Key Facts

Strabismus (squint) is a misalignment of the visual axes where the eyes do not point in the same direction. Amblyopia (lazy eye) is reduced visual acuity in one or both eyes due to abnormal visual development in childhood, not correctable with spectacles alone. Early detection and treatment during the critical period of visual development are essential.

Epidemiology

• Strabismus: affects 2-5% of children
• Amblyopia: affects 2-3% of children
• Esotropia (convergent squint) is the most common type in childhood
• Amblyopia is the most common cause of monocular visual impairment in children
• Family history increases risk (first-degree relative with strabismus: 3-5× risk)

Aetiology

• Concomitant (non-paralytic) strabismus: most common in children
  • Infantile esotropia: onset before 6 months
  • Accommodative esotropia: associated with hypermetropia, onset 2-3 years
  • Intermittent exotropia
• Incomitant (paralytic) strabismus: cranial nerve palsy (III, IV, VI), trauma, raised ICP
• Amblyopia causes: strabismus (most common), anisometropia (different refractive error between eyes), stimulus deprivation (cataract, ptosis)
• Risk factors: prematurity, family history, Down syndrome, cerebral palsy

Pathophysiology

• During the critical period (birth to 7-8 years), the visual cortex requires equal, clear, aligned input from both eyes
• In strabismus, the deviated eye sends a different image, causing diplopia and confusion
• The developing brain suppresses the image from the deviated eye to eliminate diplopia
• Chronic suppression during the critical period leads to amblyopia — failure of normal cortical development for that eye
• After the critical period, cortical plasticity diminishes and amblyopia becomes increasingly difficult to reverse

Strabismus

• Visible misalignment of the eyes (may be constant or intermittent)
• Head tilt or face turn (compensatory posture)
• Closing one eye in bright light (intermittent exotropia)
• Diplopia (in acquired strabismus; children typically suppress)
• Family may notice deviation in photographs (corneal reflections asymmetric)

Amblyopia

• Reduced visual acuity in one eye not correctable with spectacles
• Often asymptomatic — detected on screening
• Child may not be aware of monocular visual deficit
• May have associated strabismus

Types of Strabismus

• Esotropia: inward deviation (convergent squint)
• Exotropia: outward deviation (divergent squint)
• Hypertropia: upward deviation
• Hypotropia: downward deviation

Red Flags

• Absent or white red reflex (leukocoria) — retinoblastoma until proven otherwise
• New-onset squint after 6 months of age
• Sudden-onset diplopia (cranial nerve palsy — raised ICP, aneurysm)
• Acquired squint with headache and vomiting (posterior fossa tumour)
• Any squint in a child with proptosis or periorbital swelling

Bedside

• Visual acuity (age-appropriate: Kay pictures, Sheridan-Gardiner, LogMAR, Snellen)
• Cover test: cover-uncover (manifest squint) and alternate cover test (latent squint)
• Corneal light reflex (Hirschberg test): asymmetric corneal reflections indicate misalignment (1mm = ~7° deviation)
• Red reflex (Brückner test): asymmetric or absent red reflex
• Prism cover test: quantifies angle of deviation in prism dioptres
• Eye movements: assess in all nine positions of gaze
• Cycloplegic refraction: essential to identify refractive error (cyclopentolate 1%)

Bloods

• Not routinely required
• Anti-AChR antibodies if myasthenia gravis suspected
• Blood glucose (diabetic cranial nerve palsy)

Imaging

• MRI brain: if incomitant strabismus, new-onset palsy, or suspected intracranial pathology
• MRI orbits: if orbital pathology suspected
• USS eyes: if leukocoria/abnormal red reflex (retinoblastoma assessment)

Special Tests

• Orthoptic assessment: detailed binocular vision and ocular motility evaluation
• Fundoscopy: exclude retinal pathology
• OCT if structural abnormality suspected

Non-pharmacological

• Correct refractive error: spectacles (full cycloplegic correction); this alone resolves accommodative esotropia and may improve amblyopia
• Occlusion therapy (patching): patch the dominant eye for 2-6 hours/day to force use of amblyopic eye
  • Most effective before age 7 (PEDIG trials)
  • Duration depends on severity: mild (2 hours/day), moderate (4 hours/day), severe (6 hours/day)
• Prism glasses: for small-angle deviations or compensatory head posture

Pharmacological

• Atropine penalisation: atropine 1% drops to the dominant eye on weekends or daily (blurs near vision, forcing use of amblyopic eye)
  • Non-inferior to patching for moderate amblyopia (PEDIG ATS)
  • Better compliance than patching in some patients

Surgical

• Strabismus surgery: recession (weakening) or resection (strengthening) of extraocular muscles
  • Indications: constant strabismus not corrected by spectacles, significant deviation, cosmetic concern
  • Usually performed after maximum non-surgical treatment of amblyopia
  • Adjustable sutures in adults
• Botulinum toxin injection: to extraocular muscles as alternative or adjunct to surgery

Referral Criteria

• Urgent ophthalmology referral: absent/abnormal red reflex (2-week wait), new-onset squint >6 months of age, sudden diplopia
• Routine referral: any confirmed squint on cover testing, reduced visual acuity on screening
• Orthoptic assessment: all children with suspected squint

• Amblyopia: >70% achieve improved visual acuity with treatment started before age 7 (PEDIG trials)
• After age 10-12, amblyopia treatment is largely ineffective
• Accommodative esotropia: >60% controlled with spectacles alone
• Strabismus surgery: alignment achieved in 80-90% after first procedure; 10-20% require re-operation
• Untreated amblyopia is permanent — leads to lifelong monocular visual impairment
• Binocular vision may not be fully restored even with successful treatment if strabismus was long-standing
• Long-term patching compliance is the main determinant of treatment success

UK Vision Screening Programme

PEDIG Amblyopia Treatment Studies — Key Findings

Types of Cover Test Responses