Each eye is moved by six extraocular muscles arranged in three antagonist pairs. The brain — via cranial nerves III (oculomotor), IV (trochlear), and VI (abducens) — sends precisely coordinated signals to both eyes simultaneously, ensuring they always point at the same object. This coordination is called binocular fusion, and it is what allows stereo depth perception.

• Medial rectus — turns the eye inward (adduction). Supplied by CN III.
• Lateral rectus — turns the eye outward (abduction). The only muscle supplied by CN VI.
• Superior rectus — turns the eye upward and inward. CN III.
• Inferior rectus — turns the eye downward and inward. CN III.
• Superior oblique — depresses, abducts, intorts the eye. The only muscle supplied by CN IV — the smallest cranial nerve.
• Inferior oblique — elevates, abducts, extorts the eye. CN III.

A squint develops when this coordinated system breaks down. The cause is rarely a structural failure of a muscle — the muscles themselves are typically intact. The problem is in the neural programming that coordinates them: an uncorrected refractive error that forces the brain to over-accommodate (farsightedness → convergent squint), an imbalance in tonic innervation, a neurological condition affecting cranial nerve supply, or an unknown developmental variance in the binocular fusion circuitry.

1. Uncorrected refractive error (most common cause in children). Farsightedness (hyperopia) is the primary cause of accommodative esotropia — the most common type in India. When a farsighted child tries to focus on near objects, the brain triggers excessive convergence (the eyes turn inward as part of the accommodation-convergence reflex). Without glasses to correct the underlying hyperopia, the convergence becomes excessive, the eye drifts inward, and a squint develops. Critically, this type of squint can often be fully corrected by glasses alone — no surgery required.

2. Prematurity. Premature infants have a significantly elevated risk of strabismus — 30–40% of strabismus cases in India involve prematurely born babies. Prematurity is associated with retinopathy of prematurity (ROP), intraventricular haemorrhage, and neurodevelopmental differences that affect the binocular fusion circuitry. Any premature infant should have a formal paediatric ophthalmology assessment at the corrected age of 3–4 months.

3. Family history. A child with a first-degree relative who had strabismus has a 30% risk of developing it themselves. The genetic basis is polygenic — multiple genes influence the development of binocular fusion. A positive family history should lower the threshold for early formal ophthalmological examination.

4. Neurological and systemic conditions. Cerebral palsy (a common cause of strabismus in India given birth asphyxia rates), Down syndrome (~50% develop strabismus), hydrocephalus, brain tumours (cranial nerve VI palsy causing esotropia is a false localising sign of raised intracranial pressure), and thyroid eye disease (causing a restrictive strabismus in adults) all cause or contribute to misalignment.

5. Unknown developmental variance. In many children who are otherwise healthy, have no refractive error, no family history, and no neurological condition, strabismus develops for reasons that remain incompletely understood — attributed to subtle differences in the binocular fusion circuitry that develops during the first years of life.

The amblyopia-strabismus relationship runs in both directions. Strabismus causes amblyopia by suppression of the deviated eye. Amblyopia, in turn, can make strabismus harder to treat — because the reduced visual acuity in the amblyopic eye weakens binocular fusion, which is one of the forces that keeps eyes aligned.

The correct management sequence is:

1. Correct the refractive error — glasses first, always.
2. Treat the amblyopia — patching or atropine to the better eye.
3. Align the eyes — surgery or botulinum toxin when indicated.
4. Maintain binocular vision — post-surgical monitoring and, in selected cases, binocular treatment (dichoptic therapy).

The sequence matters. Operating on a squint before treating amblyopia may fail because the poor vision in the amblyopic eye cannot sustain the binocular fusion needed to maintain alignment post-operatively.

The cover test is the most important clinical tool in strabismus assessment. It requires no equipment beyond a cover paddle and an accommodative target (a small picture or letter). It reveals:

• Cover test — cover one eye; watch the other. If the uncovered eye moves to take up fixation, the covered eye was deviated. This reveals a manifest tropia (constant squint visible without cover).
• Uncover test — uncover the eye after covering it; watch if the uncovered eye moves to re-take fixation. Movement on uncover reveals phoria (latent squint that appears only when binocular vision is disrupted).
• Alternate cover test — rapidly alternate cover between eyes; the total deviation (phoria + tropia) is revealed.

A complete strabismus assessment also includes:

• Cycloplegic refraction — mandatory. Using TRIDILATE (tropicamide + phenylephrine + lidocaine) for rapid reliable mydriasis, followed by cyclopentolate for full cycloplegia — the true refractive error must be measured to identify accommodative esotropia before any surgical decision.
• Ocular motility assessment — movement of the eye in all nine positions of gaze, identifying muscle under- or over-action.
• Angle measurement — prism cover test quantifies the angle of deviation in prism dioptres at distance and near — the data that determines surgical dose.
• Binocular function — stereoacuity testing (Randot, Titmus) quantifies the degree of binocular fusion and depth perception — important for outcome measurement and prognosis.
• Dilated fundus examination — using FLUROSCÉNE strips for corneal surface evaluation plus dilated posterior segment assessment to exclude organic causes of poor vision that might be mimicking amblyopia.

