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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- Classic presentation: sudden-onset thunderclap headache peaking within 1–5 minutes — "worst headache of my life"
- CT head within 6 hours of onset has ~100% sensitivity — if >6 hours and CT negative, LP for xanthochromia is mandatory
- Most common cause: ruptured berry (saccular) aneurysm (~85%). Most common site: anterior communicating artery
- Nimodipine 60 mg every 4 hours PO for 21 days — reduces cerebral vasospasm
- Definitive treatment: endovascular coiling (preferred over surgical clipping per ISAT trial)
Overview
Subarachnoid haemorrhage (SAH) is bleeding into the subarachnoid space between the arachnoid and pia mater. Approximately 85% of non-traumatic SAH is caused by rupture of a saccular (berry) aneurysm, most commonly at the anterior communicating artery, posterior communicating artery, or middle cerebral artery bifurcation. Other causes include perimesencephalic (non-aneurysmal) SAH (~10%), arteriovenous malformations, and rarely vasculitis or coagulopathy. SAH is a neurological emergency with ~30–50% overall mortality, and ~50% of survivors have significant long-term disability.
Epidemiology
The incidence of SAH in the UK is approximately 8 per 100,000 per year. Peak incidence is age 50–60 years, and it is 1.6 times more common in women. Risk factors include hypertension, smoking, excessive alcohol, family history (first-degree relative with SAH), polycystic kidney disease, connective tissue disorders (Ehlers-Danlos type IV, Marfan syndrome), and previous history of SAH. Berry aneurysms are found incidentally in ~3% of the population but most never rupture.
Clinical Features
Symptoms
Thunderclap headache: sudden-onset, maximal intensity within 1–5 minutes — "worst headache of my life"
Often occipital, may radiate to neck
Nausea and vomiting
Neck stiffness (may take 6–12 hours to develop)
Photophobia
Loss of consciousness — occurs in ~50% at ictus
Seizures — occur in ~10%
Sentinel headache: preceding minor headache days-weeks before (warning leak) — missed in up to 25% of cases
Signs
Meningism: neck stiffness, positive Kernig and Brudzinski signs (may be absent early)
Reduced GCS — range from fully alert to coma depending on severity
Third nerve palsy (ptosis, dilated pupil, eye "down and out") — posterior communicating artery aneurysm
Sixth nerve palsy — false localising sign of raised ICP
Subhyaloid (preretinal) haemorrhage on fundoscopy — Terson syndrome
Focal neurological signs may develop later from vasospasm or hydrocephalus
Investigations
First-line
Non-contrast CT headIMMEDIATE. Within 6 hours of onset: sensitivity ~100% (appears as hyperdense blood in subarachnoid spaces/basal cisterns). After 6 hours sensitivity drops — if negative, LP is required
CT angiography (CTA)Performed alongside CT if SAH confirmed — identifies aneurysm location and morphology for treatment planning
Second-line
Lumbar punctureIf CT negative and >6 hours from onset. Wait ≥12 hours from symptom onset. Look for xanthochromia (yellow discolouration from bilirubin in CSF — detected by spectrophotometry). Raised opening pressure, uniformly blood-stained CSF
Specialist
Digital subtraction angiography (DSA)Gold standard for aneurysm detection if CTA negative but high clinical suspicion — repeat at 1–2 weeks if initial DSA negative
MRI brain/MRAMay help if diagnosis uncertain or non-aneurysmal SAH suspected
1
Immediate management
- ABCDE approach. Admit to neurosciences centre or neurocritical care
- Analgesia: IV paracetamol, codeine. Avoid NSAIDs before aneurysm is secured
- Nimodipine 60 mg PO every 4 hours for 21 days — reduces delayed cerebral ischaemia from vasospasm
- Bed rest, quiet environment, avoid straining (risk of rebleed)
- Maintain euvolaemia — avoid fluid restriction and dehydration
2
Definitive aneurysm treatment
- Endovascular coiling is preferred over surgical clipping where anatomy permits (ISAT trial — better outcomes at 1 year)
- Aim for early aneurysm securement within 24–48 hours of diagnosis to prevent rebleeding
- Decision between coiling vs clipping by MDT (neuroradiologist + neurosurgeon)
3
Managing complications
- Vasospasm/delayed cerebral ischaemia (days 4–14): nimodipine, maintain euvolaemia, induced hypertension if needed
- Hydrocephalus (acute or chronic): EVD for acute; VP shunt for chronic
- Rebleeding: highest risk in first 24 hours — early aneurysm securement is critical
- Seizures: treat acutely but routine prophylaxis not recommended
- Hyponatraemia: SIADH or cerebral salt wasting — monitor Na closely
Complications
- Rebleeding: Highest risk in first 24 hours. Mortality ~70% with rebleed. Early aneurysm securement is the most important intervention
- Vasospasm and delayed cerebral ischaemia: Peak days 4–14. Monitored by serial transcranial Doppler. Nimodipine reduces incidence
- Hydrocephalus: Acute (blood obstructing CSF flow) or chronic (communicating). May require EVD or VP shunt
- Hyponatraemia: SIADH or cerebral salt wasting — common and clinically important
- Cardiac complications: Troponin rise, ECG changes (deep T-wave inversion, long QT), stress cardiomyopathy
- Long-term: Cognitive deficits, fatigue, depression, headache in survivors
UKMLA Exam Tips
- 1Thunderclap headache = SAH until proven otherwise. CT head within 6 hours has ~100% sensitivity
- 2If CT negative and >6 hours from onset: LP is MANDATORY — look for xanthochromia (do LP ≥12 hours after onset)
- 3Xanthochromia = yellow CSF from bilirubin — distinguishes true SAH from traumatic tap
- 4Nimodipine 60 mg PO every 4 hours for 21 days — the only drug proven to improve outcomes (reduces vasospasm)
- 5Commonest aneurysm site: anterior communicating artery. PComA aneurysm → third nerve palsy
- 6Coiling preferred over clipping (ISAT trial) — better outcomes for most aneurysm configurations
- 7Do NOT confuse with meningitis: both cause headache + neck stiffness, but SAH has thunderclap onset; meningitis evolves over hours
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