Describe the ocular motor nerve palsies as they present to the squint clinic.

Third Nerve Palsy in Squint Practice

• A complete third nerve palsy gives complete ptosis (levator), the eye down and out (unopposed lateral rectus and superior oblique), a dilated unreactive pupil and loss of accommodation
• The pupil rule governs the urgency: the parasympathetic fibres run superficially in the nerve and are supplied by pial vessels, so compression affects the pupil early while ischaemia (diabetic, hypertensive) infarcts the core and spares the pupil
• A pupil-involving third nerve palsy is a posterior communicating artery aneurysm until proved otherwise, and requires same-day CT or MR angiography. It is one of the few genuine emergencies in ophthalmology

Fourth and Sixth Nerve Palsies

• Fourth (trochlear) nerve — the longest and thinnest cranial nerve, and the only one to decussate and emerge dorsally from the brainstem.
• Features — vertical and torsional diplopia, worse on looking down and to the opposite side — hence difficulty reading and descending stairs.
• PARKS three-step test identifies the palsied cyclovertical muscle: (1) which eye is higher in the primary position; (2) is the deviation worse on right or left gaze; (3) is it worse on right or left head tilt — the bielschowsky head tilt test. A right superior oblique palsy gives a right hypertropia, worse on left gaze and worse on right head tilt
• Congenital fourth nerve palsy is common and frequently presents in adulthood as a decompensating palsy. It is identified by a head tilt in old photographs, a large vertical fusional amplitude (built up over years, and normally only 2 to 3 prism dioptres) and facial asymmetry. Ask for photographs and measure the vertical fusion range
• Sixth (abducens) nerve — has the longest intracranial course, ascending the clivus and passing over the petrous apex.
• Features — esotropia with limited abduction and horizontal diplopia worse at distance and on gaze to the affected side; with a face turn toward the affected side
• Causes — raised intracranial pressure; microvascular; trauma; cavernous sinus disease; gradenigo syndrome (petrous apicitis complicating otitis media, with sixth nerve palsy and facial pain); and nasopharyngeal carcinoma, which must be considered in an adult in India and warrants examination of the postnasal space
• In a child, a sixth nerve palsy is a pontine glioma until proved otherwise and requires urgent imaging; it is not a diagnosis to observe