A berry aneurysm is a small, round bulge that forms on an artery in the brain, typically at a point where the vessel branches. The name comes from its shape: a tiny sac hanging off a blood vessel like a berry on a stem. About 3% of adults have one without knowing it, since most never cause symptoms or rupture. When they do rupture, though, the consequences can be severe.
Where Berry Aneurysms Form
Berry aneurysms develop along a ring of arteries at the base of the brain called the circle of Willis. This network supplies blood to every region of the brain, and the points where arteries split into branches are structurally weaker than the rest of the vessel wall. Over time, the constant pressure of blood flow can push outward at these weak spots, creating a balloon-like pouch.
The most common locations are where the anterior communicating artery meets the anterior cerebral artery, at the branching point of the middle cerebral artery, and where the internal carotid artery meets the posterior communicating artery. These junctions handle significant blood flow, which helps explain why aneurysms cluster there rather than forming randomly throughout the brain.
Who Is at Risk
Several factors increase the chance of developing a berry aneurysm. Smoking and high blood pressure are the two most significant modifiable risks. Polycystic kidney disease, a genetic condition that causes cysts to grow in the kidneys, is strongly associated with brain aneurysms as well. Connective tissue disorders that affect the structural integrity of blood vessel walls also raise the risk.
Family history matters considerably. If you have a first-degree relative (parent, sibling, or child) who had a brain aneurysm, your own risk is higher, and screening may be recommended even without symptoms. Among people with a family history of brain aneurysms, smoking multiplies the risk of rupture by a factor of eight, making it by far the single most dangerous combination.
Symptoms of Unruptured Aneurysms
Most small berry aneurysms produce no symptoms at all. They’re frequently discovered by accident during brain imaging done for an unrelated reason. However, a large or growing aneurysm can press on surrounding brain tissue and nerves, causing noticeable problems: pain above and behind one eye, numbness or weakness on one side of the face, a dilated pupil, or double vision. These symptoms tend to develop gradually as the aneurysm enlarges.
What Happens When One Ruptures
A ruptured berry aneurysm causes bleeding into the space surrounding the brain, a condition called subarachnoid hemorrhage. The hallmark symptom is a sudden, explosive headache, often described as the worst headache of a person’s life. It comes on in seconds, not gradually, and may be accompanied by nausea, vomiting, stiff neck, sensitivity to light, confusion, or loss of consciousness.
This is a medical emergency. Mortality rates from subarachnoid hemorrhage have been declining over recent decades (dropping roughly 2.5% per year), thanks to improvements in emergency care and surgical techniques, but it remains a life-threatening event. Many survivors face lasting disability. The good news is that the vast majority of berry aneurysms never rupture, and doctors now have good tools for estimating which ones pose the greatest threat.
How Rupture Risk Is Assessed
Size is the single biggest predictor of whether an aneurysm will burst. The annual rupture risk for aneurysms between 3 and 4 millimeters is about 0.36%. That rises to 1.69% per year for those 7 to 9 millimeters, 4.37% for those 10 to 24 millimeters, and a striking 33.4% per year for aneurysms 25 millimeters or larger.
Location also matters. Aneurysms in the front part of the brain’s circulation that are smaller than 7 millimeters have an extremely low rupture rate. The same size aneurysm in the back of the brain’s circulation carries a higher risk, around 2.5% per year. Doctors use a scoring system called PHASES that combines population background, high blood pressure, age, aneurysm size, previous bleeding history, and aneurysm location to estimate the five-year rupture probability. This score helps guide the decision between monitoring and treatment.
How Berry Aneurysms Are Detected
When a doctor suspects an aneurysm, imaging is the first step. CT angiography (a specialized CT scan using contrast dye) is fast and widely available, with a sensitivity of about 88% for detecting aneurysms. MRI-based techniques are also used, particularly for screening people with risk factors, and have the advantage of avoiding radiation. Catheter-based angiography, where a thin tube is threaded through the blood vessels to the brain, remains the most accurate method and is considered the gold standard, though it’s more invasive and typically reserved for cases where other imaging is inconclusive or when treatment planning requires precise detail.
Treatment: Clipping vs. Coiling
Two main approaches exist for treating berry aneurysms, and the choice depends on the aneurysm’s size, shape, and location, as well as the patient’s overall health.
Surgical clipping involves opening a small window in 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. Recovery takes at least four to six weeks since it requires accessing the brain directly. The major advantage is durability: clipped aneurysms rarely come back, which means less follow-up imaging over the years.
Endovascular coiling is less invasive. A catheter is inserted through a puncture in the leg and guided through the blood vessels up to the aneurysm. Tiny coils are packed inside the aneurysm sac, causing the blood inside to clot and sealing it off from circulation. Because there’s no skull incision, recovery is typically about one week. The tradeoff is a higher chance the aneurysm can re-open over time, so periodic follow-up angiograms are recommended to catch any regrowth early.
When Treatment Is Recommended
Not every berry aneurysm needs to be treated. For small aneurysms (under 7 millimeters) in the front circulation, particularly in patients with no history of brain bleeding, the rupture risk is low enough that regular monitoring with periodic imaging is often the preferred approach. Treatment carries its own risks, and for very small, stable aneurysms, those risks can outweigh the benefit.
Treatment is more strongly considered when an aneurysm is 7 millimeters or larger, located in the posterior circulation, or has been documented growing on follow-up imaging. Guidelines also recommend treating aneurysms at smaller sizes in people with a family history of brain aneurysms, since their baseline risk is already elevated. Any aneurysm that shows enlargement during monitoring should generally be treated regardless of its current size, as growth signals instability in the vessel wall.
Screening for Family Members
If you have two or more first-degree relatives who have had brain aneurysms, routine screening with noninvasive imaging is generally recommended. Screening can also be appropriate with even one affected relative if additional risk factors like smoking, high blood pressure, or polycystic kidney disease are present. Because these aneurysms develop silently, screening is the only way to catch them before they become dangerous. The typical approach is a brain MRI, which can be repeated every few years if the initial scan is normal.