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Textbook/Neurosurgery/Subarachnoid haemorrhage diagnosis and grading
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Subarachnoid haemorrhage diagnosis and grading

Recognise suspected aneurysmal subarachnoid haemorrhage, use the time-dependent CT and lumbar-puncture diagnostic pathway correctly, identify the culprit aneurysm and apply clinical and radiological grades as descriptors rather than treatment gates.

26 min full readUK scopeSources checked 13 Sept 2026Clinical review pending
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Thunderclap headache is an emergency

A headache reaching maximal intensity within minutes, especially with neck pain, vomiting, photophobia, collapse, seizure, reduced consciousness or focal deficit, may be aneurysmal SAH even when the patient looks well after the event.

Action: Arrange urgent senior assessment and non-contrast CT, give effective analgesia while documenting sedating and pupillary effects, and discuss confirmed SAH urgently with a specialist neurosurgical centre because rebleeding risk is highest in the first 24 hours.

Open the sections you need. The overview is shown first.
01Core principlesThe concepts and mechanisms needed to understand the subject.

Spontaneous aneurysmal SAH follows rupture of an intracranial arterial aneurysm into the subarachnoid space. Blood spreads through basal cisterns and sulci and may enter ventricles or brain parenchyma. The abrupt pressure surge explains thunderclap headache, vomiting, transient loss of consciousness and sometimes cardiac or respiratory disturbance. Meningeal irritation causes neck stiffness and photophobia, but these can develop after the first hours and their absence cannot exclude an early bleed.

Diagnosis depends on timing and test quality. Modern non-contrast CT is highly accurate early, which is why NICE separates scans performed within 6 hours from later scans. The early rule applies only when CT is genuinely within that window and is reported and documented by a radiologist. Beyond 6 hours, sensitivity falls as blood redistributes and becomes less dense; bilirubin formation then makes spectrophotometric CSF analysis useful after 12 hours. LP remains a procedure with contraindications and is not automatic after every negative early scan.

Grading serves communication and risk estimation. Clinical scales such as WFNS or Hunt-Hess describe initial neurological severity; CT scales such as modified Fisher estimate clot burden related to vasospasm and delayed cerebral ischaemia. Grade can change after resuscitation or CSF diversion, and prediction is imperfect. NICE therefore rejects using a severity score alone to determine transfer or treatment. The diagnosis, aneurysm anatomy, physiological state, complications and patient goals remain central.

Key points

  • This pathway concerns spontaneous aneurysmal SAH, not traumatic subarachnoid blood. Suspect it after unexplained thunderclap headache or compatible collapse, seizure, reduced consciousness or focal signs.
  • Urgent non-contrast CT is first line. If CT within 6 hours is negative and is reported and documented by a radiologist, NICE advises against routine LP; seek specialist advice and consider alternative diagnoses.
  • If negative CT was done more than 6 hours after onset, consider LP; wait at least 12 hours from onset and diagnose SAH when CSF spectrophotometry shows elevated bilirubin. Do not use visual inspection alone.
  • Once SAH is confirmed, perform CT angiography without delay. If CTA does not identify the cause but aneurysmal SAH remains suspected, discuss digital subtraction angiography with the specialist team.
  • WFNS grades clinical severity mainly from GCS and focal motor deficit; modified Fisher grade describes CT blood burden and ventricular blood to estimate vasospasm/DCI risk. Neither scale replaces serial examination or holistic decisions.
  • Urgently discuss transfer to a specialist neurosurgical centre. The culprit aneurysm is usually secured early by endovascular coiling or surgical clipping through a specialist multidisciplinary decision.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Define thunderclap preciselyRed flag

Ask how quickly pain reached maximum intensity, the exact onset time and what the person was doing. Exertion, sexual activity or Valsalva may precede rupture, but spontaneous onset is also common. “Worst ever” wording is less useful than time to peak.

Look beyond headacheRed flag

Assess consciousness, seizure, meningism, photophobia, vomiting, focal weakness, speech, eye movements and pupils. A third-nerve palsy with a dilated pupil raises concern for a posterior communicating artery aneurysm but does not replace vascular imaging.

Consider warning bleeds and mimics

A preceding sudden headache can represent a minor leak. Migraine, reversible cerebral vasoconstriction, cervical artery dissection, venous thrombosis, pituitary apoplexy, meningitis and spontaneous intracranial hypotension remain important alternatives.

Assign a clinical grade

After initial stabilisation and before heavy sedation when possible, record GCS and focal motor deficit for WFNS. Grade I is GCS 15 without motor deficit; II is GCS 13–14 without deficit; III is GCS 13–14 with deficit; IV is GCS 7–12; V is GCS 3–6.

