A brain aneurysm is a weak, bulging spot on an artery in the brain where the vessel wall has thinned and ballooned outward with blood. Somewhere between 1% and 6% 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 are only discovered incidentally during imaging for something else. The danger comes if the aneurysm ruptures, which causes bleeding into the space around the brain and can be life-threatening within minutes.
How a Brain Aneurysm Forms
Arteries have layered walls designed to handle the constant pressure of blood flow. The innermost structural layer, a thin elastic membrane, gives the artery its strength and flexibility. In a brain aneurysm, that elastic layer tears or degrades, and once damaged, it cannot repair itself. The proteins that normally keep the wall strong (elastin and collagen) break down, and the artery loses its ability to hold its shape under pressure.
The primary force behind this damage is the mechanical stress of blood flow itself. The constant pulsing of blood through the artery creates vibrations and friction along the vessel wall, particularly at points where arteries branch or curve. Over time, this stress triggers enzymes that further weaken the wall. Muscle cells from deeper layers begin migrating through the defect, and the artery gradually balloons outward, forming the aneurysm.
Types of Brain Aneurysms
Most brain aneurysms are saccular, sometimes called “berry” aneurysms because of their shape. These form a rounded, blood-filled sac that pouches off one side of the artery, typically at branching points near the base of the brain. They are by far the most common type in adults.
Fusiform aneurysms bulge outward on all sides of the artery rather than forming a distinct pouch. They look more like a widened segment of the vessel. Mycotic aneurysms are rarer and caused by infections that weaken the artery wall from within, leading to a bulge at the site of damage.
Symptoms of Unruptured Aneurysms
Small unruptured aneurysms typically produce no symptoms at all. Larger ones can press on surrounding nerves or brain tissue, creating warning signs that include headaches, vision changes, a dilated pupil in one eye, numbness or tingling on one side of the face, pain above and behind one eye, or seizures. These symptoms develop gradually as the aneurysm grows, and they reflect the physical pressure the bulge puts on nearby structures rather than any bleeding.
What a Rupture Feels Like
When a brain aneurysm ruptures, it causes a type of bleeding called subarachnoid hemorrhage, where blood spills into the space between the brain and the thin tissues covering it. The hallmark symptom is a sudden, explosive headache, often described as the worst headache of your life. Both the severity and the instant onset are key features. This is not a headache that builds over hours.
Along with the headache, rupture commonly causes neck stiffness, sensitivity to light, nausea and vomiting, a rapid decline in consciousness that can progress to coma, seizures, and focal neurological problems like double vision or weakness on one side of the body. Ruptured aneurysms are medical emergencies. Survival rates vary significantly depending on where the aneurysm is located: for aneurysms in the front part of the brain’s circulation, roughly 57% of patients survive to 30 days, while aneurysms in the back of the brain carry a much grimmer prognosis, with only about 11% surviving that same window.
Risk Factors
The major modifiable risk factors for developing a brain aneurysm are high blood pressure, smoking, and heavy alcohol consumption. High blood pressure accelerates the mechanical damage to artery walls, and it also increases the risk that an existing aneurysm will rupture. Smoking contributes to chronic inflammation in blood vessel walls, compounding that damage over time.
Non-modifiable risk factors include increasing age, being female, and family history. If a first-degree relative has had a brain aneurysm, your own risk is elevated. Several inherited conditions also raise the likelihood, most notably autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome, Marfan syndrome, and neurofibromatosis. These conditions affect the connective tissue that gives artery walls their structural integrity.
How Brain Aneurysms Are Found
The two main noninvasive imaging methods are CT angiography (CTA) and MR angiography (MRA). Both use contrast dye to visualize blood vessels in detail, but they have a notable limitation with small aneurysms. For aneurysms 5 mm or larger, CTA detects about 94% and MRA about 86%. For aneurysms smaller than 5 mm, those numbers drop sharply to 57% and 35%, respectively. This means very small aneurysms can be missed on initial screening. When doctors need a definitive answer, they may use a catheter-based angiogram, which remains the gold standard for detection.
Treatment: Clipping vs. Coiling
The two primary treatments for brain aneurysms are surgical clipping and endovascular coiling. Clipping is the more traditional approach: a surgeon opens a section of the skull and places a small metal clip across the base of the aneurysm to stop blood from flowing into it. Coiling is less invasive. A catheter is threaded through an artery in the leg or wrist up into the brain, and tiny platinum coils are packed into the aneurysm. The coils trigger clotting inside the bulge, sealing it off from the inside.
Recovery tends to be faster with coiling. For ruptured aneurysms, patients who undergo clipping stay in the hospital an average of 2.7 days longer than those treated with coiling, and they are more likely to need inpatient rehabilitation afterward. However, clipping has a meaningful long-term advantage: it produces lower rates of the aneurysm coming back. In one study, the probability of needing retreatment within seven years was 3.2% after clipping compared to 4.9% after coiling. Clipping also achieves more complete closure of the aneurysm, which means less risk of residual blood flow into the weakened area.
When Treatment Is Recommended
Not every brain aneurysm needs to be treated. The decision hinges largely on size, location, and your personal risk profile. There is ongoing debate about the exact size threshold that warrants intervention, but a few guideline patterns emerge. The American Heart Association notes that incidental aneurysms smaller than 10 mm carry an apparently low risk of bleeding. Some experts set the bar lower: Japanese neurosurgical guidelines recommend considering treatment for aneurysms larger than 5 mm, and several studies have found that rupture risk begins increasing once an aneurysm exceeds 4 to 5 mm in diameter.
For small, asymptomatic aneurysms, doctors often recommend a watch-and-wait approach with regular imaging to track any growth. Your individual risk factors matter in this decision. Someone with a family history of rupture, uncontrolled high blood pressure, or an aneurysm in a high-risk location may be advised to treat at a smaller size than someone without those factors. Quitting smoking and managing blood pressure are the two most impactful things you can do to reduce the risk of an existing aneurysm rupturing.