Describe fourth and sixth nerve palsies.

Fourth (trochlear) Nerve Palsy

• The trochlear nerve supplies the superior oblique alone, whose actions are intorsion, depression in adduction, and abduction
• It is unique in three ways, and all three are examinable: it is the only cranial nerve to decussate and to emerge from the dorsal surface of the brainstem; it has the longest intracranial course; and it is the thinnest cranial nerve. Together these make it the most vulnerable to closed head injury, in which the palsy is frequently bilateral
• Symptoms — vertical and torsional diplopia, worse on looking down and to the opposite side — so the patient complains particularly of reading and going downstairs.

Sixth (abducens) Nerve Palsy

• The abducens nerve supplies the lateral rectus alone, and is the most commonly affected ocular motor nerve
• Features — esotropia in the primary position; limited abduction; horizontal diplopia, maximal on looking toward the affected side and at distance; and a compensatory face turn toward the affected side
• IT IS the classical false localising sign. Its long intracranial course over the petrous ridge means that raised intracranial pressure from any cause stretches it — so a sixth nerve palsy does not indicate a lesion at that site, and the fundus must be examined for papilloedema in every case
• Causes — raised intracranial pressure; microvascular, which is the commonest in an older diabetic and recovers within 3 to 6 months; trauma; cavernous sinus disease; and in children, post-viral palsy and pontine glioma
• Nasopharyngeal carcinoma must be considered in India and South-East Asia, where it is common. It invades the skull base and may present with an isolated sixth nerve palsy, serous otitis media and a cervical node — so the nasopharynx and the neck must be examined and referral made for endoscopy
• Gradenigo syndrome — petrous apicitis complicating otitis media, with the triad of sixth nerve palsy, facial pain in the trigeminal distribution, and otorrhoea
• Differential of limited abduction — and each is regularly mistaken for a sixth nerve palsy: DUANE retraction syndrome (congenital, with globe retraction and narrowing of the palpebral fissure on adduction, from aberrant innervation of the lateral rectus by the third nerve); thyroid eye disease with a tight fibrotic medial rectus, which is restrictive; myasthenia gravis, which can mimic any ocular motility disorder; medial orbital wall fracture with entrapment; and convergence spasm