Horner Syndrome

Overview

Horner syndrome is the clinical triad of ipsilateral ptosis, miosis, and anhidrosis, produced by an interruption anywhere along the three-neuron oculosympathetic pathway that runs from the hypothalamus down to the iris and eyelid. The same triad can come from a congenital anomaly or a cluster headache, or from a carotid artery dissection or an apical lung tumor invading the sympathetic chain, so the diagnosis is inseparable from the question of where the lesion sits.

The pathway threads through several anatomic neighborhoods on its way down: brainstem, cervical spinal cord, lung apex, neck, and skull base. A Horner pupil is therefore never just a pupil finding. Confirm the diagnosis pharmacologically first, then localize which of the three neurons was interrupted; that localization sets both the differential diagnosis and how urgently the workup needs to move.

The Oculosympathetic Pathway

The pathway is taught as three neurons in series, and the anatomy at each stage is what makes the differential diagnosis for that order of lesion make sense.

First-order neuron (central)

Begins in the posterolateral hypothalamus and descends uncrossed through the brainstem and cervical spinal cord, synapsing at the ciliospinal center of Budge-Waller (spinal levels C8–T2). Lesions here are central: brainstem stroke (classically the lateral medullary, or Wallenberg, syndrome), demyelinating disease, or a cervical cord lesion. A first-order Horner syndrome rarely travels alone; it's almost always accompanied by other neurologic signs that make the localization obvious.

Second-order neuron (preganglionic)

Exits the spinal cord at T1, arches over the apex of the lung near the subclavian artery, then climbs through the neck to synapse in the superior cervical ganglion, close to the carotid bifurcation at the angle of the mandible. That path over the lung apex is why an apical lung tumor, a Pancoast tumor, is a classic cause of second-order Horner syndrome, and why chest imaging belongs in the workup of an otherwise-unexplained preganglionic lesion.

Third-order neuron (postganglionic)

Leaves the ganglion and runs along the internal carotid artery, through the cavernous sinus alongside cranial nerve VI, then joins the ophthalmic division of cranial nerve V to reach the iris dilator and Müller's muscle. This long stretch beside the internal carotid is exactly where internal carotid dissection does its damage, making dissection the single most urgent cause of Horner syndrome to rule out.

Clinical Features

Each element of the triad follows from a structure the sympathetic fibers normally support:

• Mild ptosis (1–2 mm). Sympathetic fibers innervate Müller's muscle (the superior tarsal muscle), which adds a small amount of upper-lid elevation on top of the levator palpebrae's much larger contribution (cranial nerve III). Denervating Müller's muscle alone leaves the levator untouched, so ptosis in Horner syndrome stays mild; it never reaches the complete ptosis a third-nerve palsy can produce.
• Miosis. Loss of sympathetic tone to the iris dilator leaves the parasympathetic sphincter unopposed, so the pupil is smaller than normal. The Horner pupil can't dilate the way the normal pupil does, so the anisocoria is worse in dim light: the normal eye widens in the dark while the affected eye lags behind, and the size gap grows.
• Anhidrosis, loss of facial sweating, occurs only with central and preganglionic lesions. Sudomotor fibers split off the pathway at the superior cervical ganglion and travel with the external carotid artery, apart from the third-order fibers that continue along the internal carotid. A postganglionic lesion therefore spares sweating entirely, which becomes a useful clue at the bedside.

Two subtler findings round out the exam. Reverse (upside-down) ptosis is a slight elevation of the lower lid, from loss of sympathetic tone to the inferior tarsal muscle. Dilation lag is the affected pupil dilating more slowly than the normal pupil in the first few seconds after the room lights go down, reflecting lost active dilator drive rather than any mechanical limitation. Together, the narrowed palpebral fissure and the raised lower lid can make the eye look sunken, a pseudoenophthalmos with no true posterior displacement of the globe.

Congenital Horner syndrome adds one more distinctive finding: iris heterochromia, with the affected iris staying lighter than the fellow eye. Sympathetic input drives melanocyte stimulation and iris stromal pigmentation during infancy, so a pathway injured early in life (birth trauma to the brachial plexus or neck is the classic cause) leaves that iris under-pigmented. A lesion arriving later, once pigmentation is already set, won't produce this finding.

Diagnosis & Pharmacologic Testing

Pharmacologic testing answers two separate questions, and different drugs answer each one: first confirming the diagnosis, then localizing which neuron is affected.

Confirming the diagnosis

Apraclonidine, a weak alpha-1 agonist, is now the preferred confirmatory agent. It barely affects a normal pupil, but the Horner pupil's iris dilator has undergone denervation supersensitivity: chronic sympathetic loss upregulates alpha-1 receptors until they respond even to weak agonist activity. The result is a reversal of the anisocoria: the miotic, Horner-affected pupil dilates, while the normal pupil barely responds to the drug's weak agonism and may even constrict slightly. Apraclonidine has largely displaced cocaine testing, which works differently, blocking norepinephrine reuptake so the normal pupil dilates while the Horner pupil, releasing no norepinephrine to begin with, does not. The switch is mostly practical: cocaine's controlled-substance status and limited availability make apraclonidine, an already-stocked glaucoma medication, far easier to keep on hand.

Localizing the lesion

Once the diagnosis is confirmed, hydroxyamphetamine distinguishes a preganglionic lesion (first- or second-order) from a postganglionic one (third-order). It works by releasing stored norepinephrine from intact postganglionic nerve terminals. If the third-order neuron is undamaged, the pupil dilates no matter where upstream the lesion actually sits. If the third-order neuron itself is damaged, there's no stored norepinephrine left to release, and the pupil stays put. The distinction carries real clinical weight: third-order lesions are more often benign (cluster headache, viral illness, an idiopathic cause), while first- and second-order lesions carry meaningfully higher odds of serious underlying disease.

Etiology & Urgent Workup

Most causes of Horner syndrome are benign, but one presentation must never be missed: acute, painful Horner syndrome with ipsilateral neck or facial pain. That combination is the classic picture of internal carotid artery dissection, a vision- and life-threatening emergency that can progress to stroke. Any patient presenting this way needs urgent vascular imaging, typically CT angiography or MR angiography of the head and neck, which reliably shows the intimal flap, luminal narrowing, or intramural hematoma of a dissection. Duplex ultrasound is the wrong first test; it can miss high cervical or more distal dissections and doesn't have the sensitivity this emergency demands.

Other causes span the length of the pathway: brainstem stroke and demyelinating disease for first-order lesions, apical lung tumor and thoracic surgery or trauma for second-order lesions, and cluster headache, viral illness, or an idiopathic cause for third-order lesions. Congenital Horner syndrome, most often from birth trauma to the neck or brachial plexus, presents with the iris heterochromia described above and is generally a benign, static finding, but only once other causes are excluded. In an infant with new-onset Horner syndrome, that workup must rule out neuroblastoma.

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