General · EdgepediaSeptember 17, 20267 min read

Subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is bleeding into the subarachnoid space, the fluid-filled area between the brain and the thin membranes (arachnoid and pia mater) that cover it. It is a medical emergency: the most common non-traumatic cause is rupture of a cerebral aneurysm, and the condition carries high mortality and long-term disability.¹^•²

Key fact	Detail
Share of stroke	SAH accounts for about 2–5% of all strokes, with disproportionate death and disability²
Global burden (GBD 2021)	Roughly 700,000 new cases, nearly 8 million prevalent cases, about 350,000 deaths, and more than 10 million disability-adjusted life years worldwide in 2021²
Leading cause	Intracranial aneurysms account for 70–85% of non-traumatic SAH³
Hallmark symptom	A sudden, severe "thunderclap" headache, often described as the worst headache ever¹
First test	Non-contrast head CT; lumbar puncture if the CT is normal¹
Untreated rebleeding	Around 30% of hospitalized patients rebleed within the first month if the aneurysm is not secured⁴
Main treatments	Neurosurgical clipping (craniotomy) and endovascular coiling¹

Causes and risk factors

Bleeding reaches the subarachnoid space in two broad ways. Spontaneous hemorrhage most often comes from the rupture of an intracranial aneurysm, a localized dilation of a cerebral artery; aneurysms cause 70–85% of non-traumatic cases.³ Other causes include arteriovenous malformations, bleeding disorders, sickle cell disease, brain tumors, cerebral venous thrombosis, use of blood thinners, and unknown (idiopathic) causes.¹ Traumatic SAH accompanies head injury, commonly with other intracranial bleeding; in older people it often follows falls, and in younger people motor vehicle crashes are the most common injury.¹

Risk factors for aneurysmal SAH include an unruptured brain aneurysm, high blood pressure, smoking, use of illicit drugs such as cocaine and methamphetamine, use of blood thinners such as warfarin, a history of polycystic kidney disease, fibromuscular dysplasia and other connective tissue or autoimmune disorders, and a strong family history of aneurysms.¹ Conditions that raise the risk include Ehlers-Danlos syndrome, Marfan syndrome, neurofibromatosis type 1, and polycystic kidney disease.⁵

Symptoms and diagnosis

The main symptom is a severe headache that starts suddenly, often called a thunderclap headache; many people describe it as the worst headache of their life, and it may begin after a popping or snapping sensation in the head. Other symptoms include decreased consciousness, photophobia, confusion, nausea and vomiting, neck stiffness, seizures, and vision problems.¹

Diagnosis rests on urgent imaging. A non-contrast head CT scan is done immediately; a small bleed can produce a normal scan, in which case a lumbar puncture may follow. Cerebral angiography, CT angiography, and transcranial Doppler ultrasound are used to identify the source of bleeding and to monitor blood flow in the brain's arteries.¹^•⁶ Clinical grading scales and CT remain central to acute assessment, though they give limited insight into the evolving secondary injury processes.²

Complications

Two complications dominate the course after aneurysmal SAH. Rebleeding is the most serious: about 30% of patients admitted to hospital rebleed within the first month if the aneurysm is not treated, and a second bleed from a cerebral aneurysm carries a much worse outlook.⁴^•¹ Delayed cerebral ischemia (DCI) develops when segments of arteries in spasm (vasospasm) reduce blood flow to the brain; it typically appears days after the hemorrhage and is detected through neurologic decline, transcranial Doppler monitoring, angiography, and perfusion imaging.⁶ The presence of a large volume of subarachnoid blood or a large clot is a proven predictor of vasospasm.⁷

