What Is a Subarachnoid Hematoma?

A subarachnoid hemorrhage (often called subarachnoid hematoma in informal usage) is bleeding into the fluid-filled space surrounding the brain, most commonly caused by the rupture of a weakened arterial bulge known as an aneurysm. It accounts for roughly 5% of all strokes but punches far above its weight in terms of severity, striking people as young as their mid-40s and frequently leaving survivors with lasting neurological problems.¹ Unlike most strokes, which tend to cluster in older adults with long histories of high blood pressure or diabetes, subarachnoid hemorrhage can devastate otherwise healthy people with little warning.

What Actually Happens Inside the Skull

The brain floats in cerebrospinal fluid that circulates through the subarachnoid space, a thin compartment between two of the membranes wrapping the brain. Arteries running through this space can develop small balloon-like weak spots called aneurysms. When one of those aneurysms bursts, high-pressure arterial blood rushes into the subarachnoid space, bathing the brain’s surface in blood. The sudden spike in pressure inside the skull can injure brain tissue immediately, and the blood itself triggers a cascade of harmful reactions over the following days and weeks.

Most aneurysms that rupture sit along a ring of arteries at the base of the brain. Studies consistently find that the anterior communicating artery is the single most common rupture site. One autopsy-based study found aneurysms there in over half of all cases.² Anatomical variations in the brain’s arterial ring may contribute to uneven blood flow and wall stress, though the statistical link between specific vessel variants and rupture risk remains debated.³

The Hallmark Symptom

The textbook presentation is a thunderclap headache, sometimes described by patients as the worst headache of their life, reaching full intensity within seconds. This sudden-onset severe headache is so closely associated with subarachnoid hemorrhage that any emergency physician hearing the description immediately considers it in their differential diagnosis.⁴ Nausea, vomiting, neck stiffness, brief loss of consciousness, and sensitivity to light frequently accompany the headache. Some people experience a seizure at onset.

A common and dangerous misconception is that the headache always comes on during exertion. While straining, heavy lifting, or sex can trigger a rupture, many subarachnoid hemorrhages happen during ordinary activity or even rest. Another pitfall: roughly one in eight patients who later present with a major bleed recall a milder “sentinel headache” days or weeks earlier, likely caused by a small warning leak from the aneurysm. That earlier headache is easy to dismiss as a migraine or tension headache, which is part of why subarachnoid hemorrhage has a well-documented misdiagnosis rate in emergency departments.

How Doctors Confirm the Diagnosis

A non-contrast CT scan of the head is the first-line test and is extraordinarily good at picking up subarachnoid blood if performed soon after the headache starts. A prospective study found that CT performed within six hours of symptom onset identified every single case of subarachnoid hemorrhage in a cohort of over 950 patients, yielding a sensitivity of 100%.⁵ A systematic review and meta-analysis confirmed this, reporting pooled sensitivity above 98% for scans done in that early window.⁶ Modern multi-slice CT scanners extend that high sensitivity even further: one large analysis showed sensitivity of nearly 100% for aneurysmal subarachnoid hemorrhage out to 24 hours, and about 99% at 48 hours.⁷

The trouble starts when people present late. That same prospective study showed sensitivity dropping to about 86% for scans done more than six hours after the headache began.⁸ Blood in the subarachnoid space gets diluted and broken down over time, making it progressively harder for CT to spot. When the CT is negative but suspicion remains high, the next step is a lumbar puncture to look for blood products in the spinal fluid.

When a Lumbar Puncture Is Needed

Lumbar puncture (spinal tap) searches for red blood cells and, more specifically, xanthochromia, a yellowish discoloration of the fluid caused by the breakdown of hemoglobin. The challenge is distinguishing true subarachnoid bleeding from a “traumatic tap,” where the needle itself nicks a small vein and introduces blood into the sample. One large study found that an adjusted red blood cell count above a specific threshold, combined with xanthochromic (yellow-tinged) fluid, identified subarachnoid hemorrhage in nearly 79% of cases, while patients with low red cell counts and clear fluid were consistently in the non-hemorrhage group.⁹

How xanthochromia is detected matters, too. Spectrophotometry, which measures the precise wavelength of light absorbed by hemoglobin breakdown products, is highly sensitive and specific. But most laboratories in North America still rely on visual inspection of the fluid, which is less reliable and can be thrown off by other causes of yellow discoloration, such as high protein levels or jaundice.¹⁰ If your hospital uses visual inspection, a borderline result should be interpreted cautiously.

