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What causes it?
Ptosis, or drooping of the upper eyelid, has four possible causes: third cranial nerve palsy, Horner's syndrome, myasthenia gravis, and musculotendinous disorder of the levator.

Third cranial nerve palsy: Damage to the nerve anywhere in its course can lead to transmission failure to the levator palpebrae superioris.

The chief worry is nerve compression by an expanding cerebral aneurysm which could rupture imminently. Other signs of third cranial nerve palsy (dilated, poorly reactive pupil, reduced ocular movements, ocular misalignment) are usually present, but they may be subtle!

Horner's syndrome: If the sympathetic pathway is lesioned, there is impaired transmission to Müller's muscle, a small muscle that sits above the upper lid tarsal plate.

The sympathetically-innervated Müller's muscle makes a relatively small contribution to lid elevation, so that the ptosis of Horner's syndrome is always mild—never more than 22 mm.

An important clue to Horner's syndrome is that the pupils are usually of unequal size (anisocoria). The smaller pupil is on the side of the lesion and both pupils react normally to light.

The sympathetic pathway to Müller's muscle is very, very long! It runs through the brain stem, spine, paraspinal region, chest, neck and head! A lesion anywhere in this pathway could cause Horner's syndrome. Here is a paraspinal tumor causing Horner's syndrom and no other abnormalities!

Myasthenia gravis: In this autoimmune neuromuscular junction disorder, antibodies chew up the muscle endplate receptors for acetylcholine, the chemical messenger of the third cranial nerve. The ptosis typically fluctuates, being least prominent after sleep.

Musculotendinous disorder of the levator: The lesion in these cases involves the muscle or its long tendon that inserts on the tarsal plate.

Congenital muscle dysplasia is a common cause.

Among acquired causes, there are inflammation, trauma, and degenerative stretching of the tendon with aging.

What to do?
Acute ptosis demands immediate evaluation by an ophthalmologist to rule out third cranial nerve palsy. Chronic ptosis should be evaluated non-emergently.

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