Saturday, 9 October 2010

MRCP revision battle 28.3: Third nerve palsy

Cranial nerve III = oculomotor nerve arises in the rostral midbrain at the level of the superior colliculus.

It has 2 adjacent nuclei:
  • oculomotor nucleus - somatic fibres (eye movements)
  • edinger-westphal nucleus - visceral fibres (pupillary constriction)

The oculomotor nerve passes between the posterior cerebral and superior cerebellar arteries, then on through the cavernous sinus and out through the superior orbital fissure.

In the orbit it splits into:
  • superior branch - supplies levator palpebrae
  • inferior branch - supplies medial rectus, inferior rectus and inferior oblique muscles and carries the visceral fibres

Complete 3rd nerve palsies tend to be peripheral rather than central in origin as the nucleus is big.

Classic 3rd nerve palsy:
  • ptosis
  • 'down and out' pupil
  • dilated pupil

If the pupil is spared it is sometimes referred to as a 'medical' third nerve palsy, whereas if it is fixed and dilated it is a 'surgical' third nerve palsy.

The reasoning behind this is that the visceral constrictive fibres run on the outside of the nerve so are spared in vascular aetiologies.

Causes of third nerve palsy:
  • diabetes (75% pupil-sparing)
  • temporal arteritis
  • SLE
  • MS
  • cavernous sinus thrombosis
  • amyloid
  • posterior communicating artery aneurysm (usually painful)
  • tumour

Webers syndrome: ipsilateral third nerve palsy with contralateral hemiplegia --> signifies a midbrain stroke.

After that pleasant forray into the world of neurology on to something completely different - G6PD deficiency...