The Danger Is Not Over After the First Bleed

Surviving the initial hemorrhage is only the first hurdle. The unsecured aneurysm can re-rupture at any time, and rebleeding carries a devastating mortality rate. A meta-analysis found that higher systolic blood pressure on admission, particularly above 160 mmHg, roughly doubled the risk of rebleeding before the aneurysm could be treated.¹¹ This is why blood pressure management in the emergency department is treated with such urgency.

Even after the aneurysm is secured, a second threat looms: delayed cerebral ischemia, which typically develops between four and fourteen days after the bleed. For years, doctors focused on vasospasm, a narrowing of brain arteries visible on imaging, as the sole culprit. Current understanding is more nuanced: delayed cerebral ischemia appears to involve not just large-vessel spasm but also small-vessel dysfunction, inflammation, disruption of the brain’s waste-clearance pathways, and abnormal electrical activity in brain tissue.¹² Delayed cerebral ischemia is a major driver of poor outcomes, which is why patients spend days to weeks in intensive care even when the initial bleeding has been controlled.

Securing the Aneurysm

There are two primary ways to stop a ruptured aneurysm from bleeding again. Neurosurgical clipping involves opening the skull and placing a tiny metal clip across the neck of the aneurysm, cutting off blood flow into it. Endovascular coiling, a less invasive approach, threads a catheter through a blood vessel in the groin up to the aneurysm, then fills it with soft platinum coils to promote clotting inside the sac. The first clipping procedure was performed in 1937; the introduction of the operating microscope in the 1970s dramatically improved safety and access to deep-seated aneurysms.¹³

A Cochrane review of randomized trials comparing the two approaches found that endovascular coiling produced better functional outcomes. At one year, about 24% of patients treated with coiling had a poor outcome (death or dependency), compared with 32% in the clipping group. That advantage persisted at ten years. The trade-off is a higher rebleeding rate with coiling: roughly twice the risk at one year and nearly three times the risk at ten years.¹⁴ When an aneurysm is suitable for either technique, coiling is generally preferred, but the choice depends on the aneurysm’s shape, location, and the patient’s overall condition.

Newer devices called flow diverters represent a further evolution. These mesh-like tubes are deployed inside the parent artery, diverting blood flow away from the aneurysm and encouraging it to scar shut over time. They are particularly useful for wide-necked, giant, or oddly shaped aneurysms that do not lend themselves well to coiling.¹⁵

Nimodipine and Medical Management

Nimodipine, a calcium channel blocker, is one of the few medications shown to improve outcomes after subarachnoid hemorrhage. A meta-analysis found that patients receiving nimodipine had a substantially higher rate of favorable outcomes and fewer adverse reactions compared with controls, along with improvements in brain blood flow velocity and consciousness level scores.¹⁶ It can be given intravenously or by mouth; a network meta-analysis found both routes appear similarly effective at preventing delayed cerebral ischemia and poor outcomes.¹⁷

Beyond nimodipine, management in the intensive care unit involves close monitoring of blood pressure, fluid balance, temperature, blood sugar, and oxygen levels. Seizure prophylaxis, pain management, and deep vein thrombosis prevention are all part of the bundle. One complication that often catches non-specialists off guard is the heart: a surge of stress hormones after subarachnoid hemorrhage can stun the heart muscle, causing temporary cardiac dysfunction that mimics a heart attack on monitoring and lab tests.¹⁸ This “neurogenic stunned myocardium” usually resolves on its own but can complicate fluid management and hemodynamic support.

The Sodium Problem

Low sodium levels (hyponatremia) develop in a significant proportion of patients after subarachnoid hemorrhage, and managing them is trickier than it might sound. Textbooks traditionally blame cerebral salt wasting, a condition in which the brain signals the kidneys to dump sodium, leading to volume depletion. However, a prospective study of 100 patients with mild-to-moderate subarachnoid hemorrhage found that the most common cause was actually inappropriate antidiuretic hormone secretion (SIADH), which was present in over 70% of cases, while true cerebral salt wasting was not identified in a single patient.¹⁹ Acute glucocorticoid deficiency also accounted for a portion of cases.

