Sudden severe headache: causes, red flags and when to call 999

Thunderclap headache is defined by its speed rather than by its pain level: it reaches maximum intensity in about a minute or less. Doctors treat it as a possible bleed around the brain first, because the consequences of missing one are serious and treatment works best early.

Unlike many symptoms, the priority order here is to rule out dangerous causes first, even though most are not found.

• Subarachnoid haemorrhage: bleeding from a ruptured aneurysm or other vessel around the brain, often with neck stiffness, vomiting, light sensitivity or collapse.

• Other vascular causes: reversible cerebral vasoconstriction syndrome, cervical artery dissection (neck pain, sometimes after neck strain), cerebral venous thrombosis, pituitary apoplexy, and stroke.

• Hypertensive crisis: very high blood pressure with headache, confusion or vision change.

• Meningitis or encephalitis: headache with fever, neck stiffness, rash or drowsiness.

• Primary headache disorders: primary thunderclap headache, exertional or cough headache, and sexual-activity headache, which are diagnoses made after dangerous causes are excluded.

• Migraine or cluster headache that starts abruptly and intensely.

A first-ever or very different headache, onset during exertion, loss of consciousness, neck stiffness, vomiting, a seizure, confusion, any weakness or speech problem, double vision, or a headache that starts after a head or neck injury all increase concern. Age over 50 and a family history of aneurysm or polycystic kidney disease also raise the suspicion.

• Call 999 immediately: a headache that peaks within a minute, or any sudden severe headache with collapse, confusion, seizure, weakness, slurred speech or facial droop.

• Do not drive yourself to hospital; ambulance crews can pre-alert the hospital.

• Tell staff the exact time it began and whether it was at its worst straight away.

• If it was a thunderclap headache that has now eased, you should still be assessed urgently; improvement does not exclude a bleed.

You can expect observations, a neurological examination, and a non-contrast CT scan of the head, ideally within six hours of onset when it is most sensitive. If the scan is normal, doctors may recommend a lumbar puncture to look for blood breakdown products in the spinal fluid, or a CT or MR angiogram to look at the vessels, depending on the timing and your risk.

• CT head: quick and sensitive for fresh blood.

• Lumbar puncture: detects bleeding that CT can miss, particularly after the first hours.

• CT or MR angiography: shows an aneurysm or a narrowed or torn vessel.

• Blood tests and blood pressure: look for infection, clotting issues and hypertensive causes.

If the work-up is clear, a specialist may diagnose a primary headache disorder and advise on triggers and treatment. Recurrent thunderclap-type headaches should be reviewed by neurology. Self-care does not apply in the first instance; the priority is urgent assessment. Afterwards, track triggers such as exertion, and avoid excessive painkillers.

This is an NHS emergency pathway. Private hospitals with 24-hour emergency departments exist but many cannot manage a brain haemorrhage on site and will transfer you, so 999 remains the right call.