Squint (Strabismus)

Epidemiology

• Affects ~4% of the population
• Amblyopia co-occurs in approximately 50% of strabismus patients
• Amblyopia without strabismus accounts for the other 50% of amblyopia cases

Classification

Direction of Deviation

Constancy

• Tropia: manifest deviation (always present)
• Phoria: latent deviation (only manifest when fusion is disrupted, e.g., by cover test)

Comitance

• Concomitant (comitant): angle of deviation is the same (<5Δ variability) in all positions of gaze — implies no muscle or nerve pathology
• Incomitant: angle varies with gaze position — suggests a paretic or restrictive cause

Esotropia (top) — corneal reflex displaced temporally; Exotropia (bottom) — corneal reflex displaced nasally

Esotropia — Detailed Classification (Table 18.1, Kanski)

Accommodative

• Refractive (fully accommodative): due to uncorrected hypermetropia (+2 to +7D), normal AC/A ratio; presents 18 months–3 years; fully corrected by spectacles
• Partially accommodative: reduced but not eliminated by full spectacle correction
• Non-refractive (convergence excess): high AC/A ratio — near deviation > distance deviation; bifocals may help

Non-accommodative

• Early-onset (essential infantile) esotropia: onset <6 months; large angle (>30Δ); alternating fixation; cross-fixation in lateral gaze; may mimic bilateral 6th nerve palsy
• Microtropia: small angle (<5Δ), often with amblyopia and eccentric fixation
• Sensory esotropia: due to poor vision in one eye
• Consecutive esotropia: follows over-correction of exotropia
• Acute-onset esotropia: sudden onset in older children/adults; needs neurological evaluation
• Cyclic esotropia: esotropia alternates on a regular 48-hour cycle
• Divergence insufficiency/paralysis

Exotropia

• Constant (early-onset): large, constant angle; frequently associated with neurological anomalies
• Intermittent exotropia: most common type overall; begins as exophoria breaking into exotropia with fatigue, bright light, or inattention; children often close one eye in sunlight
  • Distance excess: larger deviation for distance
  • Basic: equal deviation near and distance
  • Convergence insufficiency: larger deviation for near
• Sensory exotropia: secondary to visual loss in the deviating eye
• Consecutive exotropia: follows over-correction of esotropia

Causes

• Central: abnormalities of oculomotor nuclei / cranial nerves (III, IV, VI palsies)
• Refractive: uncorrected hypermetropia (esotropia) or myopia (exotropia)
• Anatomical: structural muscle abnormalities
• Sensory: reduced vision in one eye
• Restrictive: orbital pathology (e.g. thyroid eye disease), fibrosis syndromes

Consequences — Amblyopia

• Strabismic: monocular suppression of the deviating eye
• Anisometropic: difference in refractive error ≥1D between eyes
• Stimulus deprivation: media opacity (cataract), ptosis covering the pupil
• Bilateral ametropic: high symmetric hypermetropia
• Meridional: uncorrected astigmatism

Diagnosis

Clinical Tests

1. Corneal light reflex (Hirschberg test): penlight held at 33 cm; normally symmetric reflex in both pupils. Decentration indicates deviation (~1 mm displacement ≈ 7Δ)
2. Cover test (most important): patient fixates a target; cover one eye and observe the other for movement
   • Cover-uncover test: detects tropias
   • Alternating cover test: detects phorias
3. Red reflex: helps detect media opacities causing deprivational amblyopia
4. Cycloplegic refraction: mandatory in all children with esotropia (atropine or cyclopentolate)
5. Prism and alternate cover test: quantifies the angle

Warning Signs Requiring Urgent Evaluation

• Any esotropia in infancy — rule out retinoblastoma (white reflex / leukocoria)
• Acute-onset diplopia — may indicate intracranial pathology (CN III/IV/VI palsy, raised ICP)

Treatment

Amblyopia Treatment (First)

• Patching (occlusion) of the better-seeing eye — most effective; duration depends on age and severity
• Atropine penalisation (1% atropine to the better eye): equally effective to patching; useful when compliance with patches is poor
• Both modalities must be started before age 7–8 years for best outcomes; treatment before age 2–3 years gives the best prognosis
• Failure after 6 months of effective occlusion → further improvement unlikely

Refractive Correction

• Full cycloplegic refraction with spectacles is mandatory
• Fully accommodative esotropia: spectacles alone correct the deviation
• Convergence excess: bifocals reduce the near deviation
• Myopic exotropia: full myopic correction may control exodeviation

Surgery

• Early-onset esotropia: bilateral medial rectus recession — goal is alignment within 8Δ (peripheral fusion with central suppression acceptable)
• Exotropia: lateral rectus recession ± medial rectus resection
• Timing: most surgeons operate before age 2 years for infantile esotropia to maximise binocularity

Botulinum Toxin

• Can be injected into the antagonist muscle (e.g., medial rectus in esotropia) as an alternative or adjunct to surgery, particularly for small to moderate deviations

Key Points