The distinction matters because the treatments are almost opposite. Cerebral salt wasting requires aggressive fluid and salt replacement, while SIADH calls for fluid restriction. Giving large volumes of fluid to someone whose problem is SIADH can worsen the sodium imbalance. This diagnostic confusion between the two conditions continues to present challenges in neurointensive care.²⁰

How Doctors Predict Who Will Do Well

Several grading scales help clinicians gauge severity and predict outcomes. The Hunt and Hess scale and the World Federation of Neurosurgical Societies (WFNS) scale both assign grades based on a patient’s level of consciousness and the presence of neurological deficits. A study comparing these scales on 1,000 patients found that all were reasonably good at predicting functional outcome, with higher grades consistently correlating with worse results. Patients graded at the higher end of either scale (Hunt and Hess 4-5 or WFNS 3-5) had substantially increased odds of poor outcome.²¹ A separate validation study confirmed that these scales perform similarly well, with no statistically significant difference in their ability to predict 90-day outcomes.²²

The Fisher scale, which grades the amount of blood visible on the initial CT scan, adds further prognostic information. Higher Fisher grades are associated with both an increased risk of vasospasm and higher mortality. One study found that the highest Fisher grade, WFNS grade 5, and Hunt and Hess grade 5 all predicted the greatest six-month mortality.²³ In practice, these scales are not used in isolation but combined with patient age, aneurysm characteristics, and imaging findings to guide treatment decisions.

The Benign Variant Most People Have Never Heard Of

Not all subarachnoid hemorrhages come from aneurysms. About 10-15% of cases involve bleeding that collects around the brainstem in a pattern called perimesencephalic hemorrhage. Angiography in these patients shows no aneurysm, and the bleed is thought to originate from a small vein rather than an artery. The prognosis is dramatically better. A follow-up study of 37 patients with perimesencephalic hemorrhage found that none experienced rebleeding, and none had lasting neurological deficits over follow-up periods ranging from 18 months to 7 years.²⁴ This stands in stark contrast to aneurysmal hemorrhage, where rebleeding and delayed complications are constant concerns. Recognizing this pattern on the initial CT can spare patients the anxiety and repeated angiography associated with an elusive aneurysm hunt, though repeat imaging is still done to rule out a small aneurysm that may have been missed.

Family History and Screening

Subarachnoid hemorrhage has a genetic component. Having a first-degree relative who experienced an aneurysmal bleed raises your own risk significantly. A systematic review found that people with two or more affected first-degree relatives had an average aneurysm prevalence of about 13%, compared with roughly 3% in the general population. Even having one affected relative bumped that average to about 5%, and to as high as 19% when additional risk factors were present.²⁵ A screening study of first-degree relatives in Thailand found unruptured aneurysms in about 13% of those tested, with siblings at somewhat higher risk than children of affected patients.²⁶

Whether to screen is a practical question with cost implications. A cost-effectiveness analysis found that for people with two or more affected relatives, screening with brain imaging starting at age 20 and repeating every seven years until age 80 was cost-effective.²⁷ More recent work suggests screening may also be worthwhile for individuals with even a single affected first-degree relative, particularly if they have additional risk factors like smoking or hypertension.²⁸ The screening tool of choice is magnetic resonance angiography (MRA), which uses no radiation and no contrast dye in most cases, making it safe for repeated use.

Subarachnoid Hemorrhage During Pregnancy

Although rare, aneurysmal subarachnoid hemorrhage during pregnancy poses unique challenges. The physiological changes of pregnancy, including increased blood volume and cardiac output, may raise the risk of aneurysm rupture. Management requires a multidisciplinary team spanning neurosurgery, obstetrics, neuroradiology, and critical care.²⁹

The general principle is that securing the ruptured aneurysm takes priority over obstetric concerns, because the mother’s survival is a prerequisite for fetal survival. Early transfer to a center with neurocritical care and high-risk obstetric services is considered essential.³⁰ The exception is when the pregnant person is near or at term, at which point the sequence of treatment, whether to deliver first or secure the aneurysm first, becomes a case-by-case discussion.³¹ Both clipping and coiling have been used successfully in pregnant patients. The ideal treatment approach remains unsettled, which underscores why individualized care at an experienced center is so important.

Long-Term Cognitive Effects in Survivors

Surviving subarachnoid hemorrhage and returning to physical independence does not mean the brain fully recovers. Many survivors experience subtle but persistent cognitive problems, including difficulties with memory, attention, executive function, and processing speed. These deficits can linger for years and may be severe enough to prevent a return to previous work or social roles. A 2025 review characterized long-term cognitive decline as a defining feature of survivorship, noting that subarachnoid hemorrhage survivors face an increased risk of dementia compared with the general population.³² Fatigue, depression, and anxiety are also common and may compound the cognitive problems.

Research into why these lasting deficits occur is still evolving. Animal studies have implicated inflammation and impaired clearance of damaged cells as contributors to long-term brain injury. One recent study in mice showed that blocking a specific inflammatory pathway improved both sensorimotor and cognitive recovery after subarachnoid hemorrhage, reduced immune cell invasion into brain tissue, and decreased neuronal death.³³ Findings like these are early-stage, but they point toward potential therapeutic targets that could one day improve long-term quality of life for the hundreds of thousands of people who survive subarachnoid hemorrhage each year.