Assign a blood-burden grade

Modified Fisher grading uses CT thickness and intraventricular blood: 0 no SAH/IVH; 1 thin SAH without IVH; 2 thin SAH with IVH; 3 thick SAH without IVH; 4 thick SAH with IVH. Use it to communicate DCI risk, not to decide transfer alone.

Recognise early complicationsRed flag

Rebleeding, acute hydrocephalus, intraparenchymal extension, seizures and cardiopulmonary disturbance can worsen consciousness. Repeat examination and imaging are required when grade or physiology changes.

Red flags requiring action

  • Unexplained thunderclap headache requires urgent assessment even when neurological examination is normal or pain improves.
  • Collapse, seizure, reduced consciousness, focal deficit, cranial-nerve palsy, meningism or repeated vomiting strengthens suspicion and may indicate high-grade disease or a complication.
  • A negative CT more than 6 hours after symptom onset is less reliable; consider lumbar puncture, performed at least 12 hours after onset, for spectrophotometric bilirubin when it is safe.
  • Do not perform LP in cardiorespiratory instability, evolving focal signs, impaired consciousness suggesting mass effect, significant bleeding risk or an unsafe structural context.
  • Confirmed SAH with falling GCS, worsening focal signs, hydrocephalus or rebleeding requires immediate neurocritical and neurosurgical action, independent of the recorded grade.
  • A poor WFNS or Hunt-Hess grade must not be used alone to refuse transfer or aneurysm treatment; reversible hydrocephalus, seizure, sedation and physiology can alter the clinical grade.
03Interpreting evidenceInformation, measurements and their limitations.
Reasoning sequence

Consider the information, its meaning and its limitations before deciding what follows.

  1. 01
    Urgent non-contrast CT head
    Why
    Detect acute subarachnoid blood, intraventricular or parenchymal extension, hydrocephalus and mass effect.
    Interpretation and limitations
    Blood is often hyperdense in basal cisterns, fissures and sulci. A positive scan establishes SAH. For a negative scan, record onset-to-scan time and reporter: within 6 hours with radiologist reporting, NICE advises no routine LP; after 6 hours, consider LP.
  2. 02
    Lumbar puncture with CSF spectrophotometry
    Why
    Detect bilirubin after a negative later CT when clinical suspicion of SAH remains and the procedure is safe.
    Interpretation and limitations
    Allow at least 12 hours after symptom onset. Elevated bilirubin supports SAH; absence prompts reconsideration of alternative diagnoses. A traumatic tap and visual xanthochromia are not equivalent to validated spectrophotometric bilirubin analysis.
  3. 03
    CT angiography of the head
    Why
    Identify the culprit aneurysm and define anatomy for treatment after SAH is confirmed.
    Interpretation and limitations
    Aneurysmal SAH is diagnosed when CTA shows an aneurysm and the blood distribution is compatible with rupture. An incidental aneurysm may not explain the bleed; pattern and vascular anatomy must agree.
  4. 04
    Digital subtraction angiography
    Why
    Resolve persistent suspicion when CTA is negative or inconclusive and provide treatment-level vascular detail.
    Interpretation and limitations
    DSA is invasive but remains the reference investigation for small or complex vascular lesions. NICE advises considering it after specialist discussion when CTA has not found a suspected aneurysm.
  5. 05
    Serial neurological examination and repeat CT
    Why
    Detect rebleeding, hydrocephalus, seizure-related decline and evolving mass effect after diagnosis.
    Interpretation and limitations
    Recalculate the clinical description after resuscitation and interventions. Worsening consciousness or new deficit prompts immediate imaging and specialist action; a numerical grade never overrides deterioration.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked case: thunderclap pathwaySudden severe headache with a normal first examinationAn adult reports an unexplained headache reaching maximal intensity within one minute and has no persistent focal deficit.
  1. 1Arrange urgent senior assessment and non-contrast CT, recording exact onset-to-scan time; give effective analgesia and document opioid timing because sedation and pupil effects influence subsequent examination.
  2. 2If CT shows subarachnoid blood, urgently discuss transfer with a specialist neurosurgical centre and obtain CTA without delay. If CT is negative, choose the next step from the 6-hour timing and radiologist-reporting boundary.
  3. 3For a negative CT within 6 hours that is reported and documented by a radiologist, do not routinely offer LP; discuss with a senior decision-maker, seek specialist advice and evaluate alternative dangerous causes.
  4. 4For a negative CT after 6 hours with ongoing suspicion, consider LP when safe, timing it at least 12 hours after onset and sending spectrophotometry for bilirubin.
  5. 5If SAH is confirmed, assess clinical and CT severity, hydrocephalus and rebleeding while the specialist team plans early aneurysm securing.
02Grading pathwayDescribe severity without denying careAneurysmal SAH is confirmed and the team needs a shared severity description.
  1. 1Record GCS, focal motor deficit and confounders to assign WFNS, and document CT thickness and IVH to assign modified Fisher grade.
  2. 2Use the grades for communication, prognosis and complication surveillance, but base transfer and treatment on the whole patient and repeat grading after reversible causes such as hydrocephalus or seizure are treated.
  3. 3Escalate any falling GCS, new deficit or hydrocephalus immediately regardless of the initial numerical category.
03Negative vascular studySAH confirmed but CTA inconclusiveCT or CSF establishes SAH but CTA does not show a compatible culprit aneurysm.
  1. 1Review the blood distribution and CTA quality with neuroradiology and the neurosurgical centre, considering non-aneurysmal perimesencephalic and other vascular causes.
  2. 2Consider DSA when an aneurysmal source remains suspected and agree whether follow-up vascular imaging is needed if the first angiographic evaluation is negative.
  3. 3Continue neurological and hydrocephalus surveillance while the cause is clarified because a negative first CTA does not erase the confirmed haemorrhage.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
  • Repeat GCS components, pupils, focal signs and vital observations, distinguishing opioid or sedative effects from true neurological change.
  • Watch for abrupt recurrent headache, new deficit or reduced consciousness indicating rebleeding, particularly in the first 24 hours before the aneurysm is secured.
  • Monitor fluid balance, sodium, cardiac rhythm and oxygenation; aneurysmal SAH can produce natriuresis, arrhythmia and cardiopulmonary complications.
  • After aneurysm treatment, monitor for delayed cerebral ischaemia through trained serial examination and specialist vascular or perfusion tests when the examination is limited.
  • Plan cognitive, behavioural, visual, physical and vocational rehabilitation because apparently good motor recovery can coexist with persistent deficits.
06Special situationsVariants, exceptions and circumstances that change the usual approach.

