Makindo Medical Notes

Squint (strabismus) is a misalignment of the visual axes such that the two eyes do not fixate the same point in space. It may be constant or intermittent, latent (phoria) or manifest (tropia), and either concomitant (angle of deviation roughly equal in all gaze positions) or incomitant (angle varies with gaze, e.g., due to extraocular muscle or cranial nerve dysfunction).

👶 Epidemiology & Risk Factors

• Onset typically in infancy/early childhood; prevalence ~2–4%. Family history is common.
• Risk factors: prematurity/low birth weight, high hypermetropia, anisometropia, congenital cataract, retinopathy of prematurity, cranial nerve palsies, syndromes (e.g., Down syndrome), and neurological disease.
• UK context: vision screening around age 4–5 years aims to detect amblyopia/strabismus early to exploit the critical neuroplastic window.

🧠 Pathophysiology (Why eyes turn?)

• Concomitant strabismus (usually childhood): due to abnormal sensory fusion & binocular development (e.g., accommodative esotropia from uncorrected hypermetropia → excess accommodative convergence).
• Incomitant strabismus (any age): mechanical or neurogenic limitation of one muscle (III, IV, VI palsies; thyroid eye disease; trauma; myasthenia; Brown/Duane syndromes).
• Prolonged misalignment in a child suppresses the deviating eye → amblyopia (cortical underdevelopment of visual acuity) unless treated during the sensitive period.

🧂 Nomenclature & Types

• Esotropia (inward), Exotropia (outward), Hypertropia/Hypotropia (vertical), Cyclotropia (torsional).
• Accommodative esotropia: improves with full hypermetropic correction (± bifocals if high AC/A ratio).
• Intermittent exotropia: often worse with distance/fatigue; may present with closing one eye in bright light.
• Pseudoesotropia: apparent crossing from wide epicanthic folds—Hirschberg reflex is central/symmetric.

🩺 History & Symptoms

• Age of onset, constant vs intermittent, precipitating illness/trauma, family history, refractive error, symptoms of diplopia, headaches/ast asthenopia.
• Child may have no diplopia (suppression), abnormal head posture (torticollis), eye rubbing, or poor school performance.
• Adults typically report new diplopia—always pathological until proven otherwise.

👁️ Examination (Stepwise)

1. Visual acuity (monocular, age-appropriate charts). Check fixation preference in infants (central–steady–maintained).
2. Ocular alignment:
   • Hirschberg corneal reflex: reflex offset from pupil center suggests angle (≈1 mm ≈ 15–22 prism dioptres).
   • Cover–uncover test: detects tropia (movement of the uncovered eye when the fellow is covered).
   • Alternate cover test: breaks fusion to reveal phoria and total deviation; quantify with prisms.
   • 4-prism-dioptre base-out test: assesses foveal suppression/microtropia.
3. Ocular motility in nine gaze positions; look for underactions/overactions, nystagmus, pain.
4. Binocular function (stereopsis: Frisby/TNO; Worth 4-dot for suppression/diplopia).
5. Anterior/posterior segment incl. red reflex (rule out leukocoria), fundus for maculopathy.
6. Refraction—cycloplegic (cyclopentolate) in children is essential.

🧪 Key Investigations

• Cycloplegic refraction (cornerstone in paediatric strabismus).
• Orthoptic measurements: prism cover test angle at distance/near; AC/A ratio (gradient method).
• Neuro work-up if indicated: MRI brain/orbits (acute CN palsy, neurological signs, papilloedema, suspected mass), thyroid function, myasthenia tests (ice pack, AChR antibodies) guided by clinical context.

🚩 Red Flags (Urgent UK referral)

• Acute onset painful ophthalmoplegia or pupil-involving CN III palsy → same-day to eye emergency (rule out aneurysm).
• New diplopia in adults, post-trauma strabismus, or rapidly progressive deviation.
• Leukocoria or absent red reflex (retinoblastoma/cataract), ptosis, proptosis, or systemic/neurological symptoms.
• Any infant with persistent misalignment beyond ~3 months of age.

🧩 Common Clinical Patterns (Pearls)

• Accommodative esotropia: onset 6–36 months; improves with full hyperopic correction; may need bifocals if near angle >> distance.
• Congenital/infantile esotropia: large, constant angle within first 6 months; limited abduction may mimic VI palsy; consider early bilateral medial rectus recessions ± botulinum.
• Intermittent exotropia: decompensates with distance, fatigue; assess control (Newcastle or LACTOSE scales); start with observation/orthoptics → surgery if poor control/symptoms.
• CN VI palsy: horizontal diplopia worse on distance gaze to affected side; causes include microvascular (diabetes, hypertension), raised ICP, trauma, tumours.
• Superior oblique palsy (CN IV): vertical/torsional diplopia, head tilt away; positive Bielschowsky head-tilt test.

