Subarachnoid haemorrhage

Subarachnoid haemorrhage: recognising thunderclap headache in AU practice

By Dr HB Lo, FACRRM 7 min read

Subarachnoid haemorrhage (SAH) is bleeding into the subarachnoid space, most commonly from a ruptured intracranial aneurysm. AU incidence is approximately 5–10 per 100,000 per year; case fatality is around 30%.

The cardinal symptom is a thunderclap headache — peak severity within one minute. Any thunderclap headache requires immediate emergency transfer. Around 10–40% have a sentinel bleed — a severe headache days to weeks before the main rupture.

Non-contrast CT within 6 hours has sensitivity approaching 99% for SAH. If CT is negative, lumbar puncture for xanthochromia from 12 hours post-onset is required.

A headache that demands immediate action

The thunderclap headache — reaching maximum severity within one minute — is one of the most important clinical presentations in Australian general practice. Subarachnoid haemorrhage accounts for a substantial proportion of thunderclap headaches, and every missed diagnosis represents an avoidable death or preventable disability.

Australian incidence of SAH is approximately 5–10 per 100,000 per year, with around 30% case fatality. Women are slightly more commonly affected (F:M ~1.6:1), with peak age 50–60. Most cases result from rupture of a saccular intracranial aneurysm.

The GP’s role is recognition and immediate Category-1 transfer. This article explains how to make that recognition reliably.

A. Core clinical — the AU general-practice framework

Pathophysiology and causes

Aneurysmal SAH (~80–85%) — saccular aneurysms typically form at arterial bifurcations: anterior communicating artery (~30%), posterior communicating artery (~25%), middle cerebral artery bifurcation (~20%), basilar tip (~10%). Rupture delivers arterial blood under pressure into the subarachnoid space.

Non-aneurysmal SAH (~15–20%) — perimesencephalic (blood confined around the brainstem; better prognosis), AVM, vasculitis, reversible cerebral vasoconstriction syndrome (RCVS), cervical artery dissection.

Complications of aneurysmal SAH that drive morbidity and mortality include:

  • Rebleeding — highest risk in the first 24 hours (~4% on day one); justifies securing the aneurysm early
  • Vasospasm — peaks days 4–14; causes delayed cerebral ischaemia
  • Hydrocephalus — blood blocks arachnoid granulations; acute (~20%) or chronic (~10%)

Risk factors

  • Smoking — approximately 3× increased incidence; the single strongest modifiable risk factor
  • Hypertension (~2.5×)
  • Heavy alcohol use (~2×)
  • Family history — first-degree relative with SAH increases risk approximately 5×
  • Polycystic kidney disease (ADPKD) — approximately 10× increased risk
  • Connective tissue disorders — Ehlers-Danlos type IV, Marfan, fibromuscular dysplasia
  • Cocaine and amphetamine use — acute trigger

The sentinel bleed

Ten to forty per cent of aneurysmal SAH are preceded by a sentinel bleed — a small aneurysm leak causing a sudden severe headache, typically lasting hours to days, that resolves spontaneously. Patients with a sentinel bleed who are correctly diagnosed and treated have substantially better outcomes than those who present only after the major rupture.

Never dismiss a sudden severe headache that resolved on its own. If the onset was sudden and the headache was unlike the patient’s usual pattern, investigate. The resolution of the headache does not rule out SAH — it may indicate the patient is between the sentinel bleed and the catastrophic rupture.

Thunderclap headache: what to recognise

The typical SAH presentation:

  • Onset — sudden, reaching peak within 1 minute; many patients use phrases like “hit by a bat” or “explosion in the head”
  • Severity — “worst headache of life” in approximately 80% of cases
  • Associated symptoms — vomiting (~70%), neck pain/stiffness (~75%, may be delayed), photophobia, loss of consciousness (~10–25%), seizure (~10–20%), focal neurological deficit (~25%)
  • Onset during exertion — Valsalva, sex, defaecation; occurs in ~25%

Examination findings: hypertension and bradycardia (Cushing reflex if raised ICP); meningism (neck stiffness, Kernig, Brudzinski — may be absent in early presentation); third cranial nerve palsy with posterior communicating artery aneurysm (pupil-involving CN III palsy is SAH until proved otherwise); subhyaloid haemorrhage on fundoscopy (Terson syndrome — pathognomonic, seen in 10–40%).

The Ottawa SAH Rule

For alert adults over 15 with a non-traumatic headache reaching peak within one hour, any one of the following criteria mandates CT brain and lumbar puncture:

  • Age ≥40
  • Neck pain or stiffness
  • Witnessed loss of consciousness
  • Onset during exertion
  • Thunderclap onset (peak within one second)
  • Limited neck flexion on examination

Sensitivity 100%, specificity ~15%. Use as a safety net — not as the ceiling for clinical suspicion.

GP actions

Any thunderclap headache = Category-1 transfer, regardless of subsequent resolution.

