An aneurysm is a bulge or balloon-like swelling in the wall of a blood vessel, caused by a weakened spot that allows the vessel to stretch outward under the pressure of blood flow. About 5.4% of people worldwide are living with an unruptured brain aneurysm alone, and many never know it. Aneurysms can form in arteries throughout the body, but the most common and dangerous locations are the brain and the aorta, the large artery that runs from your heart through your chest and abdomen.
How an Aneurysm Forms
Artery walls are built to handle constant blood pressure, but certain conditions break down that structural integrity over time. The supportive tissue in the vessel wall loses its balance: cells that normally maintain the wall’s strength start behaving abnormally, and enzymes that break down proteins become overactive. The result is a patch of artery wall that can no longer hold its shape under pressure. Blood pushes against this weakened spot, and the wall stretches outward like a worn section of a garden hose.
The two main shapes an aneurysm can take are saccular and fusiform. A saccular (or “berry”) aneurysm bulges out on one side of the vessel, like a berry hanging from a stem. These are the most common type found in the brain and can range from a few millimeters to more than two centimeters across. A fusiform aneurysm, by contrast, causes the entire circumference of the vessel to widen, creating a spindle-shaped swelling. A rarer third type, called a mycotic aneurysm, results from an infection in the artery wall itself.
Where Aneurysms Develop
Brain Aneurysms
Unruptured brain aneurysms are surprisingly common. A meta-analysis covering more than 300,000 people found a worldwide prevalence of 5.4%. The rate is higher in women (5.4%) than in men (4.1%), and higher in people over 45 (5.3%) compared to those under 45 (3.2%). Most of these aneurysms are small, cause no symptoms, and are discovered incidentally during imaging for something else entirely.
When an unruptured brain aneurysm does cause symptoms, it’s usually because it has grown large enough to press on nearby nerves or brain tissue. This can produce a drooping eyelid, double vision, or pain above or behind the eye. But many people feel nothing at all until, and unless, a rupture occurs.
Abdominal Aortic Aneurysms
The aorta is the body’s largest artery, and aneurysms in its abdominal section are the most common type of aortic aneurysm. Many start small and stay small. Others grow over time, sometimes quickly. An expanding abdominal aortic aneurysm can cause deep, constant pain in the belly or side, back pain, or a throbbing sensation near the belly button. The primary driver is atherosclerosis, the gradual buildup of fatty deposits on artery walls. High blood pressure, blood vessel inflammation, and, rarely, infection or trauma can also be responsible.
Risk Factors
Your risk of developing an aneurysm rises with age. Abdominal aortic aneurysms are most common in adults over 65. Men develop aortic aneurysms more often than women, though when women do have one, it is more likely to rupture at a smaller size.
Cigarette smoking is one of the strongest modifiable risk factors, especially for abdominal aortic aneurysms. Smoking accelerates growth and increases the chance of rupture. High blood pressure is the leading risk factor for thoracic aortic aneurysms (those in the chest portion of the aorta) and contributes to abdominal ones as well. Other medical conditions that raise risk include coronary heart disease, peripheral artery disease, COPD, obesity, unhealthy cholesterol levels, and chronic kidney disease.
Genetics play a significant role. One in ten people with an abdominal aortic aneurysm has a family history of the condition, and if you have a first-degree relative (parent, sibling, or child) with one, your own risk is roughly one in five. Several inherited connective tissue disorders, including Marfan syndrome, Ehlers-Danlos syndrome, and Loeys-Dietz syndrome, substantially increase the risk of thoracic aortic aneurysms because they affect the structural proteins that keep artery walls intact.
Symptoms of a Rupture
A ruptured brain aneurysm announces itself with a sudden, extremely severe headache, often described as the worst headache of a person’s life. This is sometimes called a “thunderclap headache” because it reaches peak intensity within seconds. Other symptoms that can accompany it include nausea and vomiting, a stiff neck, blurred or double vision, sensitivity to light, seizures, confusion, and loss of consciousness. This is a life-threatening emergency.
Sometimes a brain aneurysm leaks a small amount of blood before fully rupturing. A leaking aneurysm can cause a sudden, severe headache that persists for days or even up to two weeks. This is essentially a warning sign that a full rupture may follow.
A ruptured abdominal aortic aneurysm typically causes sudden, intense pain in the abdomen or back, along with dizziness and a rapid drop in blood pressure. Like a brain aneurysm rupture, it requires immediate emergency care.
How Aneurysms Are Found
Because most aneurysms produce no symptoms, they are often detected during imaging done for another reason. Once suspected, specific imaging tools confirm the diagnosis and guide decisions about monitoring or treatment.
For brain aneurysms, CT angiography (a specialized CT scan that highlights blood vessels) can detect aneurysms as small as 3 millimeters, with sensitivity and specificity rates of 96 to 98%. MR angiography, which uses magnetic fields instead of radiation, is often preferred for long-term monitoring of treated aneurysms and for screening people with a family history, since it avoids repeated radiation exposure. It is less practical in emergencies because the scan takes longer and requires the patient to stay very still.
For abdominal aortic aneurysms, a simple ultrasound of the abdomen is the standard screening tool. 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). Men in that age range who have never smoked may still be offered screening on a case-by-case basis.
Treatment Options
Not every aneurysm needs immediate treatment. Small, stable aneurysms are often monitored with periodic imaging to track their size. Treatment decisions depend on the aneurysm’s location, size, growth rate, and the patient’s overall health.
Brain Aneurysm Repair
Two main approaches exist for treating brain aneurysms. Surgical clipping involves opening a small section of the skull and placing a tiny titanium clip at the base of the aneurysm to seal it off from blood flow. Recovery takes at least four to six weeks, but the advantage is a lower chance of the aneurysm coming back, which means less follow-up imaging over time.
Endovascular coiling is less invasive. A catheter is threaded through a blood vessel in the leg up to the aneurysm in the brain, where tiny coils are packed inside the aneurysm to block blood flow into it. Recovery is significantly shorter, typically about one week. The trade-off is a higher chance that the aneurysm can recur, so you will likely need routine follow-up imaging to check for any regrowth.
Aortic Aneurysm Repair
International guidelines recommend considering surgical repair when an abdominal aortic aneurysm reaches 5.5 centimeters in diameter for men or 5.0 centimeters for women. Below those thresholds, the risk of the surgery itself generally outweighs the risk of rupture, so regular monitoring is the standard approach. Repair options include open surgery or a less invasive procedure where a reinforcing graft is placed inside the aorta through a catheter, similar in concept to endovascular coiling but adapted for a much larger vessel.
For aneurysms that don’t yet meet the threshold for repair, managing the underlying risk factors is the primary strategy. Controlling blood pressure, quitting smoking, and treating high cholesterol can slow growth and reduce the chance of rupture.