Increased contour interaction

• Positional inaccuracy or distortion

• Distortion of retinotopic maps

What is the crowding phenomenon in amblyopia?

Crowding is disproportionately poorer recognition of an optotype when it is surrounded by nearby contours or other optotypes compared with when it is presented alone.

It represents abnormal contour interaction, in which neighboring contours interfere with identification of the target.

What mechanisms are thought to produce abnormal crowding in amblyopia?

Proposed mechanisms include:

• Abnormal or excessive lateral inhibition

• Abnormally large receptive fields

• Abnormal eye movements

These mechanisms cause confusion between the target and its surrounding contours.

In which type of amblyopia is crowding especially prominent?

Crowding is generally greater in strabismic amblyopia.

Approximately 66% to 90% of strabismic amblyopes demonstrate abnormal crowding or contour interaction.

Why can isolated optotype acuity overestimate vision in amblyopia?

An isolated optotype removes the surrounding contours responsible for crowding, allowing the patient to perform better than under real-world or full-chart conditions.

Therefore, amblyopia testing should use:

• A full chart, or

• Single optotypes with crowding/surround bars

What features should a good visual acuity test for amblyopia have?

It should include:

• Enough lines to measure reduced acuity

• LogMAR progression

• Equal numbers of optotypes per line

• Optotypes of similar difficulty

• Contour interaction through a full chart or crowding bars

Acuity should not be based on isolated, uncrowded letters or symbols.

Which optotypes should be used to assess amblyopia at different ages?

• Children younger than approximately 7 years: LEA or HOTV

• Older children and adults: ETDRS or Sloan letters

• Standardized pediatric examination: ATS HOTV

Regardless of age, use a full chart or optotypes with crowding bars and LogMAR progression

Which acuity tests should be avoided when evaluating amblyopia?

Avoid tests that use isolated, uncrowded letters, symbols, or lines without contour-interaction bars.

Examples listed in the slides include:

• Allen figures

• Sjögren hand

• Sailboat or lighthouse charts

• Tumbling E

• Landolt C

• Nonstandard Snellen presentations

What must be ruled out before attributing reduced visual acuity to amblyopia?

Rule out an ocular or optical explanation, including:

• Uncorrected refractive error

• Corneal distortion

• Ocular media abnormalities

• Ocular pathology

Refraction, ocular health testing, and, when appropriate, multiple-pinhole testing help distinguish these causes.

Why should HOTV and ETDRS acuities not be treated as perfectly interchangeable in amblyopia?

Compared with ETDRS, crowded HOTV may:

• Overestimate acuity by approximately 0.68 lines in the amblyopic eye

• Overestimate acuity by approximately 0.25 lines in the better eye

• Underestimate the interocular acuity difference by approximately 0.4 lines

The discrepancy is most relevant when acuity is worse than approximately 0.3 logMAR.

What is the overall structure of the ATS HOTV visual acuity protocol?

The ATS protocol uses:

1. Screening

2. Phase I

3. Reinforcement if needed

4. Phase II retesting

5. Assignment of acuity as the lowest level passed

It uses single HOTV optotypes with surround bars.

How is the screening portion of the ATS acuity protocol performed?

• Begin at approximately 20/100 or 20/400, depending on the protocol or patient.

• Present progressively smaller optotypes.

• Continue until the patient misses an optotype.

How is Phase I of the ATS protocol performed?

• Begin 2 LogMAR levels larger than the level missed during screening.

• If the patient reaches 20/20 during screening, begin Phase I at 20/30.

• Continue toward smaller optotypes.

• A level is passed with 3 of 3 or 3 of 4 correct.

What happens if a child misses two letters at an ATS acuity level?

Provide reinforcement by showing three larger letters.

Then proceed to Phase II, in which the last level missed during Phase I is presented again.

• If passed, continue to smaller levels.

• Stop when the patient fails a level.

• Recorded visual acuity is the smallest level passed in Phase I or Phase II.

