ptosis

Ptosis results from dysfunction of the muscles responsible for eyelid elevation: the levator palpebrae superioris (innervated by CN III) and the superior tarsal (Müller's) muscle (innervated by sympathetic fibers). Key causes include: (1) CN III palsy - ptosis with 'down and out' eye position, mydriasis (if compressive, e.g., PCOM aneurysm) or sparing of pupil (if ischemic, e.g., diabetic mononeuropathy); (2) Horner syndrome - mild ptosis due to Müller's muscle denervation, accompanied by miosis and anhidrosis from sympathetic chain lesion (e.g., Pancoast tumor, carotid dissection); (3) Myasthenia gravis - fatigable ptosis worsening with sustained upgaze, due to autoantibodies against postsynaptic ACh receptors at the neuromuscular junction; (4) Myotonic dystrophy and other myopathies - bilateral ptosis from muscle weakness; (5) Congenital ptosis - due to levator muscle dysgenesis. Clinical evaluation includes assessing pupil size/reactivity, extraocular movements, fatigability, and associated symptoms to localize the lesion. Ptosis is a classic 'flag' symptom tested extensively on board exams to differentiate neurologic emergencies (aneurysm) from benign or chronic conditions (Horner, myasthenia).