Thunderclap headache: an approach to a neurologic emergency.

Thunderclap headache (TCH) refers to an excruciating headache of instantaneous onset. Recognition and accurate diagnosis of this headache are important because it can be caused by various serious underlying brain disorders such as subarachnoid hemorrhage, intracranial hematoma, cerebral venous sinus thrombosis, cervical artery dissection, ischemic stroke, pituitary apoplexy, acute arterial hypertension, spontaneous intracranial hypotension, third ventricle colloid cyst, and intracranial infections. Patients with TCH who have evidence of reversible, segmental, cerebral vasoconstriction of circle of Willis arteries and normal or near-normal cerebrospinal fluid evaluation are considered to have reversible cerebral vasoconstriction syndrome. Primary TCH is diagnosed when no underlying etiology is identified. In this review, we discuss the differential diagnosis of TCH, outline the characteristics and diagnostic criteria for primary TCH, offer a pathophysiologic hypothesis for primary TCH, and detail the diagnostic evaluation of the patient presenting with TCH.