What practical techniques improve the reliability of pediatric amblyopia acuity testing?

• Use a matching or lap card

• Watch carefully for peeking

• Confirm that the child is looking before presenting the optotype

• Use encouragement and reinforcement

• Allow the child’s best attempt

• To reduce fatigue, show one optotype per line until a miss, then move up two lines or to 20/30 and test threshold formally

How can vision be assessed in an infant who cannot complete optotype acuity testing?

Use preferential-looking testing, such as Teller Acuity Cards, to estimate grating acuity.

An interocular difference of approximately 0.5 to 1 octave may be significant, but an abnormal result should be retested before diagnosing amblyopia.

Why can Teller Acuity Cards miss amblyopia?

Teller cards measure grating acuity, not recognition acuity.

In amblyopic eyes:

• Grating acuity is often better than optotype acuity.

• The discrepancy becomes greater with more severe amblyopia.

• Sensitivity is approximately 80%, so a normal result does not completely exclude amblyopia.

What infant acuity measure may be more sensitive to amblyopia than grating acuity?

Vernier acuity cards may have greater sensitivity to amblyopia than standard preferential-looking grating acuity cards.

What aspects of fixation should be documented in an infant or young child?

Assess whether fixation is:

• Central, steady, and maintained, abbreviated C&S&M

• Fixes and follows, abbreviated F&F

• Preferred equally or unequally between the eyes

• Maintained after the preferred eye is uncovered

Fixation preference may provide an estimate of interocular visual asymmetry when optotype acuity cannot be obtained.

How is fixation preference assessed in a patient with manifest strabismus?

1. Identify the fixing and deviating eyes.

2. Cover the preferred fixing eye.

3. Allow the deviating eye to take up fixation.

4. Uncover the preferred eye.

5. Observe how long the previously deviating eye maintains fixation.

Rapid return to the preferred eye indicates a stronger fixation preference.

How is fixation preference tested when there is no strabismus or only a small-angle deviation?

Use the induced tropia test:

• Place a 10 to 15 prism-diopter vertical prism before one eye while the child fixates a target.

• Repeat with the prism before the other eye.

• Observe fixation behavior and preference.

The prism creates a vertical separation that permits comparison of fixation by the two eyes.

How is fixation preference graded from Grade 0 to Grade 4?

• Grade 0, absolute: Preferred eye immediately resumes fixation after uncovering.

• Grade 1, strong: Nonpreferred eye holds fixation only momentarily.

• Grade 2, moderate: Nonpreferred eye holds until the next blink.

• Grade 3, slight: Nonpreferred eye maintains fixation through the next blink.

• Grade 4, none: Equal alternation between the eyes.

How do fixation-preference grades relate to the expected interocular acuity difference?

Lower fixation-preference grades generally correspond to a larger interocular acuity difference:

• Grade 4: approximately 0 logMAR

• Grade 3: approximately 0.10 logMAR

• Grade 2: approximately 0.17 logMAR

• Grade 1: approximately 0.30 logMAR

• Grade 0: approximately 0.80 logMAR

How reliable is fixation-preference testing for detecting amblyopia?

Fixation preference has limited sensitivity overall:

• Any amblyopia: approximately 31%

• Anisometropic amblyopia: approximately 20%

• Strabismic amblyopia: approximately 80%

It is therefore more useful for strabismic amblyopia than anisometropic amblyopia.

When is fixation-preference testing most clinically useful, and what is its main limitation?

It is most useful when testing shows either:

• A very strong or absolute preference, or

• No fixation preference with free alternation

Intermediate findings are less definitive. False-positive results are common, possibly because normal ocular dominance can resemble an amblyopic fixation preference.

Do not diagnose or exclude amblyopia from fixation preference alone.

How do contrast-sensitivity losses differ between anisometropic and strabismic amblyopia?

• Anisometropic amblyopia: More uniform and extensive loss across the spatial-frequency range

• Strabismic amblyopia: More asymmetric and generally less severe loss, concentrated at higher spatial frequencies

In general, greater amblyopia severity is associated with greater contrast-sensitivity loss.

