A brain aneurysm is a weak, bulging spot on the wall of an artery in the brain where blood pressure causes the vessel to balloon outward. About 1 in 50 people in the United States has one, roughly 6.7 million people. The vast majority never know it, because most brain aneurysms are small, cause no symptoms, and never rupture.
The danger comes if the aneurysm wall thins enough to tear open, spilling blood into the surrounding brain tissue. That event, called a subarachnoid hemorrhage, is a medical emergency. But understanding the full picture, from how aneurysms form to when they need treatment, puts the actual risk in perspective.
How a Brain Aneurysm Forms
Normal arteries in the brain have a layered wall: an inner lining of cells, a springy elastic layer, and an outer layer of smooth muscle cells reinforced with collagen. An aneurysm develops when those layers weaken and lose their structure. The smooth muscle cells become disorganized, the elastic fibers break down, and the wall gradually thins.
Over time, the weakened area stretches outward under the constant push of blood pressure. As the bulge grows, the wall continues to remodel. Muscle cells are replaced by collagen, and the tissue becomes increasingly sparse. The thinnest point is typically at the dome of the aneurysm, the farthest point from the artery, which is also where ruptures most often occur.
Types of Brain Aneurysms
Most brain aneurysms are saccular, sometimes called berry aneurysms. These look like a small, round sac attached to one side of an artery, similar to a berry hanging from a stem. They form at branching points where arteries divide, because those junctions bear the most hemodynamic stress.
Fusiform aneurysms are less common. Instead of bulging on one side, the entire artery wall expands outward in all directions, creating a widened, spindle-shaped segment. Mycotic aneurysms are rarer still and develop when an infection, usually originating in the heart valves, spreads through the bloodstream and weakens an artery wall in the brain.
Symptoms of an Unruptured Aneurysm
Small unruptured aneurysms typically cause no symptoms at all. They’re often discovered by accident when imaging is done for an unrelated reason, like a head injury or chronic headaches. A larger unruptured aneurysm, however, can press on nearby brain tissue or nerves and produce noticeable problems:
- Pain above and behind one eye
- A dilated pupil in one eye
- Double vision or other vision changes
- Numbness on one side of the face
- Seizures
These symptoms develop gradually and reflect the physical pressure of the bulging artery on surrounding structures, not bleeding.
What a Ruptured Aneurysm Feels Like
A ruptured brain aneurysm announces itself with a sudden, explosive headache, often described as the worst headache of a person’s life. It comes on in seconds, not minutes. Along with this headache, rupture can cause nausea and vomiting, a stiff neck, blurred or double vision, sensitivity to light, confusion, seizures, and loss of consciousness.
Sometimes an aneurysm leaks a small amount of blood before fully rupturing. This “sentinel leak” produces a sudden, severe headache that can persist for days or even up to two weeks. It’s essentially a warning sign that a larger rupture may follow, which makes it critical to recognize.
Who Is at Higher Risk
High blood pressure and smoking are the two most well-established modifiable risk factors, both for developing a brain aneurysm and for triggering rupture. Each raises the odds of rupture independently. People who smoke are roughly 57% more likely to experience a rupture than nonsmokers, and those with high blood pressure face about a 51% increased risk. When both factors are present together, the combined risk more than doubles compared to having neither.
Other factors that increase susceptibility include a family history of brain aneurysms (particularly a first-degree relative who had one), certain inherited connective tissue disorders, and being female, since women develop brain aneurysms more often than men. Age also plays a role; most aneurysms are diagnosed in people between 35 and 60.
How Likely Is a Rupture?
For most people with a small, incidentally discovered aneurysm, the rupture risk is low. A large meta-analysis found that aneurysms 10 mm or smaller that were monitored without treatment ruptured about 1.1% of the time over roughly 3.7 years. For the smallest aneurysms, those 3 mm or less, the rate was around 0.8% over a similar period. In practical terms, out of every 1,000 small aneurysms managed with observation, between 8 and 15 are estimated to rupture over about four years.
Larger aneurysms carry higher risk. Size is one of the strongest predictors of rupture, along with location in the brain, irregular shape, and whether the aneurysm is growing on follow-up imaging.
When Treatment Is Recommended
Not every brain aneurysm needs intervention. Guidelines from the American Association of Neurological Surgeons recommend that small, incidentally found aneurysms under 5 mm generally be managed conservatively with monitoring. This means periodic imaging to check for growth, along with blood pressure control and smoking cessation.
Treatment is more strongly considered for aneurysms larger than 5 mm in patients under 60, and for aneurysms over 10 mm in nearly all patients under 70. Any aneurysm causing symptoms, regardless of size, is typically treated. Other factors that push toward intervention include a family history of rupture, aneurysm location in higher-risk areas of the brain, and evidence of growth over time.
Clipping vs. Coiling
Two main approaches are used to treat brain aneurysms, and both aim to stop blood from flowing into the weakened bulge.
Microsurgical clipping is the more traditional procedure. A neurosurgeon opens a small section of the skull, locates the aneurysm, and places a tiny titanium clip across its neck. This seals off the aneurysm from the normal artery permanently. Recovery takes at least four to six weeks, but the advantage is durability: clipped aneurysms have a lower chance of coming back, meaning less follow-up testing over the years.
Endovascular coiling is less invasive. A catheter is threaded through a blood vessel in the leg up to the brain, where tiny coils are packed inside the aneurysm to block blood flow. Alternatively, a stent or flow diverter can be placed inside the artery to redirect blood away from the aneurysm entirely. Because no skull incision is needed, recovery is much faster, typically about one week. The tradeoff is a higher chance that the aneurysm could partially refill over time, which means routine follow-up imaging is necessary to catch any regrowth early.
The choice between the two depends on the aneurysm’s size, shape, and location, as well as the patient’s age and overall health. In many cases, both options are viable, and the decision is made collaboratively between the patient and their neurosurgical team.
Living With an Unruptured Aneurysm
Being told you have a brain aneurysm can be frightening, but for most people with small, stable aneurysms, the condition is managed with regular monitoring rather than immediate surgery. The practical steps that make the biggest difference are controlling blood pressure, quitting smoking if you smoke, and avoiding heavy straining or stimulant drugs that spike blood pressure suddenly.
Follow-up imaging is typically scheduled at regular intervals, often annually at first, then less frequently if the aneurysm remains unchanged. Many people with monitored aneurysms go their entire lives without ever needing treatment.