Amblyopia, Stereopsis, and Color Vision

Fusion

The process by which two eye images are combined into a single perception. Neurons and extraocular muscles work together to maintain a clear, single image.

Motor Fusion

The use of vergence eye movements to position the eyes to corresponding points.

Sensory Fusion

The neurophysiological/psychological process of combining two images in the visual cortex.

Version

A binocular movement or rotation of the eyes in the SAME direction.

Vergence

A binocular movement of the eyes in the OPPOSITE direction.

Deviation

A difference in eye position between the two eyes.

Phoria (Heterophoria)

A deviation that can be compensated for with vergence movements. Not present in full binocular viewing.

Tropia (Strabismus)

A deviation that cannot be compensated for with vergence movements. Present in both binocular and monocular viewing.

Purpose of Cover Test

To measure fixation deviations, assess presence or absence of motor fusion, and determine the magnitude of demand on the patient's fusional vergence system.

Additional Purposes of Cover Test

Assess eye position, measure direction and magnitude of binocular vision issues, and diagnose neuro pathway issues.

Cover Test Materials Needed

Visual acuity chart, near target, occluder, overhead lamp, horizontal and vertical prism bars or loose prisms.

Cover Test Setup

Patient wears habitual correction. Use appropriate target size. Full room illumination. Examiner holds occluder without blocking the target.

Glasses Effect on Cover Test

Myopes are more likely to show false exo. Hyperopes are more likely to show false eso. Accommodation is linked to convergence.

Cover Test Procedure

Two parts: Cover-Uncover test (differentiates phoria vs tropia) and Alternating Cover test (determines direction and size). Both done at distance and near. Cover-Uncover is done first.

Cover-Uncover Test (Cover phase)

Differentiates phoria from tropia. Watch the uncovered eye. Phoria: visual axes align with both eyes open. Tropia: one eye aligned, the other misaligned.

Cover-Uncover Test (Uncover phase)

Determines if tropia is alternating or unilateral. Alternating: both eyes can fixate when uncovered. Unilateral: patient habitually fixates with one eye.

Cover-Uncover Interpretation

No movement = phoria. Movement = tropia. Both eyes move = alternating tropia. Only one eye moves = constant tropia.

Alternating Cover Test

Determines direction and magnitude of phoria or tropia. Does not differentiate between phoria and tropia. Performed after cover-uncover.

Alternating Cover Test Procedure

Cover each eye alternately for 3-5 seconds while patient views a target. Observe movement of uncovered eye. Use prism bars until no movement is seen. Reversal = too much prism.

Bracketing

Increase prism until reversal of movement occurs, then step down. Used to verify orthophoria or confirm measurement.

Recording Cover Test

Record direction, eye, frequency, magnitude, and comitancy. Use standardized shorthand.

Comitant vs Noncomitant Deviations

Comitant: deviation the same in all gaze positions. Noncomitant: deviation varies ≥5 prism diopters, often due to stroke, tumor, or trauma.

Cover Test Terminology - Constant

A tropia is always present in all conditions.

Cover Test Terminology - Intermittent

A tropia is sometimes present, depending on conditions such as fatigue.

Cover Test Terminology - Alternating

A tropia that switches between right and left eye fixation.

Cover Test Recording Notes

No symbol for hypo deviations. Record the hyper eye. Lateral phorias do not designate an eye, but vertical phorias do. Use apostrophe (') to indicate near.

Cover Test Magnitude

Recorded in prism diopters, written before direction, using Δ symbol. Example: 12Δ EP' = 12 prism diopter esophoria at near.

Examples of Cover Test Notation

12Δ XP' = 12 prism diopter exophoria at near. 25Δ LET = 25 prism diopter left esotropia. 4Δ RHT = 4 prism diopter right hypertropia. 10Δ IRXT = 10 prism diopter intermittent right exotropia.

Amblyopia Definition

Unilateral or bilateral reduction in best corrected visual acuity in an otherwise normal eye caused by atypical visual experience early in life. Prevalence 1-5.5% of children.

Criteria for Amblyopia Diagnosis

Unilateral: ≥ 2 logMAR line difference. Bilateral: 20/40 or worse in better eye at age 4+. Under 4 years: worse than 20/50.

Severity of Amblyopia

Moderate: 20/40 - 20/80. Severe: 20/100 or worse.

Age of Amblyopia Diagnosis

Typically diagnosed between ages 3-8. Upper sensitive period is \~8 years. Difficult before age 3 due to variable VA.

