An aneurysm is a bulge or ballooning in the wall of a blood vessel, caused by a weak spot that allows the vessel to expand beyond its normal diameter. They can form in arteries throughout the body, but the most common and dangerous locations are in the brain and the aorta, the large artery running from the heart through the chest and abdomen. About 1 in 50 people in the United States has an unruptured brain aneurysm, and most never know it.
How an Aneurysm Forms
Artery walls have three layers: an inner lining, a muscular middle layer, and a tough outer layer. The middle layer does the heavy lifting, using smooth muscle cells and elastic fibers to absorb the constant pressure of blood flow. When that middle layer weakens, the wall stretches outward like a worn spot on a tire.
The weakening usually involves two processes happening together. First, the smooth muscle cells in the wall die off, which is a hallmark of aneurysm tissue. Second, the structural scaffolding between cells, particularly the elastic fibers that let the artery flex with each heartbeat, breaks down. Inflammation accelerates both problems, attracting immune cells that release enzymes capable of chewing through the artery’s structural proteins. Over time, the weakened section balloons outward, and the thinner it stretches, the more vulnerable it becomes to rupture.
Types of Aneurysms by Shape
Aneurysms come in two basic shapes. A saccular aneurysm looks like a balloon or berry hanging off one side of an artery, connected by a narrow neck. These are the most common type found in the brain. A fusiform aneurysm affects the entire circumference of the vessel, causing a section of the artery to widen in all directions, like a snake that swallowed an egg. Fusiform aneurysms are more common in the aorta.
Where Aneurysms Occur
Brain Aneurysms
Most brain aneurysms form along the circle of Willis, a ring of arteries at the base of the brain where several major blood vessels meet. The junctions and branch points in this network are natural weak spots. Aneurysms frequently develop where one side of the arterial ring carries more blood than the other, creating asymmetric pressure that stresses the vessel wall.
An estimated 6.8 million Americans have an unruptured brain aneurysm. The vast majority remain small and cause no symptoms. Larger ones, however, can press on surrounding brain tissue and nerves, causing pain above or behind one eye, a dilated pupil, double vision, numbness on one side of the face, or seizures.
Aortic Aneurysms
The aorta is the body’s largest artery, and aneurysms can develop along its entire length. Abdominal aortic aneurysms (in the belly) are more common. Thoracic aortic aneurysms (in the chest) tend to be more dangerous because of their proximity to the heart. Most aortic aneurysms grow slowly and silently over years. Some people notice a pulsing sensation near the navel or deep, steady pain in the back or abdomen, but many feel nothing at all until the aneurysm becomes large or tears.
What a Rupture Feels Like
When a brain aneurysm ruptures, blood spills into the space around 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. Other symptoms include nausea, vomiting, a stiff neck, blurred vision, sensitivity to light, confusion, seizures, and loss of consciousness. Ruptured brain aneurysms are fatal in roughly 50% of cases.
Sometimes an aneurysm leaks before it fully ruptures. A leaking aneurysm can cause a sudden, severe headache that lasts days to two weeks, acting as a warning sign before a larger bleed.
A ruptured or dissecting aortic aneurysm produces sudden, severe “tearing” pain in the chest or back. Ascending aorta problems tend to cause chest pain; descending aorta problems cause back pain. The pain can migrate as the tear extends. Low blood pressure is a grave sign, often indicating the artery has ruptured or blood is compressing the heart.
Risk Factors
Smoking is one of the strongest risk factors. Studies of human aortic tissue show that smokers have significantly less elastin, the key structural protein that keeps artery walls flexible. Their arteries also contain higher levels of the enzymes that break down wall tissue, more inflammatory cells, and signs of cellular stress, all while the body’s repair mechanisms work poorly.
High blood pressure puts constant extra force on artery walls, accelerating wear on already vulnerable spots. Family history matters too: having a first-degree relative who suffered a ruptured brain aneurysm increases your own risk, and the added risks from smoking and high blood pressure stack on top of that genetic predisposition. Women with a family history appear particularly vulnerable to the effects of smoking.
Other contributing factors include age (risk rises over 40), heavy alcohol use, and certain connective tissue disorders that weaken blood vessel walls from birth.
How Aneurysms Are Found
Many aneurysms are discovered incidentally during brain or body imaging done for unrelated reasons. When doctors suspect an aneurysm, screening typically starts with either an MRA (magnetic resonance angiography) or a CTA (CT angiography). Both are noninvasive and highly sensitive, though that sensitivity also means they occasionally flag something that turns out not to be an aneurysm at all.
If either scan raises suspicion, the gold standard for confirmation is a digital subtraction angiography, or DSA. This involves threading a thin catheter through an artery, usually from the leg, to inject dye directly at the site. It provides the most detailed images but carries a small risk: between 0.1% and 0.5% of patients develop lasting neurological effects, and 1% to 4% experience complications from the catheter itself.
Treatment for Brain Aneurysms
Small, unruptured brain aneurysms that aren’t causing symptoms are often monitored with periodic imaging rather than treated immediately. When treatment is needed, there are two main approaches.
Surgical clipping involves opening a small window in the skull and placing a tiny titanium clip across the neck of the aneurysm, pinching it shut so blood can no longer fill it. Recovery takes at least four to six weeks.
Endovascular coiling is less invasive. A neurosurgeon threads a catheter from a puncture site in the leg up through the blood vessels and into the aneurysm, then fills it with soft platinum coils. The coils trigger clotting inside the aneurysm, sealing it off from the inside. Because there’s no skull incision, recovery is typically about one week.
Both procedures aim to prevent rupture by stopping blood flow into the aneurysm while preserving normal circulation. The choice between them depends on the aneurysm’s size, shape, and location.
Treatment for Aortic Aneurysms
Aortic aneurysms are managed based on size. The traditional thresholds for recommending repair are 5.5 centimeters in diameter for men and 5.0 centimeters for women. Below those sizes, the risk of surgery generally outweighs the risk of rupture, so doctors monitor growth with regular imaging.
Recent research suggests these thresholds may actually be too aggressive for some patients. For a 60-year-old man in average health, the optimal repair size to minimize death from aneurysm-related causes may be closer to 6.9 centimeters. For a 60-year-old woman in average health, it may be around 6.1 centimeters. Patients in poor overall health may benefit from waiting even longer, since they face higher surgical risks. These findings are prompting a reconsideration of the one-size-fits-all approach, with more weight given to a patient’s age, sex, and overall health when deciding whether and when to operate.
Below the surgical threshold, management focuses on slowing growth: controlling blood pressure, quitting smoking, and monitoring with imaging every 6 to 12 months.