cranial nerve III palsy

Last reviewed 07/2021

Paralysis of the third cranial nerve affects the medial, superior, and inferior recti, and inferior oblique muscles.

The eye is incapable of movement upwards, downwards or inwards, and at rest the eye looks laterally and downwards owing to the overriding influence of the lateral rectus and superior oblique muscles respectively. The reduced response of levator palpebrae superioris results in ptosis - a drooping of the upper eyelid.

A third nerve palsy with pupillary sparing is often termed a medical third palsy and often has an ischaemic or diabetic aetiology.

Full assessment of oculomotor nerve function involves testing of movement, reaction to light, and accommodation. If all of these are normal, "PERLA" may be written in the notes - pupils equal, reactive to light and normal accommodation.