An aneurysm is a bulging, weakened spot in the wall of a blood vessel where the vessel balloons outward to more than 50% beyond its normal width. They can form in arteries throughout the body, but the two most common and dangerous locations are the brain and the large artery running through the abdomen (the aorta). Most aneurysms cause no symptoms and go undetected for years. The danger comes if they grow large enough to rupture, which can cause life-threatening internal bleeding.
How an Aneurysm Forms
Artery walls are built in layers, with the middle layer providing most of the structural strength. That middle layer contains elastic fibers and smooth muscle cells that allow the artery to flex with each heartbeat while holding its shape. An aneurysm develops when this support system breaks down.
The breakdown happens through a combination of chronic inflammation, cell death, and the action of specific enzymes that chew through collagen and elastin, the two proteins responsible for an artery’s strength and flexibility. As these structural proteins degrade and smooth muscle cells die off, the arterial wall thins. Blood pressure pushing against the weakened spot causes it to stretch outward, forming a balloon-like bulge that can slowly expand over months or years.
Types and Shapes
Aneurysms come in several forms. A fusiform aneurysm bulges outward evenly on all sides of the vessel, like a snake that swallowed an egg. A saccular aneurysm (sometimes called a “berry” aneurysm in the brain) pouches out on only one side, more like a blister. Both are considered true aneurysms because all layers of the vessel wall are involved in the bulge.
A pseudoaneurysm, or false aneurysm, is different. It forms when a tear in the inner lining of the vessel lets blood seep between the wall’s layers, creating a pocket that looks like an aneurysm on imaging but isn’t actually a ballooning of the full wall. A dissecting aneurysm involves a similar tear, but the blood tracks along the length of the vessel, separating its layers rather than pushing them outward.
Where Aneurysms Occur
Brain (Cerebral) Aneurysms
Brain aneurysms most often form at branching points in the ring of arteries at the base of the brain. The most common spots are where the front-facing arteries meet near the midline, at the branching point of the middle cerebral artery, and where the internal carotid artery connects with the posterior communicating artery. These junctions experience turbulent blood flow, which stresses the vessel wall over time. Estimates of how many people carry an unruptured brain aneurysm range widely, from less than 1% to as high as 10% of the general population, with many never knowing it.
Abdominal Aortic Aneurysms
The aorta is the body’s largest artery, running from the heart down through the chest and abdomen. Aneurysms in the abdominal section are especially common in older men with a history of smoking. The U.S. Preventive Services Task Force recommends a one-time ultrasound screening for men aged 65 to 75 who have ever smoked (defined as 100 or more cigarettes in a lifetime). Studies show that among men with a normal screening result, the risk of dying from an abdominal aortic aneurysm over the next 5 to 12 years is less than 3%.
Risk Factors
Some risk factors are within your control, and some are not. Smoking and high blood pressure are the two biggest modifiable risks. Each roughly doubles to triples the likelihood of a dangerous aneurysm, and having both together raises the risk even further. In one study of people with a family history of brain hemorrhage from aneurysms, those who both smoked and had high blood pressure were 2.7 times more likely to harbor an aneurysm than those with neither risk factor.
Family history is the strongest non-modifiable risk factor. Having a first-degree relative (parent or sibling) who experienced a ruptured brain aneurysm increases your own risk roughly sixfold. Certain inherited connective tissue conditions also weaken artery walls and predispose people to aneurysms at younger ages. Older age and male sex are additional risk factors, particularly for abdominal aortic aneurysms.
Symptoms and Warning Signs
Most unruptured aneurysms produce no symptoms at all. They are often discovered incidentally during brain scans or imaging done for other reasons. Occasionally, a large unruptured brain aneurysm presses on nearby nerves, causing a drooping eyelid, double vision, or pain behind one eye.
The most dramatic symptom is the “thunderclap” headache that signals a ruptured brain aneurysm. This is typically described as the worst headache of a person’s life, reaching maximum intensity within seconds. It may be accompanied by nausea, vomiting, a stiff neck, sensitivity to light, confusion, or loss of consciousness. Before a full rupture, some people experience a warning headache days or weeks earlier, known as a sentinel headache. Research suggests that anywhere from 15% to 60% of people who suffer a major brain hemorrhage from an aneurysm had a sentinel headache beforehand, though these are frequently dismissed as a migraine or tension headache.
Abdominal aortic aneurysms are similarly silent until they become large. A growing or leaking one may cause deep, steady pain in the abdomen or lower back, or a pulsing sensation near the navel.
When Rupture Happens
A ruptured brain aneurysm causes a subarachnoid hemorrhage, meaning blood spills into the space surrounding the brain. The overall incidence of this event is roughly 7 to 8 per 100,000 people per year. It is a medical emergency with a high fatality rate. Many people who survive face a long recovery and may have lasting neurological effects.
A ruptured abdominal aortic aneurysm is equally catastrophic. Blood pours into the abdominal cavity, and without emergency surgery, survival is unlikely. This is why screening and monitoring matter so much for people at risk.
How Aneurysms Are Treated
Brain Aneurysms
Two main procedures exist for brain aneurysms, and the choice depends on the aneurysm’s size, shape, and location.
Surgical clipping involves opening a small window in the skull and placing a tiny titanium clip across the base of the aneurysm, sealing it off from blood flow. This is effective and durable, but it is an invasive procedure. Recovery takes at least four to six weeks.
Endovascular coiling is less invasive. A catheter is threaded through a blood vessel in the leg up to the brain, where tiny coils or a stent are placed inside or near the aneurysm to block blood from entering it. Because there is no skull incision, recovery is much faster, typically about one week. Some coiled aneurysms need follow-up imaging to make sure the seal holds over time.
Abdominal Aortic Aneurysms
Small abdominal aortic aneurysms are monitored with periodic ultrasounds rather than treated immediately. The traditional size threshold for recommending repair has been 5.5 cm for men and 5.0 cm for women, though recent research suggests the optimal thresholds may be somewhat higher. A 2024 study in the Journal of Vascular Surgery found that for a 60-year-old man in average health, the repair size that best minimized long-term mortality was actually closer to 6.9 cm, and for a 60-year-old woman, about 6.1 cm. These thresholds increase further with age and in patients with other serious health conditions. Your vascular surgeon will weigh the risks of the aneurysm rupturing against the risks of the repair itself.
Repair can be done as open surgery, where the weakened section of the aorta is replaced with a synthetic graft, or as an endovascular procedure, where a stent graft is threaded through the leg arteries and positioned inside the aneurysm to reinforce the wall. The endovascular approach has a shorter hospital stay and faster initial recovery, though both methods have good long-term outcomes in appropriately selected patients.
Living With an Unruptured Aneurysm
If you’ve been told you have a small, unruptured aneurysm that doesn’t yet need repair, the focus shifts to reducing the forces that could make it grow. Controlling blood pressure is the single most impactful step. Quitting smoking matters enormously, since tobacco accelerates the enzymatic breakdown of artery walls that drives aneurysm growth. Avoiding heavy straining and stimulant drugs that spike blood pressure also helps.
Regular imaging, whether ultrasound for abdominal aneurysms or MRI for brain aneurysms, tracks the size over time. Many small aneurysms remain stable for years and never require intervention. The monitoring schedule gives you and your doctor a clear picture of whether and when treatment makes sense.