Time changes the test

An early high-quality CT and a later CT do not carry the same negative predictive value. The 6-hour boundary determines whether LP is routinely considered.

Bilirubin needs time

Waiting at least 12 hours permits haem breakdown to produce detectable bilirubin; simply counting red cells is vulnerable to a traumatic tap.

Pattern identifies plausibility

An aneurysm on CTA may be incidental. The site of subarachnoid blood should be compatible with rupture from that vessel.

Two scales answer different questions

WFNS describes clinical severity; modified Fisher describes CT blood and IVH burden related to DCI risk.

Grade can improve

Hydrocephalus treatment, seizure control and resuscitation can change consciousness, so a poor initial grade is not an immutable prognosis.

07Common pitfallsFrequent interpretation and management errors.
  1. 01

    Dismissing an unexplained thunderclap headache because the patient looks well or no neck stiffness is present.

  2. 02

    Ordering LP automatically after a radiologist-reported negative CT performed within 6 hours without specialist consideration of the NICE pathway.

  3. 03

    Performing LP before 12 hours for bilirubin analysis or when mass effect, instability or bleeding risk makes the procedure unsafe.

  4. 04

    Assuming any aneurysm found on CTA is the culprit without matching it to the blood distribution.

  5. 05

    Using WFNS or modified Fisher grade alone to deny transfer or aneurysm treatment.

  6. 06

    Applying the aneurysmal pathway to traumatic subarachnoid blood or importing traumatic haemorrhage thresholds.

Practice

Two practice questions

Question 1 of 20 correct
NeurosurgeryOriginal SBA

Negative CT after eight hours

A patient has an unexplained thunderclap headache. Non-contrast CT performed 8 hours after onset is reported as normal. The patient is stable and lumbar puncture is safe. What is the best next diagnostic step under NICE NG228?

Sources and review status3 sources · checked 13 Sept 2026 · clinical review pending
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 13 Sept 2026; clinical approval remains outstanding.

  • NICE NG228 Aneurysmal subarachnoid haemorrhage recommendationsPublished 23 November 2022; assessment, 6-hour CT boundary, 12-hour LP, spectrophotometric bilirubin, transfer, CTA and DSA recommendations read 13 September 2026. Applies to spontaneous aneurysmal SAH in people 16 and over, not traumatic SAH.
  • NICE NG228 evidence review C: severity scoring systemsPublished with NG228 in 2022; WFNS, Hunt-Hess, Fisher and GCS evidence tables and committee interpretation read 13 September 2026. Supports scales as descriptors with imperfect prognostic evidence, not isolated treatment gates.
  • AHA/ASA 2023 aneurysmal SAH guideline summaryProfessional summary updated 22 May 2023; timely specialist transfer, early aneurysm treatment and grading-scale role read 13 September 2026. US professional guidance used for general disease concepts, not UK commissioning or local protocol.
Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom