A brain aneurysm is a weak, bulging spot on the wall of an artery inside the brain, where blood pressure pushes the vessel outward like a tiny balloon. Roughly 3% of the general population is living with an unruptured brain aneurysm right now, and most will never know it. The vast majority cause no symptoms and never rupture, but when one does burst, it becomes a life-threatening emergency.
How a Brain Aneurysm Forms
Brain arteries have three layers. The innermost layer is a thin sheet of cells lining the blood vessel. The middle layer, called the media, is made of smooth muscle cells woven together with elastic and collagen fibers. This middle layer gives the artery its strength and flexibility, allowing it to expand and contract with each heartbeat. Brain arteries are naturally thinner-walled than arteries elsewhere in the body, which makes them more vulnerable to damage.
An aneurysm develops when the inner elastic lining breaks down and the muscular middle layer weakens or tears at a specific spot. Blood pressure pushing against that weakened area causes the wall to stretch outward, forming a pouch or bulge. This process is driven by a combination of mechanical stress from blood flow, chronic inflammation in the vessel wall, and gradual loss of the structural proteins that hold the artery together.
Most brain aneurysms are saccular, meaning they form a round, berry-shaped pouch that balloons out from one side of the artery. These typically develop at branching points where arteries split, because blood flow hits those junctions with extra force. Less commonly, aneurysms are fusiform, where the entire circumference of the artery swells outward over a short segment.
Who Is at Risk
Smoking, high blood pressure, and poor sleep (particularly insomnia) all raise the risk of developing a brain aneurysm. A large genetic study published in the Journal of the American Heart Association found that genetic predisposition to these three factors was consistently linked to both aneurysm formation and rupture. Heavy alcohol use and chronic kidney disease also increase risk, likely because they damage the delicate lining of blood vessels over time.
Family history matters. If a first-degree relative has had a brain aneurysm, your own risk is higher. Certain inherited conditions, particularly autosomal dominant polycystic kidney disease, are associated with a significantly increased chance of developing one. Women are affected more often than men, and aneurysms are most commonly diagnosed in people between 30 and 60 years old.
Some factors appear to be protective. Regular physical activity, and somewhat counterintuitively, a higher body mass index have been proposed as risk-reducing factors in population studies, though the relationship is complex.
Symptoms of an Unruptured Aneurysm
Small unruptured aneurysms almost never cause symptoms. They’re often discovered incidentally when someone gets a brain scan for an unrelated reason. Population studies using high-resolution imaging have found that when researchers look carefully at brain scans, the prevalence of aneurysms 2 mm or larger is around 6 to 7%, far higher than the commonly cited 3% figure, because many are tiny and clinically silent.
Larger unruptured aneurysms can press on nearby brain tissue or nerves, producing noticeable symptoms. These include pain above and behind one eye, a dilated pupil, double vision or other vision changes, numbness on one side of the face, and occasionally seizures. These warning signs deserve prompt medical attention because they may indicate the aneurysm is growing.
What a Rupture Feels Like
When a brain aneurysm ruptures, blood spills into the space surrounding the brain, a condition called subarachnoid hemorrhage. The hallmark symptom is a sudden, extraordinarily severe headache, often described as the worst headache of your life. It comes on in seconds, not gradually. Doctors call this a “thunderclap headache” because of its explosive onset.
Other signs of rupture include a stiff neck, nausea and vomiting, extreme sensitivity to light, confusion or drowsiness, seizures, and sometimes drooping on one side of the face, slurred speech, or weakness on one side of the body. Some people lose consciousness. This is a medical emergency. About 25% of people who experience a ruptured brain aneurysm do not survive, and many survivors face lasting neurological damage.
How Brain Aneurysms Are Diagnosed
If a rupture is suspected, the first step is usually a CT scan, which can detect bleeding around the brain. To map the aneurysm itself, doctors use CT angiography (CTA), a scan that involves injecting contrast dye to visualize blood vessels in detail. CTA is quick, widely available, and minimally invasive.
The most precise imaging tool is digital subtraction angiography (DSA), considered the gold standard. A catheter is threaded through an artery (typically from the leg) up to the brain, and contrast dye is injected directly into the cerebral vessels. DSA provides the clearest picture of an aneurysm’s size, shape, and exact location, but it’s more invasive and carries a small risk of neurological complications. It’s typically reserved for treatment planning or cases where CTA doesn’t provide a clear enough answer.
For screening people at higher risk, such as those with a strong family history, MR angiography (which uses magnetic resonance imaging without radiation) is often used because it’s noninvasive and doesn’t require contrast dye injection into an artery.
Treatment Options
Not every brain aneurysm needs treatment. Small, unruptured aneurysms with a low risk of bursting are often monitored with periodic imaging scans. Doctors assess rupture risk using factors like the aneurysm’s size, location, shape, and whether you have high blood pressure or a history of a previous rupture. Aneurysms smaller than 7 mm generally carry a lower rupture risk, while those 10 mm or larger are more concerning.
When treatment is recommended, two main approaches exist. Microsurgical clipping involves opening the skull and placing a tiny titanium clip across the base of the aneurysm, pinching it shut so blood can no longer flow into it. This is effective and durable, but it’s an invasive procedure requiring at least four to six weeks of recovery.
Endovascular coiling is less invasive. A catheter is guided from a puncture site in the leg through the blood vessels to the aneurysm. Tiny platinum coils are packed inside the aneurysm, causing blood to clot within it and sealing it off from circulation. Recovery is significantly shorter, typically around one week. A newer variation uses flow-diverting stents, mesh tubes placed inside the parent artery that redirect blood flow away from the aneurysm, causing it to gradually clot and heal.
The choice between these procedures depends on the aneurysm’s size, location, and shape, as well as the patient’s overall health. Some aneurysms are better suited to one technique than the other.
Complications After a Rupture
Surviving a ruptured aneurysm is only the first hurdle. One of the most dangerous complications in the days that follow is vasospasm, where nearby brain arteries suddenly narrow, restricting blood flow and potentially causing a stroke. This risk peaks roughly 5 to 15 days after the initial bleed. Patients are closely monitored during this window with regular ultrasound checks of blood flow in the brain’s arteries.
Other potential complications include hydrocephalus (a buildup of fluid in the brain), repeat bleeding if the aneurysm isn’t secured quickly, and seizures. Long-term effects vary widely. Some people recover fully, while others experience lasting problems with memory, concentration, fatigue, or emotional regulation. Rehabilitation can continue for months or even years after the event.
Living With an Unruptured Aneurysm
Being told you have an unruptured brain aneurysm can be frightening, but the reality for most people is reassuring. The annual rupture rate for small aneurysms is very low, often well under 1% per year. Your doctor will likely recommend controlling blood pressure, quitting smoking if you smoke, avoiding excessive alcohol, and getting regular follow-up imaging to check whether the aneurysm is growing.
Location matters as much as size. Aneurysms on certain arteries toward the back of the brain or at specific branch points carry higher rupture risk than those on the main internal carotid artery, even at the same size. These details help doctors decide whether watchful waiting or preventive treatment is the better path. Many people with small, stable aneurysms live full lives with nothing more than periodic scans and healthy habits.