Case presentation: A 28-year-old male with history of migraineous headaches since his late teens, presented with acute onset of thunderclap headache and episodes of bilateral complete blindness, lasting from 10 minutes to an hour. Routine blood investigations, CT brain, CT angiogram and CT venogram were all normal. Lumbar puncture with CSF manometry revealed elevated CSF opening pressure suggesting a diagnosis of idiopathic intracranial hypertension. His symptoms improved completely after CSF tap. He was started on acetazolamide for prophylaxis. 

Conclusion: In migraine patients, a change in character of headache should immediately prompt the physician to search for intracranial organic pathologies. IIH should be considered in the differential diagnosis of thunderclap headache especially with visual phenomena, given the excellent response of it with early treatment.