Subarachnoid Haemorrhage: Diagnosis and Management

Introduction

Subarachnoid haemorrhage (SAH) is bleeding into the subarachnoid space, the compartment between the arachnoid mater and pia mater that normally contains cerebrospinal fluid. It accounts for around 5% of strokes but carries a disproportionately high mortality, particularly in the first hours after the bleed.¹

The dominant cause is rupture of a saccular (berry) aneurysm arising at the branch points of the arteries forming the Circle of Willis, responsible for around 85% of spontaneous cases. The remainder are due to other vascular abnormalities or are 'perimesencephalic', a benign non-aneurysmal pattern with an excellent prognosis.

SAH is a classic exam topic because the history (thunderclap headache) is so distinctive, yet the diagnosis is still frequently missed in practice when the presentation is atypical or the headache has partly settled by the time the patient is seen.

SAH is uncommon relative to other causes of headache - only around 1 in 100 patients presenting to an emergency department with headache will have one - yet it accounts for a disproportionate share of avoidable death and disability. That combination of low prevalence and high consequence is precisely what makes the diagnostic pathway so structured: the aim is a very high sensitivity approach applied to a large low-risk population, rather than clinical judgement alone deciding who is scanned.

Aetiology

• Ruptured saccular (berry) aneurysm - around 85% of cases; most occur at the anterior communicating artery, posterior communicating artery, or MCA bifurcation
• Perimesencephalic (non-aneurysmal) haemorrhage - blood confined around the midbrain on CT with a normal angiogram; benign course and low rebleed risk
• Arteriovenous malformation
• Trauma - the commonest cause of SAH overall, but conventionally considered separately from 'spontaneous' SAH
• Vasculitis, dissection of an intracranial artery, and mycotic aneurysm (from infective endocarditis) - rare causes
• Cocaine use - causes acute hypertension and can trigger rupture of an existing aneurysm

Risk factors

• Hypertension
• Smoking
• Excess alcohol intake
• Family history of SAH or intracranial aneurysm (first-degree relative)
• Autosomal dominant polycystic kidney disease - associated with berry aneurysms; screening with MR angiography is offered to those with a strong family history
• Ehlers-Danlos syndrome type IV and other connective tissue disorders
• Coarctation of the aorta
• Cocaine and sympathomimetic drug use
• Female sex and increasing age (peak incidence 45-70 years)

Clinical features

The hallmark is a sudden-onset, severe headache, maximal within seconds - the 'thunderclap headache' - often described by patients as the worst headache of their life. It is typically occipital, though location is not reliable enough to exclude the diagnosis when it differs.

• Thunderclap headache, maximal at onset (a headache that builds up over minutes is less typical)
• Neck stiffness and photophobia, developing over the hours after the bleed as blood irritates the meninges
• Nausea and vomiting
• Loss of consciousness at onset, or a brief collapse
• Seizures at presentation
• Focal neurological deficit - a third nerve palsy (ptosis, a fixed dilated pupil, 'down and out' eye) suggests compression by a posterior communicating artery aneurysm
• Sentinel headache - a milder, self-limiting headache in the days to weeks before the major bleed, thought to represent a small warning leak; recognising it retrospectively is a common exam scenario
• Terson syndrome - vitreous or retinal haemorrhage visible on fundoscopy, caused by a sudden rise in intracranial pressure

Clinical examination

• Conscious level (GCS) - reduced level of consciousness is common and correlates with severity
• Meningism: neck stiffness, Kernig's sign (pain/resistance on extending the knee with the hip flexed), Brudzinski's sign (involuntary hip flexion on neck flexion)
• Pupils and eye movements: a fixed dilated pupil with ptosis suggests a posterior communicating artery aneurysm compressing the third nerve, or raised intracranial pressure with uncal herniation
• Focal neurology: hemiparesis, dysphasia
• Fundoscopy: subhyaloid or vitreous haemorrhage (Terson syndrome), papilloedema
• Blood pressure: often markedly elevated acutely
• Cardiovascular examination: for signs of infective endocarditis if a mycotic aneurysm is suspected

Grading severity

Two scales are used clinically: the World Federation of Neurosurgical Societies (WFNS) grade, based on GCS and focal deficit, and the Fisher (or modified Fisher) grade, based on the amount and distribution of blood on CT, which predicts the risk of vasospasm. Higher grades on both scales correlate with worse outcomes and guide urgency of intervention.

The two scales answer different questions, which is why both are used. The WFNS grade, based on GCS and focal deficit, measures how badly the brain has already been injured by the bleed and is the better predictor of overall survival and functional outcome. The modified Fisher grade, based on the volume and distribution of subarachnoid blood on CT, predicts the risk of vasospasm and delayed cerebral ischaemia over the following two weeks - a complication that has not yet happened at the time of scanning.

In practice this means a patient can be a good WFNS grade, fully conscious and neurologically intact, yet carry a high Fisher grade and therefore a substantial risk of deteriorating around day 4-14. Recognising this prevents false reassurance in a well-looking patient and explains why such patients are monitored intensively rather than discharged early.