Accommodative esotropia is one of ophthalmology's most satisfying diagnoses — because the treatment is a pair of glasses, the result is often dramatic, and no surgery is needed. The mechanism: a farsighted child's visual system triggers excessive convergence to achieve focus. Prescribing the full hyperopic correction eliminates the accommodation demand, and the convergence normalises — the eyes straighten.

In practice, the ophthalmologist performs a cycloplegic refraction (cyclopentolate drops to paralyse accommodation) and measures the full underlying hyperopia. Glasses correcting the full hyperopic prescription are then worn full-time. The child is reviewed 6–8 weeks later. In pure accommodative esotropia, the eyes will be significantly straighter — or straight — with the glasses on.

Key points parents frequently ask about:

• "Will my child wear glasses forever?" Often, yes — until the hyperopia reduces naturally with eye growth (emmetropisation). Some children outgrow the need for glasses as they reach their teens, but this is not guaranteed. Do not stop glasses to "see if the squint comes back" without ophthalmological advice.
• "What if the squint doesn't fully correct with glasses?" Partially accommodative esotropia — where glasses reduce but don't eliminate the angle — still requires surgery for the residual non-accommodative component. Glasses must be maintained post-surgery for the accommodative component.
• "Can bifocals help?" Yes — bifocal spectacles or progressive lenses are used when the squint is worse at near than at distance, providing stronger near correction to reduce near-distance convergence excess.

Squint surgery moves the attachment point of one or more extraocular muscles to change their mechanical advantage — altering the pulling force on the eye and thereby changing its resting position. Two fundamental techniques:

• Recession — the muscle is detached from the eye and reattached further back (more posterior). This weakens the muscle's pull. Used when a muscle is over-acting — e.g., recessing the medial rectus to reduce convergence in esotropia.
• Resection — a segment of the muscle is removed and the shortened muscle reattached at its original insertion. This strengthens the muscle's pull. Used on the antagonist of an over-acting muscle — e.g., resecting the lateral rectus to increase abduction in esotropia.

In practice, most squint surgeries combine recession of one muscle with resection of its antagonist on the same eye (uni-ocular surgery), or recession of the same muscle on both eyes (bilateral surgery) depending on the type and angle of deviation. The surgical dose — how many millimetres to move the muscle — is calculated from the prism cover test angle measurement and standardised surgical dosage tables.

Adjustable suture technique — an important refinement available at specialist centres. The muscle is temporarily sutured in a way that allows the ophthalmologist to adjust the eye's position a few hours post-operatively, while the patient is awake. This allows fine-tuning based on the actual post-operative alignment rather than relying entirely on pre-operative dose calculations. Particularly useful for re-operations and complex cases. Requires patient cooperation, so it is typically reserved for adults and older children.

Surgery is performed under general anaesthesia in children and topical or local anaesthesia in adults. It is a daycare procedure — no overnight hospital stay is required. The eye is red, sore, and uncomfortable for 1–2 weeks post-operatively; eye drops (MOXGUARD intracameral moxifloxacin is used at the surgical and immediate post-operative stage for infection prophylaxis; topical antibiotic-steroid combinations are standard post-operative management) are prescribed for several weeks.

Re-operation rate: Approximately 10–25% of squint surgeries require a second procedure — either because of under-correction, over-correction, or a change in the deviation over time. This is a recognised feature of squint surgery, not a failure. Parents should be counselled that the goal is the best alignment achievable, and that some patients need more than one operation.

Botulinum toxin type A (Botox) injection into the over-acting extraocular muscle was introduced by Alan Scott in 1981 as a non-surgical treatment for strabismus. The toxin temporarily paralyses the injected muscle by blocking acetylcholine release at the neuromuscular junction. The effect lasts 2–4 months initially, during which the eye deviates in the opposite direction. As the toxin wears off, the muscle regains function — but the temporary deviation allows binocular fusion circuits to re-establish. In cases where this succeeds, lasting correction may occur even after the toxin's direct effect has worn off.

When botulinum toxin is used for squint in India:

• Acute esotropia in adults — where rapid correction of diplopia (double vision) is needed, and binocular fusion recovery is the primary goal.
• Small-to-moderate angle squints — particularly where surgery carries higher risk (frail patients, systemic conditions).
• Post-viral or acquired CN VI palsy — preventing contracture of the medial rectus while the lateral rectus recovers.
• Infantile esotropia (in conjunction with surgery planning) — some centres use botulinum toxin as a primary or adjunctive treatment.

Limitations: requires EMG or intraoperative endoscopic guidance for accurate placement, the effect is temporary and may need to be repeated, and it is less effective for large-angle squints than surgery. In India, botulinum toxin for squint is available at major paediatric ophthalmology and strabismus centres in metro cities.