Ocular Motility Disorders and Strabismus

Strabismus is misalignment of the visual axes. Ocular motility disorders are the broader group of conditions causing abnormal eye movement, including cranial nerve palsies, restrictive and neuromuscular disease, and supranuclear or internuclear lesions.

Basic terminology

• Heterophoria: latent deviation controlled by fusion, revealed when fusion is disrupted.
• Heterotropia: manifest deviation (esotropia inward, exotropia outward, hypertropia vertical, cyclotropia torsional), constant or intermittent.
• Comitant: the angle of deviation stays the same in all gaze positions. Incomitant: it varies with gaze direction, typical of paralytic and restrictive causes.

Epidemiology: strabismus affects 2 to 4% of children. Esotropia is commoner in Western populations and exotropia in East Asian and African populations. In adults, strabismus is acquired from neurological, endocrine, myasthenic or traumatic causes, or is a persistent childhood deviation.

Common types in children

• Infantile (congenital) esotropia: onset by 6 months, large and stable angle (30 to 50 prism dioptres), cross fixation, latent nystagmus, dissociated vertical deviation, and inferior oblique overaction.
• Accommodative esotropia: onset at 2 to 4 years, with hyperopia (+2 to +6 dioptres), either refractive (corrected by glasses) or with a high accommodative convergence to accommodation ratio.
• Intermittent exotropia: the commonest exotropia, with distance control that deteriorates with fatigue, illness or daydreaming. One eye closes in bright sunlight.
• Duane retraction syndrome: abnormal innervation of lateral rectus by the oculomotor nerve, with limited abduction, globe retraction and narrowing of the palpebral fissure on adduction. Type 1 is the commonest.
• Brown syndrome: restricted elevation in adduction from tight superior oblique tendon sheath.
• Fourth nerve palsy: congenital decompensation or acquired after head trauma.
• Möbius syndrome, congenital fibrosis of the extraocular muscles, and Down syndrome associated strabismus.

Common acquired causes in adults

• Sixth nerve palsy: microvascular ischaemia (diabetes, hypertension), raised intracranial pressure (false localising), tumour, trauma, multiple sclerosis, Gradenigo syndrome, giant cell arteritis.
• Third nerve palsy: posterior communicating artery aneurysm (pupil involved), microvascular (pupil spared in about 80%), tumour, trauma, uncal herniation, cavernous sinus lesions.
• Fourth nerve palsy: closed head trauma, microvascular disease, congenital decompensation.
• Thyroid eye disease: restrictive myopathy, inferior and medial rectus most affected.
• Myasthenia gravis: fluctuating ptosis and diplopia, mimic of any pattern.
• Others: internuclear ophthalmoplegia (medial longitudinal fasciculus lesion, multiple sclerosis in the young, stroke in the old), skew deviation, orbital floor fracture with entrapment, orbital myositis, convergence insufficiency, decompensated phoria, and sagging eye syndrome in older adults.

Sensory consequences: in children, suppression and amblyopia (strabismic amblyopia in about 50% of esotropia) develop within the critical period (to around 7 to 8 years). Adults experience diplopia and visual confusion, not suppression.

History

• Age at onset (infantile versus acquired), constant or intermittent, family history, prematurity, neurodevelopmental delay, trauma, systemic disease, headaches and previous eye surgery.
• Children: eye turn noticed by parents, head tilt or turn, squinting in sunlight, clumsiness, poor depth perception. Look for photographs of long-standing deviation.
• Adults: diplopia (horizontal, vertical or oblique, worse at distance or near), whether monocular (not strabismus, think refractive error or cataract) or binocular (resolves on covering either eye), variability through the day, and associated neurological symptoms.

Examination

• Visual acuity with age-appropriate tests, pupil responses, head posture, and eyelid position.
• Hirschberg corneal reflex (1 mm of displacement is about 15 prism dioptres). Pseudostrabismus from epicanthal folds and wide nasal bridge shows symmetric reflexes.
• Cover/uncover and alternating cover tests, prism and cover test in primary and nine gaze positions, with near and distance measurements.
• Ocular motility in versions and ductions. Look for slowed saccades (myasthenia, internuclear ophthalmoplegia), globe retraction (Duane), nystagmus, and ptosis.
• Pattern of deviation: A and V patterns, and Parks-Bielschowsky three-step test for superior oblique palsy (right hypertropia that worsens on left gaze and right head tilt).