Pre-transfer stabilisation:

  • Airway — secure if GCS <8
  • Oxygen — target SpO₂ ≥94%
  • IV access
  • BP — target SBP <160; avoid sudden drops
  • Antiemetic (ondansetron 4–8 mg IV)
  • Analgesia — paracetamol; opioid if required; avoid NSAIDs (bleeding risk)
  • Pre-notify receiving ED and neurosurgery
  • Document: time of onset, peak timing, neurological exam, BP trend

Do not wait for the headache to return. Call 000, pre-notify the ED, and transfer immediately.

Hospital management (specialist)

  • Non-contrast CT brain → CT angiography if SAH confirmed
  • Transfer to centre with neurosurgery and interventional neuroradiology
  • Endovascular coiling — preferred for most aneurysms per ISAT (see below); within 24–48 hours to prevent rebleeding
  • Nimodipine 60 mg PO 4-hourly for 21 days — standard care for preventing vasospasm-related delayed cerebral ischaemia
  • BP control, EVD for hydrocephalus, DVT prophylaxis, glycaemic and electrolyte management

B. Evidence and controversies

Coiling versus clipping: ISAT

The International Subarachnoid Aneurysm Trial (ISAT, Lancet 2002) randomised 2,143 patients with ruptured intracranial aneurysms suitable for both treatments to endovascular coiling or neurosurgical clipping. At one year, the rate of death or dependence was lower with coiling (23.7% vs 30.6%). Long-term follow-up confirmed durable benefit of coiling in suitable patients. Coiling has higher recurrence and re-treatment rates; surgical clipping is preferred for some geometries (MCA bifurcation, wide-neck aneurysms). The decision is made by the neurosurgical and neuroradiological team.

Nimodipine

Multiple randomised trials demonstrate that nimodipine 60 mg 4-hourly for 21 days reduces delayed cerebral ischaemia and improves outcome after SAH. It is not a vasodilator for established vasospasm — it is a neuroprotective agent given prophylactically. Hypotension is the main risk; monitor BP at initiation.

Tranexamic acid: ULTRA trial

The ULTRA trial (Lancet 2021) assessed short-course tranexamic acid (1g bolus then 1g TDS until aneurysm secured, maximum 24 hours) for preventing rebleeding in aneurysmal SAH. Despite reducing rebleeding numerically, overall outcome at 6 months was no different. Tranexamic acid is not recommended as routine practice in SAH.

CT timing and lumbar puncture

Non-contrast CT within 6 hours of onset has sensitivity approaching 99% for SAH at high-volume centres. Beyond 6 hours, sensitivity falls to approximately 85% at 24 hours and continues to decline. Lumbar puncture from at least 12 hours post-onset is required when CT is negative and clinical suspicion persists — xanthochromia by spectrophotometry is the diagnostic standard, detectable for up to 2 weeks.

C. SAH in Australian practice

MBS items

PBS items

Nimodipine is PBS Authority Required for SAH vasospasm prevention. Antihypertensives and statins follow standard PBS criteria for their respective indications. Antidepressants for post-SAH depression are PBS general schedule.

Driving (Austroads)

Per Austroads Assessing Fitness to Drive: private vehicle licence requires a minimum 3-month cessation period after aneurysm has been secured, with neurology clearance. Commercial licence requirements are longer and may result in permanent restriction if cognitive impairment persists. Document the discussion with patients who drive commercially.

NDIS and post-SAH rehabilitation

Post-SAH cognitive and functional impairment frequently qualifies for NDIS support in patients under 65. My Aged Care pathways apply for those 65 and over. Occupational therapy and neuropsychology are the priority referrals in the post-discharge period.

D. Special populations

Patients with ADPKD. Risk of intracranial aneurysm is approximately 10 times the background rate. Screening MRA for intracranial aneurysms is recommended for ADPKD patients with a family history of SAH or intracranial aneurysm — discuss with a nephrologist and neurologist.

Pregnant women. SAH during pregnancy requires the same immediate transfer and investigation as in any other patient. Neuroradiological and obstetric co-management is needed; coiling is generally preferred to clipping in pregnancy to minimise radiation exposure.

Family screening. If a patient has ≥2 first-degree relatives with SAH or intracranial aneurysm, screening MRA is recommended every 5–10 years for unaffected family members. Discuss yield, psychological impact, and potential for incidental findings before referring.

Aboriginal and Torres Strait Islander patients. Coordinate with ACCHO services and tertiary neurosurgery for acute transfer and rehabilitation. Closing the Gap PBS Co-payment applies to ongoing medications. Culturally safe discussions about cognitive rehabilitation and advance care planning are important in the post-acute phase.

