Benign episodic unilateral mydriasis presenting as acute postoperative anisocoria.

Acute anisocoria in the postoperative setting raises immediate concern for life-threatening intracranial or ophthalmic pathology. We report the case of a female patient in her 30s who developed a left-sided fixed dilated pupil following uncomplicated right-sided functional endoscopic sinus surgery. Urgent imaging and neurological evaluation were unremarkable. The patient had a history of migraine and previous self-resolving episodes of unilateral mydriasis associated with headaches, for which extensive investigations had previously been normal. Her postoperative headache resolved with analgesia, and the anisocoria normalised within 3 days. No regional anaesthetic block, perioperative dexamethasone, ocular exposure to topical agents or surgical complications were identified. The overall clinical picture was most consistent with benign episodic unilateral mydriasis (BEUM), although this should be regarded as a diagnosis of exclusion, and the temporal relationship with surgery should not be interpreted as proven causation. The clinical history and negative investigations strongly supported a diagnosis of BEUM, a rare migraine-associated phenomenon. This case highlights the importance of recognising BEUM in the postoperative setting, where its presentation is unusual and mandates thorough investigation to exclude sinister causes before a benign diagnosis can be made.