Syndrome clues

• Third nerve palsy: ptosis, eye down and out, mydriasis (compressive). Pupil-sparing suggests ischaemia, but incomplete palsies with pupil sparing can still be compressive.
• Fourth nerve palsy: vertical or torsional diplopia worse on downgaze (reading, stairs), head tilt to the opposite shoulder.
• Sixth nerve palsy: esotropia worse at distance and on gaze to the affected side, abduction deficit, face turn to the affected side.
• Internuclear ophthalmoplegia: impaired adduction with abducting nystagmus of the other eye, convergence preserved.
• Thyroid eye disease: lid retraction, lid lag, proptosis, conjunctival injection, restricted upgaze and abduction.
• Myasthenia gravis: fatigable ptosis, Cogan lid twitch, variable diplopia, pupils normal.
• Orbital floor fracture: restricted upgaze with pain on attempted elevation.

Red flags: leukocoria (retinoblastoma), sudden onset in a child, papilloedema, ophthalmoplegia with headache or neck pain (aneurysm, carotid dissection, cavernous sinus thrombosis), pupil involvement in a third nerve palsy, bilateral or progressive palsies, proptosis with vision loss, age over 50 with jaw claudication and headache, and associated neurological signs.

Children

• Cycloplegic refraction in every child with strabismus: cyclopentolate 1% (one drop twice, 5 minutes apart) or atropine 1% twice daily for 3 days in esotropia with high hyperopia and darkly pigmented irides. Cyclopentolate 0.5% for infants under 1 year.
• Dilated fundus examination to exclude retinoblastoma, optic nerve hypoplasia, cataract and macular disease.
• Stereoacuity (Titmus, TNO), Worth 4-dot test for fusion, Bagolini glasses.
• Neuroimaging with MRI of the brain and orbits when there is acquired non-accommodative strabismus, incomitance, neurological signs, papilloedema, or any sixth nerve palsy beyond isolated post-viral palsy.

Adults

• Prism cover test in nine positions, Maddox rod, Hess or Lees screen, and synoptophore. Forced duction testing (under topical anaesthesia) distinguishes restrictive from paretic causes, and active force generation tests assess muscle strength.
• Microvascular cranial nerve palsy in a patient with vascular risk factors and an isolated, pupil-sparing palsy: check blood pressure, glucose or HbA1c, lipids, ESR and CRP (giant cell arteritis if over 50), and observe for 3 months. No scan is needed initially if recovery begins in 6 to 8 weeks.
• MRI brain and orbits with contrast and MR angiography or CT angiography for pupil-involving or incomplete third nerve palsy, painful palsy, multiple cranial neuropathies, age under 50, or failure to improve at 3 months.
• Thyroid function and TSH receptor antibodies, with orbital MRI or CT showing enlarged muscle bellies sparing the tendons.
• Myasthenia gravis: ice pack test (improvement of ptosis by 2 mm or more after 2 minutes), acetylcholine receptor and MuSK antibodies, repetitive nerve stimulation or single-fibre EMG, and CT chest for thymoma.
• Lumbar puncture with opening pressure when papilloedema suggests raised intracranial pressure and imaging is normal.
• Orbital CT for fractures and orbital myositis.

Amblyopia and refractive correction

• Correct refractive error first and fully, with glasses worn full time for 4 to 16 weeks before considering surgery in accommodative esotropia.
• Treat amblyopia before surgery: occlusion of the better eye for 2 to 6 hours daily (patching), or atropine 1% in the better eye daily or on weekends. Binocular digital therapy is an option. Treat until visual acuity is equal or plateaus.
• Accommodative esotropia with high AC/A ratio: bifocals (+2.50 to +3.00 add) or miotics in selected cases. Refractive surgery is an option in adults.

Esotropia

• Infantile esotropia: surgery between 6 months and 2 years, bilateral medial rectus recession of 3 to 7 mm (per angle), or recession-resection for larger angles. Earlier alignment favours better binocularity. Dissociated vertical deviation needs superior rectus recession, and inferior oblique overaction needs inferior oblique weakening.
• Partially accommodative esotropia: glasses plus surgery for the residual angle.

