A brain aneurysm is a bulging, weakened spot in the wall of an artery in the brain. About 3% of adults have one without knowing it. The bulge fills with blood and, in most cases, never causes problems. But if it grows large enough or ruptures, it can become a life-threatening emergency.
What Happens Inside the Artery
Healthy brain arteries have layered walls that flex with each heartbeat. In a brain aneurysm, the inner elastic lining of the artery breaks down. Smooth muscle cells in the wall die off, inflammatory cells move in, and the wall thins and weakens. Over time, blood pressure pushes against that weak spot, causing it to balloon outward.
Most brain aneurysms form at branching points in the network of arteries at the base of the brain, known as the Circle of Willis. These junctions naturally bear more stress from blood flow, making them vulnerable. The two most common shapes are saccular, which balloons out on one side of the artery like a berry on a stem, and fusiform, which bulges outward in all directions. Saccular aneurysms are far more common and the type most associated with rupture.
Who Is at Risk
The two most significant risk factors are also the most controllable: cigarette smoking and high blood pressure. Both accelerate the weakening of artery walls over time. Cocaine use and excessive alcohol consumption also raise the risk substantially.
Some risk factors are outside your control. Women develop aneurysms more often than men, and risk increases after age 40. People of color face a higher risk of rupture specifically. A strong family history of brain aneurysms also matters, and certain genetic conditions make aneurysms more likely. These include autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome, Marfan syndrome, and fibromuscular dysplasia.
Symptoms of an Unruptured Aneurysm
Most unruptured brain aneurysms produce no symptoms at all. They’re often discovered by accident during brain imaging done for an unrelated reason. When an unruptured aneurysm does cause symptoms, it’s usually because it has grown large enough to press on nearby nerves or brain tissue. Warning signs include pain above and behind one eye, a dilated pupil, double vision or other vision changes, numbness on one side of the face, and seizures. A sudden onset of drooping eyelid or double vision from nerve compression can signal the aneurysm is expanding rapidly.
What a Rupture Feels Like
When a brain aneurysm ruptures, blood spills into the space surrounding the brain. This is called a subarachnoid hemorrhage, and it’s a medical emergency. The hallmark symptom is a thunderclap headache: an extraordinarily severe headache that strikes suddenly, reaching peak intensity in seconds. People consistently describe it as the worst headache of their life, unlike anything they’ve experienced before.
Other symptoms often accompany the headache: nausea and vomiting, a stiff neck, sudden weakness, confusion, irritability, sensitivity to bright light, dizziness, vision changes including blind spots, and decreased consciousness. Some people lose consciousness entirely. Roughly 35% of people who suffer a subarachnoid hemorrhage die within three months, and more than half of survivors have an incomplete recovery with lasting neurological effects.
How Brain Aneurysms Are Found
Two imaging techniques are commonly used. MR angiography uses magnetic resonance imaging to capture detailed pictures of brain arteries, revealing the size, shape, and location of an aneurysm without any radiation. CT angiography uses a contrast dye and X-rays to produce similar images quickly, making it especially useful in emergency situations when a rupture is suspected.
If those scans don’t provide enough detail, doctors may perform a cerebral angiogram. This involves threading a thin catheter from a blood vessel in the leg up to the brain’s arteries and injecting dye for high-resolution X-ray images. It remains the most detailed way to visualize an aneurysm.
Size, Location, and Rupture Risk
Not all brain aneurysms need treatment. The risk of rupture depends heavily on size and location. Aneurysms smaller than 7 millimeters are the most frequently detected and the least likely to rupture. In a large prospective study, small aneurysms in the front part of the brain’s circulation had essentially zero rupture risk, while those in the back of the brain carried about a 2.5% annual risk.
The numbers climb steeply with size. Annual rupture rates range from about 0.36% for aneurysms 3 to 4 mm across, to 1.69% for those 7 to 9 mm, to 4.37% for those 10 to 24 mm. Aneurysms 25 mm or larger carry an annual rupture rate of 33.4%. Location matters too: aneurysms at the back of the brain are more dangerous at every size.
Growth is another red flag. About 5.4% of small aneurysms grow at least 0.75 mm per year, and aneurysms with documented growth have an annual hemorrhage rate of 18.5%. That’s why regular imaging is essential for aneurysms being monitored without treatment. Screening every six months can detect growth before rupture in over 90% of cases, according to one observational study.
Treatment: Clipping vs. Coiling
When treatment is needed, there are two main approaches. The choice depends on the aneurysm’s size, shape, and location, as well as the patient’s age and overall health.
Surgical clipping involves opening a small window in the skull and placing a tiny titanium clip across the base of the aneurysm. This permanently seals it off from blood flow. It’s effective and durable, but it’s an invasive procedure with a recovery time of at least four to six weeks.
Endovascular coiling is less invasive. A catheter is threaded from a puncture site in the leg up through blood vessels to the aneurysm. Tiny coils are packed inside the aneurysm to block blood from filling it, or a stent is placed in the artery to redirect blood flow away from the aneurysm. Because there’s no skull incision, recovery typically takes about one week. The tradeoff is a higher chance that the aneurysm could partially refill over time, which means follow-up imaging is important.
Living With an Unruptured Aneurysm
For small, stable aneurysms, the recommended approach is often careful monitoring rather than immediate treatment. The risks of surgery can outweigh the very low rupture risk for a small aneurysm that isn’t growing. Monitoring typically involves periodic MR angiography to check for any change in size.
If you have a family history of brain aneurysms, treatment may be recommended even for smaller aneurysms than would typically trigger intervention. Quitting smoking, managing blood pressure, and avoiding cocaine are the most impactful steps you can take to lower rupture risk. If an aneurysm shows growth on follow-up imaging, treatment is generally recommended regardless of its current size.