Overview
The third cranial nerve (CN III) is known as the oculomotor nerve. Its functions involve:
- Innervation of the extraocular muscles:
- Superior rectus: elevates eyeball
- Inferior rectus: depresses eyeball
- Medial rectus: adducts eyeball
- Inferior oblique: elevates, abducts, and laterally rotates eyeball
- Levator palpebrae superioris: elevates the upper eyelid
- Parasympathetic pupillary constriction
- By supplying the ciliary muscles of the eye
- Accommodation
Pathophysiology
The parasympathetic fibres of CN III are on the outer part of the nerve. Therefore, as a general rule of thumb, compression (e.g. tumours, intracranial bleeds etc.) can push on CN III and cause a dilated, unreactive pupil (due to impaired parasympathetic function). ‘Medical’ causes such as ischaemia or diabetes tend to cause problems with eye movement over pupillary responses. Regardless, all third nerve palsies are red flags and should be investigated and managed appropriately, regardless of pupillary involvement.
Features
- Fixed dilated pupil that does not accommodate
- Ptosis due to the reduction in levator palpebrae superioris function
- The eye may be deviated “down and out” – due to unopposed lateral rectus action
Causes
- Diabetes mellitus
- Vasculitides – giant cell arteritis, systemic lupus erythematosus
- Syphilis
- Posterior communicating artery aneurysm – there is often associated pain
- A raised intracranial pressure can cause uncal herniation which may compress CNIII
