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🎯 Squint (Strabismus) - Complete NEET PG Guide

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1. DEFINITIONS - Get These Right First

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2. PSEUDO-SQUINT - The Common Trap

• Positive angle kappa (fovea temporal to anatomical centre) → nasal displacement of corneal reflex → looks like exotropia but isn't (pseudoexotropia)
• Negative angle kappa → looks like esotropia but isn't
• Most common cause: Retinopathy of Prematurity (ROP) displaces the macula

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3. THE AC/A RATIO - High-Yield Concept

• = prism dioptres (Δ) of convergence per dioptre (D) of accommodation
• Normal = 3-5Δ/D

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4. SENSORY CONSEQUENCES OF SQUINT

Confusion

• Both foveae are stimulated by different objects simultaneously
• Brain sees two superimposed but different images

Diplopia (Double Vision)

• Same object falls on non-corresponding retinal points
• Esotropia → homonymous (uncrossed) diplopia
• Exotropia → heteronymous (crossed) diplopia

How Children Adapt (Sensory Adaptations)

1. Suppression - cortex actively ignores the image from the deviating eye (prevents diplopia/confusion)
2. Abnormal Retinal Correspondence (ARC) - the deviating eye's extrafoveal point pairs with the fixing eye's fovea; the brain creates a new "false" correspondence so images merge despite the squint

ARC is tested with: Bagolini striated glasses, Synoptophore, Worth 4-dot test

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5. AMBLYOPIA - The Most Important Consequence

• Amblyopia = reduced VA in one eye (or both) due to abnormal visual experience during the sensitive period (birth to ~7-8 years)
• Squint is one of the top causes
• Sensitive period: birth to 7-8 years (most plastic before age 3)
• After 7-8 years, cortical connections become fixed - this is why early treatment matters

• Strabismic amblyopia - from suppression of deviating eye
• Anisometropic amblyopia - from unequal refractive errors
• Stimulus deprivation amblyopia - from cataract, ptosis etc (worst prognosis)

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6. COVER TESTS - Must Know

Cover-Uncover Test (detects manifest squint = tropia)

1. Patient fixes on target
2. Cover one eye - watch the uncovered eye:
   • If uncovered eye moves to take up fixation → that eye was deviating = tropia present
3. Uncover - watch the previously covered eye:
   • If it moves on uncover → it was suppressing = phoria

Alternate Cover Test (reveals total deviation including phoria)

• Shift cover rapidly back and forth between eyes, fully dissociating fusion
• More sensitive - reveals the total deviation

Prism Cover Test (measures the angle)

• Prisms are placed with base opposite to direction of deviation
• Esotropia → base-OUT prism
• Exotropia → base-IN prism
• Hypertropia → base-DOWN before the higher eye
• End-point = no movement on alternate cover testing

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7. CLASSIFICATION OF SQUINT

A. Concomitant (Comitant) vs Incomitant

B. Types of Esotropia (Convergent Squint)

Infantile (Congenital) Esotropia

• Onset before 6 months
• Large angle (>30-40Δ), constant, alternating
• Associated: cross-fixation, inferior oblique overaction, DVD (Dissociated Vertical Deviation), latent nystagmus
• No refractive error (not accommodative)
• Treatment: Surgery (bilateral medial rectus recession)

Refractive Accommodative Esotropia

• Age: 18 months - 3 years
• Cause: high hypermetropia (+2 to +7 D), normal AC/A ratio
• Accommodation to focus stimulates excess convergence
• Treatment: Full hypermetropic correction → eyes straighten with glasses
• Near and distance deviation are equal

Non-Refractive Accommodative Esotropia (High AC/A type)

• Normal refractive error, but high AC/A ratio
• Squint worse at near than distance (≥10Δ difference)
• Treatment: Bifocal glasses (reduce accommodative effort for near) + full cycloplegic refraction

Microtropia

• Small angle deviation (<10Δ), often undetectable on casual examination
• Has a central suppression scotoma
• Diagnosed with: 4Δ prism base-out test
  • Normally a 4Δ prism causes a movement; in microtropia with central scotoma - no movement seen

Consecutive Esotropia

• Esotropia after surgical overcorrection of exotropia

Sensory Esotropia

• Due to poor vision in one eye (cataract, optic atrophy, retinoblastoma)
• Always do fundus examination in all children with squint to rule out retinoblastoma!

