Describe paralytic squint with its assessment and the management of squint.

Paralytic (incomitant) Squint

• The distinction is not academic: a concomitant squint is a developmental and refractive problem, whereas an incomitant squint is a neurological or orbital sign and demands a cause be found
• Why the secondary deviation exceeds the primary — when the paretic eye is made to fix, a greatly increased innervation is required; by HERING’S law the same excess reaches the yoke muscle of the sound eye.

Special Syndromes and Restrictive Disease

• DUANE retraction syndrome — a congenital miswiring in which the lateral rectus is innervated by a branch of the third nerve instead of the sixth, so that both horizontal recti CO-contract on attempted adduction. The result is globe retraction with narrowing of the palpebral fissure on adduction, and widening on abduction. Type I (limited abduction) is commonest; type II limited adduction; type III both. It is commoner in females and on the left, may be associated with deafness and skeletal anomalies (Wildervanck), and is managed conservatively, surgery being reserved for an abnormal head posture or a squint in primary position
• Brown syndrome — restricted elevation in adduction from a tight or tethered superior oblique tendon. The forced duction test is positive, which distinguishes it from inferior oblique palsy. Congenital, or acquired from trauma, surgery or inflammation at the trochlea
• Thyroid eye disease is the commonest cause of restrictive strabismus in adults. The muscles are infiltrated and fibrosed, and the order of involvement is remembered as "I’M slow": Inferior rectus, Medial rectus, Superior rectus, Lateral rectus. Since the inferior rectus is affected first and tethers the eye down, the characteristic finding is limited elevation — which mimics a superior rectus palsy until the forced duction test is done. It may also cause a false-positive raised intraocular pressure on upgaze

Management of Squint

• The order OF treatment IS fixed, and surgery is last:
• 1. Treat any underlying disease — and in an incomitant squint this means finding the neurological or orbital cause first
• 2. Correct the refractive error fully after cycloplegia. This alone cures accommodative esotropia and is the foundation of everything else
• 3. Treat amblyopia — and this must be done before surgery, since operating on an amblyopic eye does not improve vision and the alignment is less stable
• 4. Orthoptic treatment — convergence exercises, anti-suppression and fusional amplitude training.
• 5. Prisms — to relieve diplopia in small or recovering deviations, and to test the likely effect of surgery. Fresnel press-on prisms are cheap, adjustable and ideal while a palsy is recovering
• 6. Botulinum toxin injected into the antagonist muscle, which is valuable as a temporary measure in an acute sixth nerve palsy to prevent contracture of the medial rectus while recovery is awaited
• 7. Surgery, on the principles of weakening an overacting muscle (recession, or myectomy of the inferior oblique), strengthening a weak one (resection or plication), and transposition where a muscle is completely palsied. Adjustable sutures allow refinement under topical anaesthesia in a cooperative adult