Ptosis in Horner syndrome results from loss of sympathetic innervation to which eyelid muscle?

• A. Superior tarsal muscle
• B. Levator palpebrae superioris
• C. Orbicularis oculi
• D. Ciliary muscle

Answer: (A)

In Horner syndrome, the sympathetic pathways that reach the eye include a smooth muscle in the eyelid called Müller's muscle (the superior tarsal muscle). Loss of sympathetic input weakens this muscle, causing ptosis. The main eyelid elevator, the levator palpebrae superioris, is a skeletal muscle controlled by the oculomotor nerve, so it remains functional and ptosis is typically mild. The other listed muscles aren’t responsible for this droop: the orbicularis oculi closes the eyelid and is facial-nerve–innervated, and the ciliary muscle controls accommodation, not eyelid elevation. So the ptosis in Horner syndrome comes from loss of sympathetic innervation to the superior tarsal (Müller) muscle.