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Squint (Strabismus)

Definition

Squint, formally called strabismus, is a misalignment of the eyes in one or more visual coordinates — horizontal, vertical, or rotational — such that both eyes cannot simultaneously direct their gaze at the same point in space.

— Guyton & Hall Textbook of Medical Physiology; Kanski's Clinical Ophthalmology, 10th ed.

Types

Type	Direction of deviation
Esotropia	Eye turns inward ("cross-eyed")
Exotropia	Eye turns outward ("wall-eyed")
Hypertropia	Eye turns upward
Hypotropia	Eye turns downward

When the deviation is latent (only manifest when binocular fusion is disrupted), it is called a phoria (esophoria, exophoria, etc.). When it is manifest at all times, it is a tropia.

Incomitant strabismus: the angle of deviation varies with gaze direction (usually due to muscle palsy). Concomitant strabismus: the angle of deviation stays the same regardless of gaze direction.

Causes

Childhood onset

Failure of development of normal binocular fusion mechanisms
Anisometropia (difference in refractive error between the eyes) → oculomotor imbalance
Abnormal neuronal "set" of the conjugate eye movement control pathways in early infancy
Associated conditions: cerebral palsy, retinopathy of prematurity, prematurity/low birth weight

Adult onset

Weakness or mechanical restriction of extraocular muscles
Cranial nerve palsies (III, IV, VI)
Systemic disease: diabetes mellitus, stroke, brain tumours, trauma

Sensory Consequences

When a squint develops, two troublesome perceptions arise:

Confusion — Two different images are superimposed because separate objects stimulate corresponding retinal points (including both foveae) simultaneously.
Pathological diplopia (double vision) — The same object stimulates non-corresponding retinal points in the two eyes, producing two images in different positions.
- In esotropia → homonymous (uncrossed) diplopia
- In exotropia → heteronymous (crossed) diplopia

Sensory adaptations (children only — up to age 6–8 years)

Due to visual cortical plasticity, children adapt by:

Suppression — active cortical inhibition of the image from the deviating eye
Abnormal retinal correspondence (ARC) — a new pairing between the non-foveal point of the deviated eye and the fovea of the fixating eye

Persistent unilateral suppression leads to strabismic amblyopia ("lazy eye") — reduced best-corrected visual acuity in the deviating eye with no structural pathology.

Amblyopia (Lazy Eye)

A critical complication. Types include:

Strabismic amblyopia — from continuous monocular suppression of the deviating eye
Anisometropic amblyopia — from blurred image in the more ametropic eye
Stimulus deprivation amblyopia — from media opacities (cataract, ptosis)

The earlier the onset of squint in childhood, the greater the risk of amblyopia.

Motor Adaptation — Compensatory Head Posture (CHP)

Children and adults with incomitant strabismus (e.g. muscle palsy) often adopt a head posture to maintain binocular single vision (BSV) and avoid diplopia:

Face turn — for horizontal deviations (head turns toward the field of the weak muscle)
Head tilt — for torsional/vertical deviations (e.g. right superior oblique palsy → head tilt to the left)
Chin elevation/depression — for elevator/depressor muscle weakness

Pseudo-squint

A large angle kappa (>5°) can simulate an exotropia when none exists. This is common in retinopathy of prematurity where macular displacement causes the corneal light reflex to appear nasally displaced, mimicking a squint.

Clinical Evaluation

History: age of onset, duration, variability (intermittent = some binocularity retained), diplopia type, family history (strabismus is frequently familial), birth history, prior spectacle/patch use.

Key principle: the later the onset of esotropia in childhood, the more likely an accommodative component (arising especially between 18–36 months).

Management Principles

Goal	Intervention
Correct refractive error	Spectacles (especially for accommodative esotropia)
Treat amblyopia	Occlusion (patching) of the dominant eye; must be done during the sensitive period
Restore binocular alignment	Strabismus surgery (recession/resection of extraocular muscles); adjustable sutures improve outcomes
Non-surgical alignment	Prisms; botulinum toxin injection
Orthoptic exercises	For convergence insufficiency

Organic disease must be excluded before treating amblyopia. Surgery is more likely needed the earlier the onset of the squint.

Sources: Kanski's Clinical Ophthalmology, 10th ed. | Guyton & Hall Textbook of Medical Physiology

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