What are the key refraction considerations when evaluating amblyopia?

• Perform careful retinoscopy, especially in children younger than approximately 6 years.

• Use lens bars or trial lenses as needed.

• In strabismus, occlude one eye or use prism when necessary to obtain an accurate reflex.

• Perform cycloplegic refraction to identify the full refractive error.

• Consider binocular refinement when developmentally appropriate.

What pupil findings may occur in amblyopia, and how clinically useful are they?

Reported findings include:

• Longer pupillary-constriction latency

• A mild relative afferent pupillary defect

However:

• Pupillary abnormalities occur in only a minority of patients.

• They do not correlate reliably with visual acuity.

• They do not predict response to occlusion therapy.

A substantial RAPD should therefore raise concern for disease other than uncomplicated amblyopia

What binocular vision and ocular-motor testing should be included in an amblyopia assessment?

• Versions

• Ductions when indicated

• Cover testing

• Measurement of tropia or phoria

• Assessment of comitancy

• Evaluation for A/V patterns and syndromes

• Stereoacuity testing with both random-dot and contour tests

An A/V pattern may be suggested by a difference of approximately 10 to 15 prism diopters between upgaze and downgaze.

What testing error must be avoided during stereoacuity assessment?

Do not allow the patient to inspect the stereotest before putting on the appropriate glasses.

Seeing the monocular contours without the glasses may provide clues that allow the patient to identify the correct response without true stereopsis.

How can central versus eccentric fixation be evaluated in amblyopia?

Fixation location can be assessed using:

• Visuoscopy with a direct ophthalmoscope

• MITT box

• Maxwell’s spot

These methods determine where the patient places the fixation target relative to the fovea.

How does fixation typically differ between refractive and strabismic amblyopia?

• Refractive amblyopia: Usually central but may be unsteady

• Strabismic amblyopia: Eccentric fixation is more common

• Unilateral high myopia: May be an exception in which refractive amblyopia demonstrates eccentric fixation

Eccentric fixation has been reported in approximately 23% to 82% of patients with strabismic amblyopia.

Why is identifying eccentric fixation clinically useful?

Eccentric fixation can:

• Support a diagnosis of strabismic or combined-mechanism amblyopia

• Help distinguish pure refractive amblyopia from combined amblyopia

• Serve as a prognostic indicator

What forms of suppression should be assessed in a patient with amblyopia?

Evaluate for both:

• Central suppression

• Peripheral suppression

Suppression testing complements acuity, fixation, strabismus, and stereoacuity assessment when evaluating binocular function.

A child performs much better with an isolated HOTV letter than with a line of HOTV letters. What does this indicate?

This indicates the crowding phenomenon, reflecting abnormal contour interaction.

The isolated optotype reduces interference and may overestimate functional acuity. Use a full chart or single optotypes with standardized surround bars.

A preverbal child freely alternates fixation between the two eyes. How should this be interpreted?

Free alternation corresponds to approximately Grade 4 fixation preference and suggests little or no interocular acuity difference.

However, fixation testing is not perfectly sensitive, so amblyopia cannot be excluded without age-appropriate acuity and a complete examination.

A child with no obvious strabismus shows a strong preference during the induced tropia test. What does this suggest, and what should be done next?

It suggests possible unequal visual acuity between the eyes, but it is not diagnostic by itself because ocular dominance can produce a false-positive result.

Next steps include:

• Standardized crowded monocular acuity

• Cycloplegic refraction

• Ocular health examination

• Repeat testing if reliability is questionable

A patient has a reduced pupil response and severe unilateral acuity loss presumed to be amblyopia. What is the key clinical concern?

Although a mild pupillary asymmetry can occur in amblyopia, pupil findings do not reliably correlate with its severity.

A clear or substantial RAPD should prompt investigation for retinal, optic-nerve, or other afferent disease rather than being automatically attributed to amblyopia.