Restrictive Strabismus

Strabismus caused by mechanical limitation of eye movement from fibrosis, entrapment, scarring or mass effect rather than neurological weakness.

• Causes:
  • Thyroid eye disease (most common cause in adults; inferior and medial rectus fibrosis).
  • Orbital fracture with entrapment of muscle or soft tissue (floor, medial wall); orbital haematoma or oedema.
  • Brown syndrome (tight superior oblique tendon or sheath), congenital fibrosis of extraocular muscles, Duane retraction syndrome, strabismus fixus.
  • Post-surgical or post-trauma scarring: retinal detachment buckles, glaucoma drainage implants, orbital surgery, scleral buckle, sinus surgery, local anaesthetic myotoxicity after peribulbar or retrobulbar block.
  • Orbital inflammation (myositis, idiopathic orbital inflammation), tumours, and high myopia (heavy eye syndrome with superotemporal globe herniation).
• Muscle fibrosis, adhesions and dysfunction restrict excursion in the direction opposite to the tight muscle.

• Limited ductions in the direction opposite to the restricted muscle (limited elevation from tight inferior rectus; limited abduction from tight medial rectus).
• Diplopia in primary position and gaze; abnormal head posture; ocular pain on movement; increased IOP on attempted gaze away from the restriction (above 6 mmHg increase on upgaze, raised in thyroid disease).
• Globe retraction and enophthalmos on attempted movement in Duane syndrome; pain and bruising with trauma; periorbital emphysema; infraorbital numbness in floor fracture.
• Vertical restriction pattern: hypotropia, limited elevation (thyroid and floor fracture); heavy eye syndrome: esotropia with hypotropia.

• Forced duction test (traction test, done under topical anaesthesia or general anaesthesia): resistance when moving the eye in the direction of limited movement indicates restriction.
• Forced generation test, saccadic velocity (normal initially) and Hess chart: small field, tight lines.
• Intraocular pressure on primary position and attempted gaze away from the tight muscle.
• CT orbits (fracture, muscle enlargement, entrapment: "trapdoor" sign in children) or MRI with fat suppression; thyroid function and antibodies; ultrasound for muscle thickness.
• Examine for diplopia fields, exophthalmometry, and optic nerve function.

• Thyroid eye disease: restore euthyroidism, stop smoking, steroids or teprotumumab in active phase; surgery only in inactive phase (3 to 6 months stable): recession of tight muscles (inferior rectus recession 3 to 6 mm with adjustable sutures, avoid hang back sutures where possible), then eyelid surgery.
• Orbital floor fracture with muscle entrapment: urgent surgery (within 24 to 48 hours in children with trapdoor fracture and oculocardiac reflex: nausea, vomiting, bradycardia), release entrapment, reconstruct floor with implant; adults with persistent diplopia and positive forced duction at 1 to 2 weeks.
• Brown syndrome: observation in congenital forms (spontaneous resolution common); surgical tenotomy or silicone expander if hypotropia in primary position or head posture; triamcinolone injection (20 mg) or oral NSAIDs for acquired inflammatory cases.
• Duane syndrome: observation, prisms, surgery for head turn or severe cosmetic deviation (recession of the medial rectus, Y-splitting of the lateral rectus).
• Post-surgical and post-block restriction: botulinum toxin to the antagonist; release of scar; adjustable sutures.
• Heavy eye syndrome: surgical union of superior and lateral recti (loop myopexy) with or without medial rectus recession.
• Myositis: oral prednisolone 1 mg/kg daily with taper.
• Prisms for small deviations; occlusion; botulinum toxin for diagnostic purposes and as adjunct.