C. Types of Exotropia (Divergent Squint)

Intermittent Exotropia

• Most common exotropia in children
• Eyes straight at near, deviate outwards at distance (convergence holds near fixation)
• Worse when tired or unwell
• Tends to progress if untreated

Consecutive Exotropia

• Exotropia after surgical overcorrection of esotropia

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8. SPECIAL SYNDROMES - High Yield for NEET PG

Duane Retraction Syndrome

• Congenital fibrosis/absence of 6th nerve nucleus; LR innervated by branches of 3rd nerve
• Globe retracts + palpebral fissure narrows on adduction (pathognomonic)
• Huber Classification:

• Type I is most common
• Associations: Wildervanck syndrome, Goldenhar syndrome, thalidomide
• Treatment: Most need no surgery; surgery only for significant head posture or cosmesis
• Do NOT resect the lateral rectus (worsens retraction!)

Brown Syndrome

• Restriction of elevation in adduction (cannot look up and inward)
• Due to tight/shortened superior oblique tendon (sheath) preventing it from passing smoothly through the trochlea
• Congenital Brown: superior oblique tendon abnormality
• Acquired Brown: swelling near trochlea (RA, trauma, surgery)
• Treatment: Usually conservative; surgery (superior oblique tendon lengthening) if binocular function compromised

Dissociated Vertical Deviation (DVD)

• Associated with infantile esotropia
• One eye drifts upward (and extorts) when attention is distracted or eye is covered
• No corresponding downward movement of the other eye (breaks Hering's law - hence "dissociated")
• Usually bilateral, but asymmetric
• Treatment: Superior rectus recession ± posterior fixation suture

Inferior Oblique Overaction (IOOA)

• Associated with infantile esotropia and DVD
• Eye elevates in adduction
• V-pattern exotropia often associated

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9. ALPHABET PATTERNS

• V opens Up (like the letter V) → divergent/exo up top → inferior oblique overaction
• A opens Down (like the letter A) → divergent/exo at bottom → superior oblique overaction

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10. HESS CHART

• Smaller chart = affected (paretic) eye
• Larger chart = normal (unaffected) eye
• Underaction in one area of smaller chart → overaction of the yoke muscle shown in larger chart
• Over time, secondary contracture makes charts look more equal (become concomitant)

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11. SYNOPTOPHORE (Major Amblyoscope)

1. Grade I - Simultaneous macular perception (simultaneous perception)
2. Grade II - Fusion (ability to fuse two different images)
3. Grade III - Stereopsis (depth perception)

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12. TREATMENT OF SQUINT - Sequential Approach

Step 1: Correct refractive error first

• Full cycloplegic refraction (cyclopentolate 1% or atropine 1%)
• Hypermetropia correction may fully correct accommodative esotropia

Step 2: Treat amblyopia

• Occlusion (patching) of the better eye: forces the amblyopic eye to work
• Atropine penalization: blur the better eye with atropine drops (alternative to patching)
• Must be done within the sensitive period (before 7-8 years)

Step 3: Surgery (if needed)

• After refractive error correction and amblyopia treatment
• Goals: restore binocular single vision, cosmesis, maintain/improve stereopsis

• For esotropia → weaken medial rectus (recession) and/or strengthen lateral rectus (resection)
• For exotropia → weaken lateral rectus (recession) and/or strengthen medial rectus (resection)

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13. HIGH-YIELD ONE-LINERS FOR NEET PG

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14. QUICK DIAGRAM - Extraocular Muscle Actions

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