Differential Diagnosis of Thunderclap Headache

Life-Threatening Causes

Subarachnoid hemorrhage (SAH) is the primary concern in any thunderclap headache presentation and must be excluded first, as it accounts for 10-25% of cases and carries 27-44% mortality, according to the American Heart Association 1, 2
Aneurysmal subarachnoid hemorrhage is described by 80% of alert patients as "worst headache of my life" reaching maximal intensity within seconds to one minute, as reported by the American Stroke Association 1, 2
Cervical artery dissection presents with thunderclap headache in up to 20% of cases, though gradual onset is more typical, and is associated with Horner syndrome when present, according to the American College of Radiology 3
Cerebral venous sinus thrombosis can present with thunderclap headache mimicking SAH, particularly when involving superior sagittal or lateral sinuses, as noted by the American Heart Association 4
Intracerebral hemorrhage is associated with aneurysms or vascular malformations, according to the American College of Radiology 3

Spontaneous intracranial hypotension (SIH) is a recognized presentation pattern of thunderclap headache followed by orthostatic headache, as reported by the American Academy of Neurology 5
Intracranial infection, such as meningitis or encephalitis, can present with acute thunderclap headache, particularly when associated with fever and altered mental status, according to the American College of Radiology 3, 6

For patients presenting within 6 hours of headache onset without new neurological deficits, a noncontrast head CT on a high-quality scanner interpreted by a board-certified neuroradiologist has 98.7% sensitivity for detecting SAH, as recommended by the American College of Radiology 1, 2, 7
The Ottawa SAH Rule criteria should be applied, including age ≥40 years, neck pain/stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion on examination, as suggested by the American Heart Association 1, 2
CT sensitivity for SAH is 98% with specificity 99% when performed acutely, according to the American College of Radiology 3, 6

If CT and LP are negative, brain MRI with susceptibility-weighted imaging (SWI) and vascular imaging (CTA or MRA) should be obtained to evaluate for RCVS, arterial dissection, cerebral venous thrombosis, and other vascular pathology, as recommended by the American College of Radiology 3, 7
CTA has >95% sensitivity for aneurysms ≥3mm and is faster and less invasive than conventional angiography, according to the American College of Radiology 7

Never rely on CT alone after 6 hours, as CT sensitivity drops significantly with time, and LP is mandatory when clinical suspicion remains high, as warned by the American Heart Association 1, 7
Do not skip LP based on negative CT in high-risk presentations, as failure to identify SAH leads to nearly 4-fold higher likelihood of death or disability, according to the American College of Radiology 2

Patients with first-degree relatives with aneurysms or vascular abnormalities should undergo neuroimaging even with atypical presentations, as these conditions are familial, according to the American College of Radiology 3
Autosomal dominant polycystic kidney disease (ADPKD) increases the risk for intracranial aneurysms, and patients should be educated to recognize thunderclap headache as requiring immediate medical attention, as recommended by the American College of Radiology 2