Why Early Detection is Important

Children often do not report blurry vision because they don't know normal. Early exams are critical.

Impact of Amblyopia

Reduced accommodation, fixation, stereopsis, and contrast sensitivity. Causes visual-motor deficits and suppression.

Amblyopia Risk Factors

Refractive error, strabismus (especially esotropia), premature birth, first-degree relative with amblyopia.

Types of Amblyopia

Form deprivation amblyopia, Strabismic amblyopia, Anisometropic amblyopia.

VA Testing for Amblyopia

Single letter vs crowding. Crowding reveals amblyopia more reliably. Must be consistent with optotype type across visits.

Preliminary Testing for Amblyopia

Includes stereopsis/stereoacuity tests, Worth dot fusion test, color vision testing, near point of accommodation (NPA), near point of convergence (NPC).

Near Point of Accommodation (NPA)

A subjective test of accommodative ability. Performed monocularly and binocularly. Push-up or pull-away methods.

Expected NPA Values by Hofstetter Formula

Minimum = 15 - (age/4). Average = 18.5 - (age/3). Maximum = 25 - (age/4).

NPA Push-Up Method

Cover one eye. Move target closer at \~5 cm/sec until blur reported. Measure distance in cm from spectacle plane.

NPA Pull-Away Method

Start target at spectacle plane (blurred). Move away slowly until target clears. Measure distance in cm from spectacle plane.

NPA Recording

Record in centimeters. Convert to diopters by 1 / (distance in meters).

Near Point of Convergence (NPC)

Performed binocularly. Move target closer until patient reports diplopia (break) then away until fusion regained (recovery). Expected: 5 cm / 10 cm.

NPC Recording

Record as Break/Recovery. If no diplopia, record as BON (Bridge of Nose).

Stereopsis Definition

Ability to perceive depth based on minimum retinal disparity detectable. Requires both eyes seeing slightly different images.

Binocular vs Monocular Depth Cues

Stereopsis (binocular) is innate. Monocular cues (perspective, parallax, overlay, shadow, horizon relation) are learned.

Factors Affecting Stereopsis

Clarity of retinal images, eye alignment, suppression, amblyopia, strabismus.

Stereopsis Clinical Uses

Required for vocations (pilots, police, military). Diagnosing binocular anomalies. Measuring treatment outcomes. Prognosis prediction.

Stereopsis Limitations

Not useful beyond \~660 m (20 arcsec threshold).

Stereopsis in Strabismus

Constant strabismus: zero random dot stereopsis. Intermittent strabismus: reduced stereopsis, often absent at certain distances.

Stereopsis in Amblyopia

Reduces both contour and random dot stereopsis, especially random dot.

Stereopsis Testing Indications

All children <14, young adults with reading complaints, and when final binocular assessment required.

Types of Stereo Tests

Contour stereographs (line stereopsis, monocular clues present). Randot stereographs (random dot stereopsis, no monocular clues).

Stereo Tests at Distance

Wirt circles (Polaroid filters). Random Dot "E" test (cards with or without "E").

Stereo Tests at Near

Stereo Fly (Titmus fly, animals, Wirt circles), Randot stereo (RDS + Wirt circles), Stereo Butterfly, Stereo Reindeer, TNO test.

Stereo Fly Test

Large fly target for gross stereopsis (3000 arcsec). Ask patient to pinch fly's wings. Positive = above page.

Stereo Fly Suppression Check

Ask what letter (L) and circle (R) patient sees. Missing one indicates suppression of that eye.

Stereo Animals Test

Animals at 400, 200, and 100 arcsec. Ask which pops off page (cat, rabbit, monkey).

Stereo Animals Passing Criteria

Preschool expected 120 arcsec. Kindergarten passing = 120 arcsec, borderline = 100 arcsec, fail if miss >1.

Wirt Circles

Test from 800 to 40 arcsec. Stop when two misses in a row.

Adult Stereo Expected Results

Normal adults: 40 arcsec. Fail if worse than 60 arcsec.

Recording Stereo Results

Record in arcsec. Example: Stereopsis 50" Fly N'. If key unknown, record # correct / # tested.

Randot Stereo Test

Book with four parts: RDS targets, animals, suppression test, Wirt circles (20-400 arcsec).

Randot Test Part 1

RDS targets at 500 and 250 arcsec. Patient must identify one from each set.

Randot Test Part 2

Animals at 400, 200, 100 arcsec. Same as Stereo Fly animals.