When to escalate

Immediate (000 / Category-1 transfer):

  • Any thunderclap headache — sudden onset reaching peak within 1 minute
  • Severe headache with LOC, vomiting, or meningism
  • Third nerve palsy with headache
  • Any subhyaloid haemorrhage on fundoscopy

Post-SAH — refer urgently if any:

  • New neurological symptom after hospital discharge
  • New sudden or severe headache
  • Cognitive deterioration — may indicate communicating hydrocephalus

Routine post-SAH management:

  • Neurosurgery review at 6 months and 1 year
  • Follow-up MRA/CTA per treating neurosurgeon’s protocol
  • GP coordination of smoking cessation, BP management, and psychological care

What this article is and is not

This is general health information drawn from Stroke Foundation Australia Clinical Guidelines 2022, Therapeutic Guidelines (eTG), BMJ Best Practice, and current trial evidence. It is not personal medical advice and does not replace clinical assessment and specialist management.

For AU consumer information: Stroke Foundation Australia, Brain Foundation Australia, HealthDirect.

Sources cited

  1. Stroke Foundation — AU Clinical Guidelines for Stroke Management 2022
  2. Therapeutic Guidelines (eTG)
  3. BMJ Best Practice — Subarachnoid haemorrhage
  4. Ottawa SAH Rule — MDCalc
  5. ISAT — coiling vs clipping (Lancet 2002)
  6. ULTRA — tranexamic acid in SAH (Lancet 2021)
  7. ANZAN
  8. Austroads — Assessing Fitness to Drive
  9. Stroke Foundation Australia — patient resources
  10. Brain Foundation Australia
  11. HealthDirect

Frequently asked questions

  • What is a thunderclap headache and how is it different from a migraine?

    A thunderclap headache reaches maximum severity within one minute of onset — it peaks 'like a clap of thunder.' A migraine typically builds over 30 minutes to several hours and is part of a recurrent pattern the patient recognises. The key clinical difference is speed of onset: sudden peak under one minute demands investigation for SAH and other secondary causes regardless of the patient's prior headache history. A 'worst headache of my life' description occurs in about 80% of confirmed SAH, but it is the sudden peak rather than severity alone that drives the Ottawa SAH Rule. Patients who have previously had SAH or sentinel bleeds may not describe it as their worst headache, so onset pattern is the primary discriminator.

  • What is the Ottawa SAH Rule?

    The Ottawa SAH Rule is a clinical decision aid for alert adult patients (age >15) with a non-traumatic headache that reached peak intensity within one hour. Any one of the following criteria is sufficient to require CT brain and lumbar puncture: age ≥40; neck pain or stiffness; witnessed loss of consciousness; onset during exertion; thunderclap onset (peak within one second); limited neck flexion on examination. The rule has a reported sensitivity of 100% and specificity of approximately 15%, meaning it is designed to be sensitive at the cost of some over-investigation. It supports clinical reasoning but does not replace GP judgement — any thunderclap headache warrants transfer regardless of whether all Ottawa criteria are met.

  • How reliable is CT for ruling out SAH?

    Non-contrast CT brain within 6 hours of symptom onset has sensitivity approaching 99% for SAH in experienced centres using modern scanners. Sensitivity falls to approximately 85% by 24 hours as blood becomes isodense with brain tissue. A negative CT brain within 6 hours in a well patient with good clinical context may be sufficient to rule out SAH in specialised settings, but this is a decision for the receiving ED and neurology team, not for GP reassurance. If presentation is delayed more than 6 hours, or if clinical suspicion persists despite a negative CT, lumbar puncture for xanthochromia from at least 12 hours post-onset is required. Xanthochromia (yellowish CSF by spectrophotometry) persists for up to 2 weeks.

  • What is a sentinel bleed and why does it matter?

    A sentinel bleed is a small leak from an aneurysm that occurs days to weeks before a major rupture, producing a severe sudden headache that typically resolves within hours to days. It occurs in 10–40% of aneurysmal SAH and represents a critical missed-diagnosis opportunity. Patients with sentinel bleeds who reach medical attention and are correctly diagnosed and treated have substantially better outcomes than those who present after the major rupture. The clinical message: never dismiss a sudden severe headache that resolved spontaneously, particularly if it is unlike the patient's usual headaches. Headache that 'went away' does not exclude SAH — obtain CT and consider LP.

  • What does long-term recovery look like after SAH?

    About 30% of SAH survivors have moderate to severe disability. Even in those with a 'good outcome' by standard measures, approximately 50% have lasting cognitive, mood, or executive function difficulties — fatigue, memory problems, reduced processing speed, anxiety, depression, and PTSD are all common. Return to work is often delayed or incomplete. Long-term management in general practice includes: smoking cessation (smoking is the strongest modifiable risk factor, approximately tripling incidence); BP control; follow-up MRA or CTA at 6 months, 1 year, and 5 years to monitor for aneurysm recurrence or new aneurysm formation; family screening by MRA for first-degree relatives if two or more family members have been affected; and Mental Health Care Plan referral for psychological sequelae.

Source quality

Sources grouped by evidence tier. AU primary tier first; international where AU is silent or lagging; named-author reconstruction where guidelines have not yet caught up. How tiers work.