Horner's Syndrome

Horner’s Syndrome

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.

Classic Triad

• Ptosis (upper lid, 1–2 mm) — Müller’s muscle denervation
• Inverse ptosis / lower lid elevation — inferior tarsal muscle denervation
• Miosis (small pupil) with dilation lag in dim light

Localizing the Lesion

The three-neuron sympathetic pathway is disrupted at different levels depending on the cause:

• First-order (central): hypothalamus to spinal cord — stroke, tumor, demyelination, syringomyelia
• Second-order (preganglionic): spinal cord to superior cervical ganglion — Pancoast tumor of the lung apex, carotid or subclavian artery pathology, cervical rib
• Third-order (postganglionic): superior cervical ganglion to orbit — carotid artery dissection, cavernous sinus mass, cluster headache

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.

Confirming & Localizing with Eye Drops

Pharmacologic testing both confirms Horner’s syndrome and helps localize it:

• Apraclonidine (or cocaine) test — confirmation: in Horner’s, the denervated pupil is super-sensitive, so apraclonidine drops paradoxically dilate the affected pupil and lift the lid, reversing the anisocoria — confirming the diagnosis.
• Hydroxyamphetamine test — localization: it dilates a normal or proximal-lesion pupil but fails to dilate a third-order (postganglionic) Horner’s pupil, separating a postganglionic cause from central/preganglionic ones.

Managing the Ptosis

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.