Introduction to Strabismus

Strabismus is misalignment of the visual axes so that the two foveae do not simultaneously fixate the same object. Prevalence is about 2 to 4 percent of children.

• Terms: heterotropia (manifest deviation present with both eyes open), heterophoria (latent deviation controlled by fusion, revealed on dissociation), comitant (same angle in all gaze directions) versus incomitant (angle varies with gaze), concomitant, constant versus intermittent, alternating versus unilateral.
• Normal binocular single vision needs clear images, aligned axes, functioning extraocular muscles, sensory fusion and a visual cortex able to integrate both images. Disruption at any point produces strabismus.
• Mechanisms: refractive (accommodative esotropia in hypermetropia), sensory (poor vision in one eye from cataract, retinal or optic nerve disease), motor or neuromuscular (nerve palsy, myasthenia, restriction), innervational (infantile esotropia, intermittent exotropia), mechanical (orbital fracture, thyroid disease, fibrosis), and cortical.
• Risk factors: family history, prematurity, low birth weight, maternal smoking in pregnancy, cerebral palsy, Down syndrome, craniofacial anomalies, uncorrected refractive error, retinoblastoma and other causes of visual loss.

• In children: visible eye turn, abnormal head posture, squinting in bright light, eye closure, avoidance of near tasks; suppression avoids diplopia and leads to amblyopia.
• In adults: diplopia, asthenopia, headaches and visual confusion; new onset deviation indicates acquired disease needing investigation.
• Esodeviations (inward), exodeviations (outward), hyperdeviations and hypodeviations, cyclodeviations; patterns A and V (horizontal deviation changes between up and down gaze by 10 or more prism dioptres for A pattern, 15 or more for V pattern).
• Signs of sensory loss: a strabismic eye with poor acuity, leukocoria, abnormal red reflex.
• Accommodative convergence and abnormal anomalous retinal correspondence in long standing childhood deviations.

• History: age of onset, constant or intermittent, family history, birth history, trauma, systemic illness, previous treatment.
• Visual acuity by age appropriate method (preferential looking, picture charts, Snellen), cycloplegic refraction (cyclopentolate 1 percent, 1 drop twice 5 minutes apart in children over 1 year; 0.5 percent below 1 year; atropine 1 percent twice daily for 3 days if heavily pigmented iris or high hypermetropia suspected), dilated fundus examination, red reflex and Bruckner test.
• Corneal light reflex (Hirschberg test: each millimetre of decentration is about 15 prism dioptres, 1 mm of reflex displacement = 7 degrees), cover uncover and alternate cover tests, prism and cover test for distance and near with and without correction, motility in nine positions, convergence near point, versions and ductions, head posture.
• Sensory testing: Worth four dot test, stereoacuity (Titmus, TNO, Lang), 4-prism dioptre base out test, after image tests.
• Forced duction test and saccadic velocity if restriction or paresis suspected; neuroimaging when acquired, with neurological signs or poor visual development.

• Aims: restore normal vision (treat amblyopia), correct refractive error, achieve ocular alignment, restore binocular function when possible, and eliminate diplopia or abnormal head posture.
• Spectacle correction of full cycloplegic hypermetropia in accommodative esotropia; correct myopia in exotropia.
• Amblyopia treatment first: patching of the dominant eye 2 to 6 hours daily, atropine 1 percent penalisation, or binocular treatment.
• Prisms, orthoptic exercises, occlusion for diplopia in adults.
• Botulinum toxin for acute or small angle deviations and for diagnostic testing; doses of 2.5 to 5 units per muscle.
• Surgery when non-surgical measures fail or deviation is large: recession (weakening, moves insertion posteriorly) and resection (strengthening) of rectus muscles, oblique muscle procedures, adjustable sutures in adults; timing of surgery by type (infantile esotropia before age 2 for chance of binocular function).
• Complications of surgery: over or under correction, diplopia, scleral perforation (about 1 in 1000), anterior segment ischaemia, slipped muscle, infection and conjunctival scarring.