Horner syndrome
Formal Definition
A clinical triad of ipsilateral miosis (constricted pupil), ptosis (drooping eyelid), and anhidrosis (lack of sweating on the affected side of the face), often with apparent enophthalmos, caused by disruption of the sympathetic nerve supply to the face and eye; the sympathetic pathway has three neurons (first-order from hypothalamus to ciliospinal center of Budge at C8-T2, second-order from spinal cord to superior cervical ganglion, third-order to the eye/face) and localization of the lesion determines associated findings and underlying etiology.
How It's Used on the Ward
"Horner's" — the constellation of a small pupil, droopy eyelid, and possibly decreased sweating on one side of the face; signals interruption of the sympathetic pathway somewhere along its three-neuron chain from the brain to the eye; finding new Horner's prompts imaging to find the lesion (often brainstem stroke, carotid dissection, or apical lung tumor called Pancoast tumor).
Example
""55-year-old smoker presenting with 3 weeks of left shoulder pain radiating to the medial arm and forearm, noticed 2 days ago that his left pupil looks smaller than the right and his left eyelid is droopy. Exam: left miosis (3mm vs 5mm right in dim light), left ptosis (lid covers 4mm of iris vs 2mm right), left facial anhidrosis confirmed with starch-iodine test. Diagnosis: Horner syndrome with suspected Pancoast tumor (apical lung mass). Ordered CT chest and MRI brachial plexus, urgent pulmonology referral.""
Clinical Context
Localization of lesion: (1) First-order (central, hypothalamus to C8-T2): ipsilateral facial anhidrosis, often with brainstem signs (stroke as cause — vertebral artery dissection, lateral medullary syndrome / Wallenberg). (2) Second-order (preganglionic, spinal cord to superior cervical ganglion): anhidrosis of ipsilateral face, neck, and arm; causes include Pancoast tumor (apical lung cancer), trauma, thoracic surgery. (3) Third-order (postganglionic, superior cervical ganglion to eye): minimal or no anhidrosis (postganglionic fibers to sweat glands have rerouted); causes include carotid artery dissection (often from trauma — note new headache/neck pain), cluster headache, herpes zoster. Pharmacologic localization: cocaine or apraclonidine drops confirm Horner's (pupil fails to dilate); hydroxyamphetamine distinguishes pre- vs postganglionic. Workup: MRI brain + MRA neck (rule out carotid dissection), CT chest (Pancoast).