🛠️ Management – Principles

1. Treat amblyopia: occlusion therapy (patching better eye), atropine penalisation, near tasks during patching. Most effective before ~7–8 years; diminishing returns thereafter.
2. Correct refractive error: full cycloplegic prescription; can fully correct accommodative esotropia. Consider bifocals for high AC/A ratio.
3. Orthoptic therapy: convergence exercises (e.g., pencil push-ups) for convergence insufficiency; prisms for symptomatic small deviations/diplopia.
4. Botulinum toxin: into overacting muscle in selected cases (infantile esotropia, acute nerve palsies) as diagnostic or therapeutic bridge.
5. Strabismus surgery: weaken (recession) or strengthen (resection/plication) muscles to align eyes; goal is alignment & binocular function, not spectacles independence.
6. Adults with diplopia: temporary occlusion/fresnel prisms, treat cause (e.g., microvascular palsy usually resolves in 3 months), definitive prisms or surgery once deviation stable.

🧯 UK Primary Care & Referral Notes

• Any suspected strabismus in a child → routine orthoptic/paediatric ophthalmology referral; urgent if associated with leukocoria, reduced red reflex, acute onset, or neurological signs.
• New-onset diplopia in adults → urgent same-day ophthalmology assessment; advise do not drive.
• Share-care with orthoptists is central in the NHS; early detection via health visitor/school screening improves outcomes.

🧪 Differentials

• Pseudoesotropia (epicanthic folds), nystagmus with null point head posture, convergence spasm, ocular media opacity (cataract) or macular disease causing eccentric fixation, thyroid eye disease, myasthenia gravis.

🧭 OSCE Tips (Exam-savvy)

• Do a cover–uncover and alternate cover test correctly; describe findings as “manifest esotropia, constant, comitant, angle ~20Δ by prism cover.”
• Demonstrate Hirschberg and measure with prisms; check stereopsis (Frisby/TNO) and suppression (Worth 4-dot).
• Always state you would perform cycloplegic refraction and fundus exam to exclude organic causes.

📈 Complications & Prognosis

• Amblyopia, loss of stereopsis, psychosocial impact. Earlier detection and full refractive/amblyopia therapy markedly improve binocular outcomes.
• Microvascular adult palsies often recover within 3–6 months; persistent incomitance may need prisms/surgery.

📝 Quick Case Vignette (Teaching Point)

A 3-year-old with new inward deviation when looking at near, hypermetropia +4.50D both eyes, reduced stereopsis. Full hyperopic glasses given; near angle improves but persists—bifocals prescribed for high AC/A. Amblyopia treated with patching. Take-home: correct the optics first, treat amblyopia early, then consider surgery if residual constant deviation threatens binocular development.

🔑 Take-Home Messages

• Think comitant vs incomitant: comitant → refractive/binocular development; incomitant → neurogenic/mechanical (investigate).
• Children rarely complain of diplopia—look for suppression/amblyopia; adults with new diplopia need urgent assessment.
• Management sequence: acuity → refraction → alignment (orthoptics/botox/surgery), with amblyopia therapy early.

Cases

• Case 1 — Infantile Esotropia: A 10-month-old baby presents with a constant inward deviation of the left eye noted since 6 months of age. No visual complaints yet but parents report abnormal fixation. Plan: Urgent orthoptic/ophthalmology referral to assess amblyopia risk; cycloplegic refraction; likely need for glasses ± strabismus surgery before age 2.
• Case 2 — Intermittent Exotropia: A 7-year-old child is brought in with outward drift of the right eye, especially when tired or daydreaming. Vision otherwise normal, no diplopia reported. Plan: Orthoptic assessment, refractive correction, consider patching/exercises; surgery if control poor or psychosocial impact significant.
• Case 3 — Adult Acquired Diplopia (CN VI Palsy): A 65-year-old man with new horizontal diplopia worse when looking left; exam shows impaired abduction of the left eye. History of poorly controlled diabetes and hypertension. Plan: Urgent ophthalmology/neuro referral to confirm sixth nerve palsy; investigate vascular risk factors and exclude intracranial pathology; advise on prism lenses/patching until resolved.