Levator palpebrae superioris and ptosis

The levator palpebrae superioris is a thin muscle that runs forwards inside the orbit (the bony eye socket) and attaches to the upper eyelid. When it contracts, it pulls the lid upwards and uncovers the eye. It is supplied by the oculomotor nerve, the third cranial nerve (CN III).

If the muscle, or the nerve that drives it, stops working, the lid falls. This drooping of the upper eyelid is called ptosis. The oculomotor nerve runs through the lateral wall of the cavernous sinus (a channel of veins on each side of the base of the skull) on its way to the orbit, so damage to the nerve there can stop the levator working.

Do not confuse the levator with the other muscles around the eye. The orbicularis oculi, a ring of muscle around the eye supplied by the facial nerve, closes the eyelids. The superior rectus, an eyeball muscle that shares the levator's nerve, raises the eyeball rather than the lid. Müller's muscle is a small smooth muscle in the upper lid with a sympathetic supply, and it only assists the levator. Its loss gives a mild, partial droop, as in Horner syndrome, which follows damage to the sympathetic nerves (part of the automatic nervous system).

A full CN III injury usually brings other signs too: the eye rests 'down and out'. A marked, isolated droop with normal eye movements suggests the levator is the muscle affected, though these findings do not show whether the muscle or its nerve is damaged.