Randot Test Part 3

Suppression test using boxes with R + L letters.

Randot Test Part 4

Wirt circles (20-400 arcsec) with random dot background. Most sensitive test.

Randot Recording

Record arcsec threshold. Example: 20" Randot N'. Abnormal example: 80" / 500" RDS.

Other Stereo Tests

Stereo Butterfly (child-friendly). Stereo Reindeer (cheap, less accurate). TNO (red-green anaglyph, 2000-15 arcsec). Frisby/Lang (plates, no glasses).

Flat Fusion Tests

Used if poor stereopsis or suspected strabismus. Worth 4-dot test is most common.

Worth 4-Dot Test

Uses red-green anaglyph glasses. 4 dots: 2 green, 1 red, 1 white. White dot perceived as pink or mixed color.

Worth 4-Dot Procedure

Monocular check: OD covered = 3 green, OS covered = 2 red. Binocular: normal = 4 dots.

Worth 4-Dot Abnormal Responses

2 red = OS suppressed. 3 green = OD suppressed. 5 dots = diplopia.

Worth 4-Dot Diplopia Types

Uncrossed diplopia = eso deviation (red dots right of green). Crossed diplopia = exo deviation (red dots left of green). Vertical diplopia = hyper deviation.

Worth 4-Dot Recording

Record number of dots, test distance, and lighting conditions. Example: Worth dot: 4 @ near, 5 eso @ 5 ft, 3 dots @ 10 ft in dark.

Color Vision Clinical Uses

Identify hereditary and acquired color defects, evaluate macular function, test aptitude for occupations requiring color discrimination.

Color Vision Indications

Children (especially boys), young adults before driving, law enforcement, DOT, firefighters, electricians, lab workers, older patients on medications.

Prevalence of Color Defects

\~8% of males, \~0.5% of females. X-linked inheritance. Varies by ethnicity.

Types of Color Vision Defects

Red-green (Deutan = green defect, 95%; Protan = red defect, 4%). Tritan (blue defect, <1%).

Types of Color Vision Perception

Trichromats (normal), Dichromats (missing one cone, suffix "-opia"), Anomalous trichromats (weak cone response, suffix "-anomalous"), Monochromats (two missing cones or rod-only).

Color Blindness Misconception

Most color-deficient people see colors but differently. Only rod monochromats are truly color blind and legally blind.

Deutan Confusions

Violet vs blue-purple, pink vs dull green, magenta vs brown, tan vs olive, brown vs green, orange vs yellow.

Protan Confusions

Purple vs blue-purple, magenta vs gray, tan vs olive, brown vs red, yellow-green vs yellow.

Protans Handicap

Reduced sensitivity at dusk or in dim light.

Types of Color Vision Tests

Pseudoisochromatic plates, arrangement tests (chips), matching tests (yarn).

Ishihara Plates

Most common pseudoisochromatic test. Red-green only. Multiple editions.

Ishihara Plate Types

Vanishing figures, transformed figures, hidden digits, diagnostic plates.

Ishihara Demo Plate

Plate 1, number "12" must be seen by all.

Ishihara Instructions

Ask patient to read number within 3 seconds per plate. Test monocularly. Children/illiterate: line tracing.

Ishihara Recording

Record score out of total plates per eye. Example: Color OD 12/13, OS 11/13, Method Ishihara.

HRR Plates

Test both red-green and blue-yellow defects. Includes demo plates, screening plates, and diagnostic plates.

HRR Instructions

Ask patient to identify shape (X, O, Δ) on each plate. If any missed in plates 5-10, proceed to diagnostic plates.

HRR Recording

Record number correct over total. Example: Color OD 10/10, OS 10/10, Method HRR.

Arrangement Tests

Patient orders colored chips around a color circle. More diagnostic but longer.

Matching Tests

Yarn test, patient matches yarn colors. Real-world but unreliable.

Lighting for Color Testing

Use full spectrum (5000K, CRI >85) daylight-corrected bulbs. Avoid sunlight or warm fluorescent.

Color Vision Acquired Defects

70% of ocular/systemic diseases cause color defects, often blue-yellow (tritan). Seen in glaucoma, diabetes, drug toxicity.

Acquired Color Defects Causes

Glaucoma, diabetes, medications, aging, optic nerve/retina disease.

Chromatopsia

Abnormal condition where objects appear tinged with color. Can result from drugs, chemicals, stress, UV exposure.