Hydrocephalus is also common: it is present in up to 30% of patients with intraventricular hemorrhage, arising from mechanical blockage of cerebrospinal fluid flow and from inflammation caused by blood in the CSF. Blood in the subarachnoid space can impair CSF reabsorption, causing communicating hydrocephalus, or induce intraventricular adhesions that obstruct CSF pathways, causing non-communicating hydrocephalus.⁶^•⁸ Seizures, stroke, and medical complications contribute to deterioration, and unexplained neurologic change is evaluated with urgent head CT, angiography, or EEG.⁶

Treatment

The goals of treatment are to save life, repair the cause of bleeding, relieve symptoms, prevent complications such as stroke, and prevent future bleeding.¹ For aneurysmal SAH, the aneurysm is secured by one of two methods: neurosurgical clipping, in which a craniotomy is performed and a clip is placed across the neck of the aneurysm, or endovascular coiling, in which coils are packed into the aneurysm sac through the blood vessels to promote thrombosis, sometimes with stents to hold the coils in place.¹^•⁹ Early management also includes cerebrospinal fluid drainage, control of intracranial pressure, and organ support to limit secondary brain injury.⁸

Clipping versus coiling: the trial evidence

The International Subarachnoid Aneurysm Trial (ISAT), a randomized trial of 2,143 patients with ruptured aneurysms enrolled at 43 centers between 1994 and 2002, found that 24% of patients assigned to coiling were dead or dependent at one year compared with 31% of those assigned to clipping, a relative risk reduction of 23.9% in death or dependence.¹⁰ A 2018 Cochrane review of four randomized trials (2,458 participants) reached similar conclusions: after one year, 24% of the endovascular group and 32% of the surgical group had poor functional outcome (risk ratio 0.77), and at 10 years the figures were 35% and 43% (RR 0.81).⁴

The trade-off is durability. Rebleeding was more frequent after coiling than after clipping, with a risk ratio of 1.83 at one year and 2.69 at 10 years in the randomized trials.⁴ Long-term follow-up of the UK ISAT cohort found that patients treated endovascularly were more likely to be alive and independent at 10 years (odds ratio 1.34), but recurrent hemorrhage from the treated aneurysm occurred more often after coiling.¹¹ Reviews of the two approaches conclude that coiling gives better short- and long-term functional outcomes overall, while clipping offers lower rebleeding risk and more durable occlusion, so treatment choice depends on aneurysm characteristics, patient age and condition, and local expertise.¹²^•⁹ The evidence comes mainly from patients in good clinical condition with anterior circulation aneurysms; for patients in poor condition, no reliable randomized evidence directly guides the choice.⁴

Prognosis and prevention

Outcome after SAH depends on the location and amount of bleeding, the occurrence of complications, age, and severity of symptoms at presentation; older age and more severe symptoms predict poorer outcomes. Some people recover completely, while others die even with treatment.¹ Overall, about a third of people with aneurysmal SAH die within three months, and one in five survivors remains dependent on others for daily activities.⁴

Measures that may help prevent SAH include stopping smoking, treating high blood pressure, identifying and treating an aneurysm before rupture, and avoiding illicit drugs.¹

History

The modern treatment of aneurysmal SAH developed over the twentieth century. Norman Dott performed the first open surgical treatment of an intracranial aneurysm by wrapping it, reported as 1931 in one historical account and 1933 in another, and Walter Dandy performed the first obliterative clipping of an aneurysm, dated 1937 or 1938 depending on the source.⁹^•³^•¹³ The operating microscope, introduced to aneurysm surgery by M.G. Yasargil in the 1960s, and the pterional craniotomy approach described by Yasargil and Fox in 1975 refined microsurgical clipping.⁹^•³ In 1991, Guido Guglielmi introduced detachable coils, which filled the aneurysm sac through the vessels and made minimally invasive treatment possible without craniotomy; large trials in the early 2000s then established coil embolization as an accepted alternative for ruptured aneurysms.⁹^•¹³

References

Topic: Encyclopedia › Life and health › Human health and medicine › Diseases and injuries › Nervous and sensory conditions › Stroke and cerebrovascular disease

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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