Differential diagnosis

• Migraine - can occasionally present with a sudden severe headache, but lacks the abruptness of true thunderclap onset and CT/CSF are normal
• Meningitis or encephalitis - fever and a more gradual onset, though meningism overlaps considerably
• Reversible cerebral vasoconstriction syndrome - recurrent thunderclap headaches over days to weeks, often triggered by vasoactive substances, with normal or transiently narrowed vessels on angiography
• Cervical artery dissection - neck pain with headache, sometimes with a partial Horner syndrome
• Cerebral venous sinus thrombosis - headache with a more variable onset, may have papilloedema and seizures
• Pituitary apoplexy - sudden headache with visual field loss and features of hypopituitarism
• Primary cough or exertional headache - benign but can mimic the abruptness of SAH

Investigations

First-line

Non-contrast CT head is the first investigation. Its sensitivity is very high - close to 100% - within 6 hours of symptom onset, when performed on a modern scanner and reported by an experienced radiologist, but sensitivity falls progressively thereafter as blood is resorbed.²

Lumbar puncture

If CT is negative but clinical suspicion remains, lumbar puncture is performed to look for xanthochromia - the yellow discolouration of CSF caused by bilirubin from breakdown of red cells, which distinguishes true SAH from a traumatic tap. It must be performed at least 12 hours after symptom onset, since it takes this long for oxyhaemoglobin to be metabolised to bilirubin; done too early, a traumatic tap cannot be distinguished from true SAH. Spectrophotometry is the preferred method of detecting xanthochromia in the UK.

Definitive vascular imaging

Once SAH is confirmed, CT angiography is performed to identify the aneurysm (or other vascular lesion) and plan how to secure it. Digital subtraction (catheter) angiography remains the gold standard where CT angiography is inconclusive or when endovascular treatment is planned in the same sitting.

Supporting investigations

• ECG - SAH can cause deep T-wave inversion and QT prolongation, mimicking cardiac ischaemia ('cerebral T waves')
• FBC, clotting, U&Es and group and save prior to any intervention
• Blood glucose - hyperglycaemia is associated with worse outcomes

Management

All patients with confirmed or strongly suspected SAH need immediate discussion with a specialist neurosurgical or neurovascular centre.

General measures

• Strict bed rest, analgesia and antiemetics
• Maintain normovolaemia and avoid hypotension, which risks cerebral ischaemia
• Nimodipine 60 mg orally every 4 hours for 21 days, started as soon as the diagnosis is made - a calcium channel blocker shown to reduce the risk of delayed cerebral ischaemia from vasospasm, though its exact mechanism is more complex than simple vasodilatation³
• Correct any coagulopathy and manage blood pressure to avoid extremes in either direction
• VTE prophylaxis with intermittent pneumatic compression until the aneurysm is secured

Securing the aneurysm

The ruptured aneurysm should be secured as early as safely possible, usually within 24-72 hours, to prevent rebleeding - the single greatest risk in the first days after SAH.

The ISAT trial found better outcomes with coiling over clipping in patients suitable for either technique, and coiling is now the preferred option where the aneurysm's anatomy allows it.⁴ The decision is made by a specialist neurovascular multidisciplinary team.

Managing complications

Hydrocephalus (from blood obstructing CSF drainage) is managed with an external ventricular drain, and later a ventriculoperitoneal shunt if it persists. Vasospasm causing delayed cerebral ischaemia, typically at days 4-14, is managed with induced hypertension and maintaining euvolaemia, alongside continued nimodipine; endovascular treatment (balloon angioplasty or intra-arterial vasodilators) is used for refractory cases.

Complications

• Rebleeding - highest risk in the first 24 hours, and often fatal; the main reason for early securing of the aneurysm
• Vasospasm and delayed cerebral ischaemia - typically days 4-14 after the bleed, a leading cause of morbidity in survivors
• Hydrocephalus - acute (obstructive, from clot) or chronic (communicating, from impaired CSF resorption)
• Hyponatraemia - from cerebral salt wasting or SIADH; fluid restriction is avoided in cerebral salt wasting as it worsens cerebral ischaemia, so the underlying cause must be distinguished
• Seizures
• Cardiac complications - neurogenic stunned myocardium and arrhythmias from the catecholamine surge

Red flags

Most missed subarachnoid haemorrhages are missed at first presentation, in a patient who looks well by the time they are assessed. The features below should be treated as sufficient grounds for imaging on their own, without waiting for corroborating signs.

Prognosis

SAH carries a high mortality: around 10-15% of patients die before reaching hospital, and overall mortality at one month is roughly a third. Of those who survive, a substantial proportion have persisting cognitive impairment, fatigue or mood disturbance even when physical recovery is good, and formal neuropsychological assessment and rehabilitation should be offered.

Outcome correlates strongly with the WFNS grade at presentation, the amount of blood on initial CT, and the speed with which the aneurysm is secured and complications such as vasospasm are managed.

Cognitive and psychological outcomes deserve particular emphasis because they are so often underestimated. Patients who make an apparently full physical recovery frequently report persistent fatigue, impaired concentration and memory, anxiety and low mood, and these are the symptoms that most affect return to work and quality of life. They should be asked about specifically at follow-up rather than waiting for the patient to raise them, with referral for neuropsychological assessment where they are significant.

Screening relatives is a separate question that commonly arises. Screening for unruptured aneurysms with MR angiography is generally offered only where there are two or more affected first-degree relatives, or in conditions such as autosomal dominant polycystic kidney disease, since incidental aneurysms carry their own risks of anxiety and of intervention.