Exotropia

• Intermittent exotropia: observation if well controlled and no amblyopia. Treatments: over-minus lenses (-1 to -3 dioptres), part-time patching, convergence exercises (office-based vergence therapy) and surgery for deterioration (bilateral lateral rectus recession of 5 to 9 mm, or recess-resect). Overcorrection is aimed at the early postoperative period to reduce recurrence.
• Convergence insufficiency: office-based vergence and accommodative therapy for 12 weeks, with prisms for symptomatic adults.

Paralytic strabismus

• Treat the cause. In microvascular palsies, control diabetes and blood pressure, and expect recovery in 3 to 6 months. Patch or fresnel prisms for diplopia in the interim.
• Third nerve palsy: aneurysm needs urgent endovascular coiling or clipping. Strabismus surgery after 6 months of stable deviation, with ptosis surgery after alignment.
• Sixth nerve palsy: observation for 6 months in adults with prisms or occlusion. Botulinum toxin A 2.5 to 5 units into the medial rectus in the acute phase prevents contracture and gives temporary alignment. Surgery after 6 months: medial rectus recession with lateral rectus resection for partial palsies, and vertical rectus transposition (with or without botulinum toxin to medial rectus) for complete palsies.
• Fourth nerve palsy: prism correction for small deviations, inferior oblique weakening (myectomy or recession) for superior oblique underaction, Harada-Ito procedure for torsion, and contralateral inferior rectus recession.
• Internuclear ophthalmoplegia: treat multiple sclerosis (methylprednisolone 1 g IV for 3 days in relapse) or stroke. Surgery for persistent exotropia.

Restrictive and neuromuscular disease

• Thyroid eye disease: restore euthyroidism, stop smoking, selenium 100 micrograms twice daily for 6 months in mild disease, IV methylprednisolone 4.5 g cumulative (0.5 g weekly for 6 weeks then 0.25 g weekly for 6 weeks) in moderate to severe active disease, teprotumumab 10 mg/kg then 20 mg/kg every 3 weeks for 7 further doses, or rituximab/tocilizumab in selected cases. Strabismus surgery when inactive and stable for 6 months, using adjustable sutures and recessing the tight muscle (avoid resection). Orbital decompression before strabismus surgery when proptosis is significant.
• Myasthenia gravis: pyridostigmine 30 to 60 mg three to four times daily, prednisolone 10 to 20 mg daily (titrated up to 1 mg/kg in severe disease), azathioprine 2 to 3 mg/kg/day as steroid sparing therapy, and thymectomy for thymoma or in generalised disease. Ocular surgery only when the deviation is stable.
• Orbital floor fracture with entrapment: surgical release and reconstruction as described for orbital trauma.
• Duane retraction syndrome: observation, face turn correction by surgery for significant head posture or primary position deviation. Avoid resection of the affected lateral rectus (worsens retraction). Y-splitting of the lateral rectus for upshoots and downshoots.
• Brown syndrome: observation for acquired inflammatory forms, superior oblique tenotomy or silicone expander in constant deviations.
• Sagging eye syndrome: prisms, then surgery on the lateral rectus and vertical rectus muscles.

Diplopia management in adults

• Occlusion, frosted lens, or Fresnel and ground-in prisms up to 10 to 15 prism dioptres, noting that large or incomitant deviations are poorly controlled. Botulinum toxin A injection (2.5 to 5 units, effect lasts 3 months) as a diagnostic and therapeutic adjunct to test surgical tolerance of diplopia.
• Strabismus surgery with adjustable sutures when the deviation is stable for 6 months, using forced duction testing to plan.

Surgical technique and risks

• Recession weakens the muscle by moving the insertion posteriorly, resection strengthens it by shortening, and transposition redirects the force vector.
• Complications: over- or undercorrection (15 to 30% need reoperation), infection, scleral perforation (about 0.1%), slipped or lost muscle, anterior segment ischaemia after operating on three or more rectus muscles in adults, diplopia, adherence syndrome, and oculocardiac reflex under anaesthesia (treated with atropine 0.01 to 0.02 mg/kg IV).
• Anaesthesia: children under general anaesthesia with malignant hyperthermia vigilance (association with strabismus is debated). Adults may use topical anaesthesia with sedation for adjustable procedures.

Follow-up

• Children need review every 3 to 6 months until visual maturity (about 8 years) to detect amblyopia recurrence and loss of alignment. Postoperative review at 1 day, 1 to 2 weeks, 6 weeks and 3 months.
• Adults with neuropathies are followed every 4 to 8 weeks until stable.