An aneurysm looks like a balloon or bulge pushing outward from the wall of a blood vessel. It forms when a weakened spot in an artery stretches under the pressure of blood flow, creating a pouch that can range from a few millimeters to several centimeters across. Medically, a bulge qualifies as a true aneurysm when the vessel’s diameter expands to more than 1.5 times its normal size.
The Two Basic Shapes
Aneurysms come in two distinct forms. A fusiform aneurysm bulges outward on all sides of the blood vessel, giving it a spindle or football-like shape. This is the more common type. A saccular aneurysm, sometimes called a berry aneurysm, pouches out on only one side, looking more like a small sac or berry hanging off the artery wall by a narrow neck. Berry aneurysms are the classic shape found at branching points in the brain’s arteries.
In both types, the artery wall at the bulge is thinner and weaker than normal. As the aneurysm grows, that wall stretches further, which is why size matters so much in deciding whether treatment is needed.
What You Can See or Feel on the Body
Most aneurysms are hidden deep inside the body and produce no visible signs at all. But there are exceptions. An abdominal aortic aneurysm, which forms in the large artery running through your midsection, can sometimes be felt as a throbbing or pulsing sensation near the belly button. In thin individuals, a doctor may even be able to see or feel this pulsation during a physical exam.
Aneurysms behind the knee (popliteal aneurysms) can be more visible. A healthcare provider can sometimes see and feel the enlarged artery at the back of the knee, and it may throb in time with your pulse. If one of these ruptures or sends a blood clot downstream, the signs become more obvious: swelling and bruising behind the knee, along with a leg that turns pale, cold, or numb below the blockage.
How Brain Aneurysms Show Up
A brain aneurysm is invisible from the outside, but it can produce physical signs you or someone else might notice. An unruptured aneurysm pressing on nearby nerves can cause pain above and behind one eye, a dilated pupil in that eye, double vision, or a drooping eyelid. These changes happen because the bulging vessel is physically pushing against the nerve that controls eye movement.
About 11% of people who go on to have a major brain aneurysm rupture experience a “warning headache” days or weeks beforehand. This is a sudden, unusually severe headache lasting at least an hour, caused by a tiny leak from the aneurysm. Research using specialized brain imaging has confirmed these warning headaches reflect actual small bleeds, not just recall bias. If a brain aneurysm fully ruptures, the hallmark is a sudden, explosive headache often described as the worst headache of a person’s life, sometimes with nausea, stiff neck, or loss of consciousness.
What Aneurysms Look Like on Scans
Since most aneurysms can’t be seen from the outside, imaging is how they’re actually visualized. The appearance depends on the type of scan.
On a CT angiogram (CTA), contrast dye is injected into the bloodstream and the aneurysm appears as a bright, contrast-filled pouch ballooning off the artery. The shape, size, and neck width are all clearly visible, which is why CTA is one of the primary tools for diagnosis. If the aneurysm has ruptured, a standard CT scan without contrast will show blood pooled around the brain as a bright white area against the darker surrounding tissue.
On an MRI-based scan called magnetic resonance angiography (MRA), flowing blood creates its own signal, so the aneurysm shows up as a bright spot along the artery. One limitation is that blood clots of different ages or fluid-filled cysts can sometimes mimic the appearance of an aneurysm on these scans, which is why doctors often use multiple imaging methods to confirm a diagnosis.
Very large aneurysms, sometimes called giant aneurysms, can have a more complex appearance. On CT, they may show up as a large, dense mass with calcified edges and surrounding swelling. Inside, layers of old and new blood clot give the mass a mixed appearance on MRI, with the still-open channel of blood flow visible as a dark void running through the clot.
How Size Determines Risk
The size of an aneurysm is the single most important factor in deciding how dangerous it is. For aortic aneurysms, surgical repair has traditionally been recommended once the diameter reaches about 5.5 centimeters. Updated guidelines from the American College of Cardiology have lowered that threshold to 5.0 centimeters at specialized centers with experienced surgical teams, and even lower for people with genetic connective tissue disorders.
Growth rate also matters. An aortic aneurysm that expands by half a centimeter or more in a single year is considered fast-growing and typically warrants intervention regardless of its current size. For people who are significantly smaller or larger than average, doctors may adjust these thresholds based on body size, since the same diameter aneurysm puts more relative strain on a smaller person’s aorta.
Brain aneurysms follow a different scale. Because the arteries in the brain are much smaller, an aneurysm just 7 to 10 millimeters across can carry meaningful rupture risk, and anything over 25 millimeters is classified as giant. Small, stable brain aneurysms under 7 millimeters are often monitored with periodic imaging rather than treated immediately.
After Treatment on Follow-Up Imaging
If an aneurysm has been treated with coils or other devices inserted through a catheter, follow-up scans have their own visual vocabulary. A successfully treated aneurysm appears as a tightly packed mass of coils with no contrast dye entering the sac. An incompletely treated one may show contrast still filling a portion of the aneurysm beside the coils, or seeping between them, a finding called interstitial opacification. These follow-up appearances help doctors decide whether additional treatment is needed.