Squint (Strabismus)

Definition

"Strabismus, also called squint or cross-eye, means lack of fusion or misalignment of the eyes in one or more of the visual coordinates: horizontal, vertical, or rotational." — Guyton and Hall Textbook of Medical Physiology

Types

Classification

By Comitance

• Comitant (concomitant): Angle of deviation is the same in all directions of gaze. Typically childhood-onset.
• Incomitant (paralytic/restrictive): Angle varies with gaze direction; due to extraocular muscle palsy or mechanical restriction (e.g. thyroid eye disease, blow-out fracture, nerve palsy).

By Onset

• Congenital / early-onset: Often presents within first 6 months
• Acquired: May be accommodative, paralytic, or secondary to systemic disease

Aetiology

• Failure of binocular fusion mechanisms in early development
• Refractive errors: Hypermetropia is the most common cause; accommodative esotropia typically appears between 18–36 months
• Anisometropia: Unequal refraction between the two eyes
• Extraocular muscle palsy: CN III, IV, or VI palsies
• Mechanical restriction: Thyroid ophthalmopathy, orbital floor fracture
• Systemic/neurological disease: Diabetes mellitus, stroke, brain tumours, cerebral palsy, raised intracranial pressure
• Underlying ocular pathology: Retinoblastoma, macular scarring, optic disc hypoplasia (mandatory to exclude)

Consequences

Amblyopia (Lazy Eye)

Sensory Adaptations in Children

• Suppression: Active cortical inhibition of the deviating eye's image
• Abnormal retinal correspondence (ARC): A new, anomalous correspondence develops between non-corresponding retinal points
• Microtropia: Small-angle squint (<10Δ) with partial binocular cooperation; a central suppression scotoma is characteristic

In Adults

Clinical Evaluation

History

• Age of onset (earlier onset → higher amblyopia risk; later onset → more likely accommodative)
• Variability (intermittent vs. constant)
• Type of diplopia (horizontal, vertical, cyclovertical)
• Family history (strabismus is frequently familial)
• Birth history, developmental milestones, general health

Visual Acuity

• Pre-verbal children: fixation behaviour, preferential looking tests
• Verbal children (≥3 years): crowded logMAR letter/picture tests (Kay pictures, Keeler logMAR)
• LogMAR tests preferred over Snellen for monitoring amblyopia therapy

Cover Tests

• Cover test: Detects manifest deviation — cover the fixing eye and look for movement of the deviating eye to take up fixation
• Uncover test: Detects latent deviation — adduction of the uncovered eye = exophoria; abduction = esophoria
• Alternate cover test: Reveals the total deviation (manifest + latent) by disrupting fusion
• Prism cover test: Quantifies the angle of deviation in prism dioptres (Δ); performed in primary position, 8 diagnostic gaze positions, and near fixation

Sensory Tests

• Worth four-dot test: Differentiates binocular single vision (BSV), ARC, suppression
• Bagolini striated glasses: Detects BSV, ARC, or suppression under natural conditions
• Synoptophore: Tests 3 grades of binocularity — simultaneous perception, fusion (with motor fusion range), stereopsis; also detects ARC

Stereopsis Tests

• Titmus test: Polarized vectorgraph; fly (3000"), animals (400–100"), circles (800–40")
• TNO / Frisby: Random dot tests — best evidence of high-grade BSV

Other Tests

• Hess chart / Lees screen: Maps ocular motility in incomitant strabismus (muscle palsies, restrictive disease); primary and secondary deviation can be distinguished
• Field of binocular single vision: Goldmann or tangent screen; useful in paretic strabismus follow-up
• Diplopia charting / botulinum toxin test: For intractable diplopia
• Angle kappa: Large positive angle kappa can mimic exotropia (pseudo-squint) — important to distinguish

Refraction and Fundoscopy

• Cycloplegic refraction: Mandatory; hypermetropia is most common refractive cause
• Dilated fundoscopy: Mandatory — exclude retinoblastoma, macular scarring, optic disc hypoplasia

Management

Optical Correction

Amblyopia Treatment

• Occlusion (patching) of the dominant eye to stimulate the amblyopic eye
• Atropine penalisation as an alternative
• Best results achieved during the sensitive period (before ~8 years of age)

Prisms

• Fresnel or ground-in prisms to neutralise deviation, especially in diplopia management and in elderly adults with sagging eye syndrome (ARDE)

Botulinum Toxin

• Injected into the overacting muscle; diagnostic and therapeutic use (especially for acute paralytic strabismus and intractable diplopia)

Surgery

• Indicated when optical treatment fails to achieve alignment
• Recession (weakening): Muscle insertion moved posteriorly
• Resection (strengthening): Section of muscle removed, reattached at original insertion
• Adjustable sutures: Improve outcomes; suture tension adjusted postoperatively under topical anaesthesia
• Goal in early-onset esotropia: alignment within 8Δ with peripheral fusion and central suppression
• Inferior oblique overaction: Treated surgically if significant