A 37-year-old man presents with 5 days of left-sided headache with a retrobulbar component. Evaluation reveals a 2-mm relative ptosis on the left, anisocoria without afferent pupillary defect, and dilation lag of the left eye with no vermiform movements of the pupil. While getting a brain MRI, he develops a typical severe headache, is taken to the ER, given 10L of 100% oxygen for 20 minutes, and his headache resolves. He is put on a course of oral steroids and verapamil, with sumatriptan as needed. Within 1 week, his headaches improved significantly, although there was no change in the anisocoria or ptosis.

An isolated, painful Horner syndrome mainly either involves the region of the superior cervical ganglion and internal carotid artery or presents without imaging abnormalities. Although this patient had cluster headaches, a painful Horner syndrome should be considered a carotid dissection until proven otherwise. There might be no other neurologic signs, and in these cases, the isolated presentation especially mimics a cluster headache. Up to 20% of spontaneous carotid dissections have a Horner syndrome, and 91% of carotid artery dissections that have a Horner syndrome are painful. Diagnosis is especially important because without treatment to prevent thromboembolism, the risk of stroke nears 20%.