A subarachnoid hemorrhage (SAH) is bleeding into the space between the brain and the thin tissues that cover it. This space, called the subarachnoid space, is normally filled with cerebrospinal fluid that cushions the brain. When blood leaks into this area, it creates sudden, dangerous pressure and can damage brain tissue within minutes. In the United States, the average annual incidence is about 11.4 cases per 100,000 adults, and women are roughly 1.4 times more likely to be affected than men.
The Hallmark Symptom: Thunderclap Headache
The defining symptom is a headache so sudden and severe that people often describe it as the worst headache of their life. This “thunderclap” headache strikes without warning and peaks in intensity within 60 seconds. It can be accompanied by nausea, vomiting, seizures, altered consciousness, and sensitivity to light. Some people lose consciousness immediately.
Not everyone experiences a dramatic onset. Some have a smaller warning bleed, sometimes called a “sentinel headache,” days or weeks before a larger rupture. This earlier headache is often milder and may be mistaken for a tension headache or migraine, which means the warning sign gets missed. Any headache that comes on suddenly and feels different from anything you’ve experienced before warrants emergency evaluation.
What Causes the Bleeding
About 80 to 85% of non-traumatic cases are caused by the rupture of a saccular (often called “berry”) aneurysm. These aneurysms are small, balloon-like weak spots that form along the arteries at the base of the brain, particularly where blood vessels branch. The walls of these arteries are naturally thinner than elsewhere in the body, lacking the tough outer elastic layer that reinforces vessels in other areas. Over time, the constant force of blood flow weakens these spots further, causing them to stretch outward. When the wall becomes thin enough, it tears open.
The risk of rupture increases with the size of the aneurysm and with higher blood pressure inside the vessel. Turbulent blood flow at branching points accelerates the weakening process. The remaining 15 to 20% of non-traumatic cases come from other vascular abnormalities, including arteriovenous malformations, or from causes that can’t be identified even after thorough testing.
Traumatic subarachnoid hemorrhage, caused by head injuries such as falls, car accidents, or assaults, is actually more common overall than the spontaneous type but is classified and managed differently.
Risk Factors You Can Control
Several modifiable risk factors are strongly linked to both the formation and rupture of brain aneurysms. Cigarette smoking is one of the most significant. Smoking raises levels of fibrinogen in the blood, increasing blood viscosity and placing more stress on vessel walls. Nicotine also causes temporary spikes in blood pressure lasting two to three hours after each cigarette, and these surges have been directly associated with aneurysm rupture.
High blood pressure is another major contributor. Sustained hypertension damages the inner lining of blood vessels and increases the mechanical force pushing against weakened arterial walls. Sudden blood pressure spikes, such as those triggered by vigorous exercise or straining, can act as immediate triggers for rupture. Heavy alcohol consumption raises blood pressure and increases blood thickness, compounding the hemodynamic stress. Cocaine use is a particularly dangerous risk factor because it causes sharp, sudden elevations in blood pressure.
How It Is Diagnosed
A non-contrast CT scan of the head is the first-line diagnostic tool. Its accuracy depends heavily on how quickly the scan is performed after the headache begins. When done within six hours, CT detects subarachnoid blood with near-perfect reliability. In a study of patients presenting with acute headache, CT sensitivity within that six-hour window was 100%. After six hours, sensitivity drops to around 90 to 92%, meaning roughly one in ten cases can be missed.
If the CT scan is negative but suspicion remains high, particularly when more than six hours have passed, a lumbar puncture (spinal tap) is the next step. This test looks for blood breakdown products in the cerebrospinal fluid that confirm bleeding even when the CT appears normal. For patients scanned within six hours with a clearly normal result read by an experienced radiologist, additional spinal fluid testing may not be necessary.
Once bleeding is confirmed, a specialized imaging study of the blood vessels (typically a CT angiogram or catheter-based angiogram) pinpoints the exact location and shape of the aneurysm or other source of bleeding.
Treatment: Clipping vs. Coiling
The immediate goal of treatment is to seal off the ruptured aneurysm so it stops bleeding and can’t bleed again. Two main procedures accomplish this.
Surgical clipping involves opening the skull (a craniotomy) and placing a tiny metal clip across the neck of the aneurysm, cutting off blood flow to the weakened area. Endovascular coiling is a less invasive approach: a catheter is threaded through an artery in the groin up to the brain, and tiny platinum coils are packed inside the aneurysm to block it from the inside.
Outcomes for the two approaches are broadly similar. Research comparing the procedures shows no significant difference in one-year mortality, likelihood of needing rehabilitation, or 30-day readmission rates. Coiling does involve a shorter hospital stay, by roughly 2.7 days on average, which reflects its less invasive nature. The choice between the two depends on the aneurysm’s size, shape, and location, as well as the patient’s overall health. In recent years, coiling has become the more commonly performed procedure, accounting for about 62% of treated cases in one large study.
Dangerous Complications After the Bleed
Surviving the initial rupture is only the first challenge. One of the most feared complications is cerebral vasospasm, a narrowing of the brain’s blood vessels that reduces blood flow and can cause additional brain damage or stroke. This typically occurs between 4 and 14 days after the hemorrhage, making the first two weeks a critical monitoring window. Patients remain in an intensive care unit during this period so that any signs of vasospasm can be detected and treated quickly.
Rebleeding is another serious risk. Without treatment to secure the aneurysm, the chance of a second rupture is highest in the first 24 hours and remains elevated for weeks. This is why identifying and sealing the aneurysm as soon as possible is a priority. Other complications include hydrocephalus (a dangerous buildup of fluid in the brain), seizures, and electrolyte imbalances that can further injure brain tissue.
Survival and Long-Term Outlook
Subarachnoid hemorrhage carries a high mortality rate. Finnish registry data covering nearly 9,500 cases found that about 24% of patients died before ever reaching the hospital. Among those who were hospitalized, an additional 20% died within 30 days. Overall, roughly 39% of patients did not survive the first month.
For those who do survive, recovery varies widely. Some people return to their previous level of function, while others face lasting challenges including cognitive difficulties with memory and concentration, fatigue, mood changes such as depression and anxiety, and headaches that persist for months. The severity of the initial bleed, the patient’s age, and how quickly treatment was received all influence long-term outcomes. Rehabilitation, including physical, occupational, and speech therapy, plays a central role in recovery for many survivors.