Strabismus

Epidemiology & Risk Factors

• Prevalence: ~2–4% of pediatric population.
• Normal variant: Intermittent strabismus is normal in infants < 4 months of age (due to immature ocular motor control). Persistence ≥ 4 months is pathologic.
• Risk Factors:
  • Prematurity / Low birth weight (LBW).
  • Positive family Hx.
  • Neurologic conditions: Cerebral palsy (CP), hydrocephalus, Down syndrome.
  • Uncorrected refractive errors (e.g., hyperopia leading to accommodative esotropia).
  • Ocular pathology: Retinoblastoma, congenital cataracts.

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Clinical Features

• Ocular Misalignment:
  • Esotropia: Inward/nasal deviation (most common).
  • Exotropia: Outward/temporal deviation.
  • Hypertropia/Hypotropia: Vertical deviation (upward/downward).
• Compensatory behaviors: Head tilt, abnormal head posture, or squinting/closing one eye in bright sunlight.
• Visual symptoms:
  • Young children: Suppression of image from the deviating eye (often asymptomatic, but leads to visual loss).
  • Older children / Adults: Diplopia (double vision) or asthenopia (eye strain).

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Diagnosis

• Initial / Bedside Screening:
  • Corneal light reflex test (Hirschberg test): Asymmetric deflection of light off the corneas (normal = symmetric, central reflex).
  • Cover test / Cover-uncover test:
    • Cover test: Detects manifest deviation (tropia). The uncovered deviating eye shifts to fixate when the straight eye is covered.
    • Cover-uncover test: Detects latent deviation (phoria).
• Essential Workup:
  • Cycloplegic refraction: To assess for uncorrected refractive errors (hyperopia).
  • Dilated funduscopic exam: Crucial to rule out underlying intraocular pathology (e.g., retinoblastoma, optic nerve hypoplasia). c
• Imaging (MRI/CT Orbit & Brain):
  • Indicated if: Sudden/acute onset, paralytic/incomitant strabismus (CN III, IV, or VI palsy), presence of neurologic signs, papilledema, or suspected retinoblastoma/intracranial neoplasm.

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Differential Diagnostics

• Pseudostrabismus:
  • Diff: False appearance of esotropia due to prominent epicanthal folds or a broad, flat nasal bridge. Shows symmetric corneal light reflex and normal cover test.
• Retinoblastoma:
  • Diff: Manifests with leukocoria (white pupillary reflex) and intraocular mass visible on fundoscopic exam / ocular US / MRI.
• Cranial Nerve Palsy (CN III, IV, VI):
  • Diff: Sudden onset, limited extraocular motility (incomitant deviation depending on direction of gaze). Often associated with trauma, ↑ ICP, ischemia, or neoplasm.
• Congenital Cataract:
  • Diff: Leukocoria, absent or abnormal red reflex; visual deprivation leads to secondary sensory strabismus.

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Management

Primary goal: Prevent irreversible amblyopia and preserve binocular vision (depth perception).

1. First-Line / Conservative:
   • Refractive Correction: Prescribe corrective lenses (glasses) (curative for accommodative esotropia driven by hyperopia).
   • Occlusion Therapy (for Amblyopia):
     • Patching the sound/dominant eye for several hours daily to force stimulation of the amblyopic eye.
     • Pharmacologic Penalization: Topical atropine drops placed in the sound eye (blurs near vision, forcing use of the amblyopic eye; alternative if poor patch adherence).
2. Second-Line / Surgical:
   • Extraocular Muscle (EOM) Surgery (recession/resection):
     • Indicated for: Congenital (infantile) esotropia, persistent deviation despite full refractive/patching therapy, or large non-accommodative deviations.
     • Optimal timing: Usually performed early (prior to age 1–2) for congenital esotropia to maximize binocular fusion.
3. Refractory / Alternative:
   • Botulinum toxin (Botox) injection: Direct injection into the overacting EOM (temporary realignment or adjunct in select cases).

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Complications

• Loss of Stereopsis: Deficits in binocular depth perception.
• Contracture of Extraocular Muscles: Chronic misalignment causing mechanical shortening of the affected rectus muscle.
• Psychosocial and cosmetic impairment.

Amblyopia

• Pathophysiology: Functional ↓ visual acuity (VA) due to abnormal visual cortex development and image suppression during early childhood (< 7–8 yo).
• Etiologies:
  • Strabismic (most common): Ocular misalignment (esotropia/exotropia) causes suppression of the deviated image.
  • Refractive / Anisometropic: Unequal refractive error between eyes; visual cortex favors the clearer image. c
  • Deprivational (most severe): Physical occlusion of visual axis (e.g., congenital cataract, severe ptosis, retinoblastoma).
• Clinical Features:
  • Typically asymptomatic; detected via routine screening.
  • Squinting, head-tilting, bumping into objects, poor stereopsis (depth perception).
  • PE: Asymmetric VA (≥ 2-line difference on chart), abnormal corneal light reflex (Hirschberg), or abnormal red reflex (Brückner).
• Diagnosis:
  • Screening: Red reflex in infants; formal VA testing (LEA symbols, Snellen) starting at age 3–5.
  • Workup: Cycloplegic refraction (detects refractive errors) + dilated fundoscopy (rules out retinoblastoma, cataract, retinal pathology).
  • Criteria: ↓ VA not immediately correctable by refraction alone, in the absence of structural retinal/optic nerve defects.
• Management (critical before age 7–8 due to visual neuroplasticity):
  • Initial: Treat primary etiology (urgent cataract removal; corrective lenses for refractive error).
  • First-line: Occlusion therapy (patching) of the sound (better-seeing) eye for 2–6 hrs/day.
  • Alternative / Second-line: Atropine 1% drops instilled in the sound eye (blurs near vision; preferred if poor patch compliance).
  • Refractory / Residual strabismus: Extraocular muscle surgery after amblyopia is maximally treated.
• Complications:
  • Irreversible vision loss / blindness in the amblyopic eye if untreated past critical window (> 8 yo).
  • Reverse (occlusion) amblyopia in the sound eye due to excessive patching or atropine use.