GPAtlas🧭 When to suspect
A subarachnoid haemorrhage (SAH) is bleeding into the subarachnoid space – most often from a ruptured berry (saccular) aneurysm of the Circle of Willis. It is a neurosurgical emergency: case fatality is high, a substantial proportion of patients die before reaching hospital, and rebleeding in the first hours is the major preventable cause of early death. This is precisely why the only thing that matters in primary care is rapid recognition and immediate transfer.
Suspect SAH in anyone presenting with a thunderclap headache – a sudden, severe (“worst headache of my life”) headache that reaches maximum intensity within 1 to 5 minutes. The speed of onset is the most important diagnostic feature – more so than the absolute severity of the pain. Most thunderclap headaches are not due to SAH, but that must never deter investigation: the key clinical skill is to act on the history alone and refer immediately, even when examination and observations are entirely normal.
Every thunderclap headache also has a short list of dangerous mimics – all of which need the same-day emergency pathway.
| Cause of thunderclap headache | Pointers / why it matters |
|---|---|
| Subarachnoid haemorrhage | The must-not-miss diagnosis – sudden agonising headache ± vomiting, neck stiffness, photophobia, altered consciousness or focal deficit. |
| Intracerebral haemorrhage | Headache with a focal neurological deficit and/or reduced consciousness. |
| Cervical artery dissection (carotid/vertebral) | Younger patient; neck or face pain, Horner’s syndrome, recent trauma or neck manipulation; high stroke risk. |
| Cerebral venous sinus thrombosis | Headache ± seizures or papilloedema; prothrombotic states, pregnancy/puerperium, combined hormonal contraception. |
| Reversible cerebral vasoconstriction syndrome (RCVS) | Recurrent thunderclaps over days; post-partum or after vasoactive drugs (including cocaine, decongestants). |
| Pituitary apoplexy | Sudden headache with visual field loss or ophthalmoplegia ± features of hypopituitarism. |
| Bacterial meningitis | Fever, meningism, photophobia ± non-blanching rash. |
| Spontaneous intracranial hypotension | Orthostatic headache (worse upright, relieved lying flat) from a CSF leak. |
Raise suspicion further with known risk factors: smoking and hypertension (the strongest modifiable factors), excess alcohol, a family history of SAH or aneurysm (especially two or more first-degree relatives), autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome, coarctation of the aorta, use of sympathomimetic drugs (e.g. cocaine), increasing age and female sex.
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🧠 Clinical pearl – the sentinel headache In a minority of patients a small herald (“warning”) bleed produces a sudden, severe, self-limiting headache days to weeks before the major rupture. A “worst-ever” headache that has since settled is therefore a red flag for an impending catastrophic bleed – not reassurance – and still warrants urgent investigation. |
Source: NICE NG228 · National Clinical Guideline for Stroke
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