A headache of sudden onset — especially reaching maximum intensity within seconds to minutes — is a neurological emergency until a subarachnoid haemorrhage is excluded. The 'thunderclap headache' pattern must always be evaluated acutely.
The thunderclap headache (worst headache of life, reaching peak in seconds) has a 10–20% incidence of subarachnoid haemorrhage (SAH). However, most sudden-onset headaches are benign (primary thunderclap headache, sexual headache, exertional headache). The Ottawa Subarachnoid Haemorrhage Rule guides which patients need lumbar puncture after negative CT head.
Subarachnoid haemorrhage (SAH)
Rupture of a cerebral aneurysm — classically 'worst headache of my life', reaching maximum in seconds. 10–20% of thunderclap headaches. Medical emergency — 30% mortality within 30 days.
Primary thunderclap headache
Benign sudden-onset severe headache without structural cause — diagnosis of exclusion after SAH excluded by CT + LP.
Cerebral venous sinus thrombosis (CVST)
Sudden severe headache, may be progressive. Associated with pregnancy, OCP use, thrombophilia. MRI venography diagnostic.
Hypertensive crisis
Severe blood pressure elevation (>180/120) with severe headache, visual changes, confusion. BP measurement is immediate first step.
Meningitis / encephalitis
Sudden severe headache with fever, neck stiffness, photophobia. Bacterial meningitis progresses within hours — emergency.
In HIV patients with CD4 <100, sudden headache with focal neurological signs — ring-enhancing lesions on CT/MRI.
A thunderclap headache reaches maximum severity within 60 seconds (often seconds). It is classically described as the 'worst headache of my life'. About 10–20% are caused by subarachnoid haemorrhage — bleeding into the space around the brain from a ruptured aneurysm. Without treatment, SAH carries 30% mortality within days. Every thunderclap headache requires emergency CT head evaluation.
No. Most sudden-onset severe headaches are benign — primary thunderclap headache, sexual headache, or exertional headache are all benign conditions. However, subarachnoid haemorrhage presents identically to benign thunderclap headache and cannot be excluded clinically. CT head within 6 hours has >98% sensitivity for SAH; lumbar puncture is added if CT is negative.
Migraines typically build over 15–60+ minutes (not sudden), are often preceded by aura, are throbbing/unilateral, and are familiar — the same as previous attacks. A true thunderclap headache is sudden (seconds to maximum), is the worst ever experienced, and feels qualitatively different. A migraine patient reporting 'this one is completely different' warrants the same emergency evaluation as a thunderclap headache.
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