Intracranial hemorrhage—subarachnoid, intraparenchymal, subdural, epidural

Meningitis/encephalitis

Infections: Sinusitis, pharyngitis, otitis media

First migraine, tension-type, or cluster headache

Febrile illness-related headache (often related to upper respiratory tract infection)

Dental or Temporo-Mandibular Joint dysfunction

Hydrocephalus

Vasculitis

Intracranial hypertension—primary (pseudotumor) or secondary

Intracranial hypotension

Arterial ischemic stroke

Neoplasm

Hypertension

Ventriculo-peritoneal shunt malfunction

Toxins (carbon monoxide, lead)

Migraine or tension-type headaches

Episodic intracranial hypertension (i.e., ventricular tumor)

Neoplasm

Intracranial hypertension (primary or secondary)

Neoplasm

Abscess

Epidural or Subdural Hemorrhage

Vascular Malformation

Toxins (carbon monoxide, lead)

Chronic daily headache

Medication overuse headache

New daily persistent headache

Psychosomatic

Post-concussion syndrome

Thunderclap headache (or sudden severe headache onset) is uncommon, but recognition and accurate diagnosis of this headache are important because there is often a serious underlying brain disorder that requires specific and urgent therapy.
These include subarachnoid hemorrhage, parenchymal hemorrhage, sinovenous thrombosis, intracranial infection, arterial dissection, pituitary apoplexy, intracranial hypotension, and intermittent hydrocephalus. After a history and physical examination, diagnostic testing often begins with a noncontrast head computed tomography (CT) in which acute blood will be bright. If subarachnoid hemorrhage is suspected and the head CT is nondiagnostic, then lumbar puncture should be performed with two tubes sent for cell count (to differentiate between subarachnoid blood in which the red blood cell count will remain constant and a traumatic tap in which the red blood cell count will decrease from tube 1 to tube 4). If arterial dissection, sinovenous thrombosis, or tumor is suspected, then MRI with and without gadolinium is indicated and may require specific sequences to visualize the neck vasculature or venous sinuses. If intracranial hypertension or hypotension is suspected, then a lumbar puncture with opening pressure is indicated. Generally, neuroimaging is indicated prior to lumbar puncture since mass lesions may pose the risk of herniation with lumbar puncture. Only after appropriate evaluation can more benign etiologies be diagnosed including first or severe migraine or tension headache, cluster headache, or exertion/coital headache.