INTRODUCTION AND DEFINITION
Thunderclap headache (TCH) refers to a severe headache of sudden onset. Its explosive and unexpected nature is likened to a "clap of thunder." Although TCH initially referred to pain associated with an unruptured intracranial aneurysm , multiple etiologies are now recognized  (table 1). These include cerebral venous thrombosis, cervical artery dissection, spontaneous intracranial hypotension, pituitary apoplexy, retroclival hematoma, ischemic stroke, acute hypertensive crisis with reversible posterior leukoencephalopathy syndrome, third ventricular colloid cysts, bacterial and viral meningitis, complicated sinusitis, and reversible cerebral vasoconstriction syndromes.
In addition, the term primary TCH is often used to refer to a benign, idiopathic, and potentially recurrent headache of sudden and severe intensity with a lack of underlying pathology.
This topic will review each of the entities considered in patients with TCH, offer pathophysiologic considerations, and discuss the diagnostic evaluation of TCH.
In addition to the pain associated with the onset of subarachnoid hemorrhage from intracranial aneurysm rupture, thunderclap headache may be associated with multiple other causes (table 1).
●Reversible cerebral vasoconstriction syndromes