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OP10.2 | Heterotropia and Strabismus — SDL Guide (Part 3)

Diagnosis, Differentials, and Amblyopia Assessment

The most common diagnostic pitfall at the MBBS level is failing to distinguish pseudostrabismus from true strabismus. Pseudostrabismus is the appearance of a squint in a child with straight eyes — most commonly caused by a prominent epicanthal fold (covering the nasal sclera and mimicking esotropia) or a wide nasal bridge, combined with a positive angle kappa. In pseudostrabismus: the Hirschberg reflex is SYMMETRICALLY placed (centred or slightly temporal due to angle kappa); the alternate cover test shows NO corrective movement; ductions are full. Parents can be firmly reassured. With growth and reduction of the epicanthal fold, the apparent squint resolves.

Amblyopia assessment is mandatory in every child with strabismus. Amblyopia is defined as a reduction in best-corrected visual acuity in one eye (compared to the fellow eye or to age norms) that is not attributable to any structural abnormality of the eye or visual pathway. It is graded as:
- Mild: VA 6/9 to 6/12
- Moderate: VA 6/18 to 6/36
- Severe: VA 6/60 or worse

The key clinical test is monocular visual acuity with crowded optotypes (not a single large letter, which underestimates amblyopia due to the absence of the crowding phenomenon — isolated optotypes are easier than surrounded optotypes for an amblyopic eye). A child who steadily refuses to have the straight eye covered (cries, tries to push the occluder away) is exhibiting the fixation preference sign — a strong indication of amblyopia in the other eye.

Differentiating accommodative from non-accommodative esotropia: Try the patient's cycloplegic refraction as glasses for 6–8 weeks and recheck the angle of deviation with glasses on vs off. Full correction abolishes the deviation entirely = fully accommodative. Partial improvement = partly accommodative (surgery for residual angle). No improvement = non-accommodative (surgery required).

Management and Indications for Referral

Management of strabismus is multistep and must address both the deviation and any associated amblyopia. The sequence follows the principle: treat amblyopia first, then align the eyes.

Step 1 — Refractive correction: Prescribe the full cycloplegic refractive error. For accommodative esotropia, this alone may fully resolve the deviation. Even for non-accommodative strabismus, correcting any refractive error optimises vision and prevents refractive amblyopia.

Step 2 — Amblyopia treatment: Penalise the better (fixing) eye to force the amblyopic eye to work. Options include: direct occlusion (patching the better eye — the mainstay; patch for 2–6 hours per day depending on severity and age); pharmacological penalisation with atropine drops in the better eye (blurs near vision in that eye, forcing the child to use the amblyopic eye for near tasks — useful when the child refuses patching); optical penalisation (deliberate over-minussing of the better eye's lens). Treatment continues until VA is equal or stabilises.

Step 3 — Strabismus surgery (if needed): Surgery is indicated when the deviation persists after refractive correction and amblyopia treatment, or when the deviation is non-accommodative from the outset. The goal is cosmetic alignment and, in young children, restoration of the chance for binocular vision. Standard procedures: medial rectus recession (weakening, for esotropia), lateral rectus resection (strengthening, for esotropia), and the reverse for exotropia. Surgery in young children ideally occurs before age 3–4 for the best chance of binocular vision development.

Step 4 — Botulinum toxin: Chemodenervation of an overacting muscle (e.g. medial rectus in acute CN VI palsy — prevents contracture while awaiting recovery; or as primary treatment for small-angle esotropia).

Indications for referral (MBBS level):
- Any child with a suspected squint, regardless of age — refer to a paediatric ophthalmologist. Pseudostrabismus can be confirmed and dismissed, but true strabismus must not be delayed.
- Any child with strabismus and reduced visual acuity — urgent referral; the critical period is closing.
- Any adult with new-onset diplopia — requires exclusion of neurological, thyroid, or orbital pathology.
- Any strabismus with associated systemic features (headache, papilloedema, systemic hypertension, diabetes) — coordinate with the relevant specialty.
- Congenital esotropia with large angle — early referral for surgical planning.

Self-Assessment: Strabismus Classification and Management

Having worked through the classification, pathophysiology, examination, and management of strabismus, you now need to consolidate this learning into the clinical reasoning chain you will use at the bedside and in assessments. Strabismus is one of the most commonly examined topics in ophthalmology OSCEs precisely because it integrates anatomy (EOM anatomy and cranial nerve supply), physiology (the accommodation-convergence reflex and binocular vision development), clinical examination (cover tests, Hirschberg, ductions, versions), and management principles (refractive correction first, then amblyopia therapy, then surgery) in a single coherent clinical encounter. The examiner's expectation at the MBBS level is that you can classify any presented squint case along the five axes, interpret the cover test and Hirschberg findings correctly, identify when cycloplegic refraction is mandatory, recognise amblyopia and explain its treatment, and state the appropriate referral pathway. The key distinctions below are the most frequently tested points — if you can answer them fluently without hesitation, you are well-prepared.

Classification checklist: For any strabismus case, can you state the: direction (eso/exo/hyper), laterality (unilateral/alternating), comitance (comitant/incomitant), onset (congenital/acquired), and accommodative status?

Key distinctions to revise:
- Comitant: full ductions, equal angle all gazes, no diplopia (child), amblyopia risk.
- Incomitant: limited ductions, angle varies with gaze, diplopia (adult).
- Pseudostrabismus: symmetric Hirschberg, negative cover test, epicanthal fold.
- Amblyopia: reduced VA not correctable with spectacles; treat before age 7–8.
- Accommodative esotropia: resolves with spectacle correction of hypermetropia.

Key referral rule: ALL children with a suspected squint should be referred — even if you suspect pseudostrabismus, a specialist must confirm. The cost of a missed amblyopia is a lifetime of visual impairment in one eye.