Lesions of the oculomotor nerve (CN III)

This is the "down and out" eye syndrome. It is characterised by ptosis, a down-and-out pupil, mydriasis, absent light reflex with intact consensual constriction of the opposite eye, and failure of accommodation. Classically, this is the lesion which develops during uncal herneation, due to an ipsilateral cerebral injury. Question 26.2 from the second paper of 2011 and Question 27 from the first paper of 2019 discussed the localisation of a midbrain lesion by a CN III palsy.

Features of a Third Nerve Palsy

• Ptosis
• Down-and-out pupil
• Mydriasis
• Failure of light reflex
  (but consensual constriction of the opposite eye is intact)
• Failure of accommodation

Causes of unilateral CN III lesions:

• Uncal herneation: Pressure from herniating uncus on nerve
• Fracture involving ipsilateral cavernous sinus
• Cavernous sinus thrombosis (ipsilateral)
• Aneurysm (ipsilateral)
• Midbrain lesion (see Question 26.2 from the second paper of 2011)

Causes of bilateral CN III lesions:

• Cavernous sinus thrombosis
• Aneurysm
• Contralateral brainstem lesion (midbrain)

Excluding a midbrain (nerve nucleus) cause for a third nerve lesion

Cannot be nuclear: (authoritative source!)

• Unilateral "down and out" eye, but the contralateral superior rectus function is preserved
• Unilateral unreactive pupil
• Unilateral ptosis

Must be nuclear:

• Bilateral third nerve palsy
• Unilateral third nerve palsy, with contralateral superior rectus palsy, and bilateral partial ptosis

In addition, the college in their answer to Question 27 from the first paper of 2019 mention that "A mass lesion results in pupillary non-reactivity, in diabetes the pupil is spared". This is accurate: diabetes and other forms of microvascular disease cause a pupil-sparing oculomotor palsy. This is because the nerve's central core underoes ischemic infarction.

Exclusion of a 4th nerve lesion

• Tilt the head to the same side as the lesion.
  The affected eye will intort if the fourth nerve is intact.