Eyelid
A mild droopy lid with a small pupil and reduced facial sweating — the triad of Horner's syndrome — and why it signals a problem along the sympathetic nerve pathway.
Medically reviewed by M. Polly McKinstry, MDOculoplastic SurgeonLast updated June 2026
Part of our complete guide to Ptosis (Droopy Eyelid) — this page covers Horner’s syndrome in depth.
Horner’s syndrome is caused by interruption of the sympathetic nerve pathway supplying the eye. Because sympathetic fibers innervate Müller’s muscle (the secondary upper lid elevator) and the inferior tarsal muscle, their loss produces a mild but distinctive ptosis of 1–2 mm.
The three-neuron sympathetic pathway is disrupted at different levels depending on the cause:
New-onset Horner’s syndrome requires urgent MRI/MRA to rule out carotid dissection or intracranial mass. In infants and children, Horner’s syndrome can cause heterochromia (lighter iris on the affected side) because sympathetic tone is needed for normal melanin development in the iris stroma.
Pharmacologic testing both confirms Horner’s syndrome and helps localize it:
The first priority is always identifying and treating the underlying cause (see urgent imaging above). The mild 1–2 mm droop itself rarely needs surgery, but when it bothers a patient cosmetically, a small Müller’s-muscle (internal) resection reliably raises the lid — logical here because Müller’s muscle is precisely the muscle that lost its sympathetic supply. Upneeq® (oxymetazoline) drops, which stimulate Müller’s muscle, can also give a temporary non-surgical lift.
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