A brain aneurysm looks like a small, blood-filled bulge ballooning out from an artery, most often shaped like a berry hanging from a stem. Most are tiny, under 11 millimeters, roughly the size of a large pencil eraser. They form along the network of arteries at the base of the brain, and their appearance varies depending on their shape, size, and whether they’ve ruptured.
Shape and Physical Appearance
The most common type, called a saccular or “berry” aneurysm, looks exactly like its nickname suggests: a round or slightly oval pouch that balloons outward from one side of an artery wall. It has a narrow neck where it connects to the artery and a wider, rounded dome. Think of it like a small balloon being inflated off the side of a garden hose. The dome is the weakest point and the spot most likely to rupture.
A less common type, the fusiform aneurysm, looks different. Instead of bulging to one side, the entire artery widens in all directions, creating a spindle-shaped swelling. Picture a snake that swallowed an egg. There’s no distinct neck or pouch, just a gradual widening and then narrowing of the vessel.
A third type, the mycotic aneurysm, is caused by infection. These tend to look more irregular and lobulated, with thicker, uneven walls and sometimes a visible blood clot sitting off-center inside the bulge. Surrounding tissue often appears inflamed or swollen on imaging.
Where They Form
Brain aneurysms almost always develop at branching points where arteries fork, because these junctions experience the most stress from blood flow. The most frequent locations are the anterior and posterior communicating arteries, two key connectors in the ring of blood vessels at the base of the brain known as the Circle of Willis. These same sites are also where ruptures happen most often.
How They’re Classified by Size
According to the National Institute of Neurological Disorders and Stroke, brain aneurysms fall into three size categories:
- Small: Less than 11 millimeters in diameter, about the size of a large pencil eraser.
- Large: 11 to 25 millimeters, roughly the width of a dime.
- Giant: Greater than 25 millimeters, wider than a quarter.
Most brain aneurysms are small and never cause symptoms. Giant aneurysms are rare but can press on surrounding brain tissue, causing neurological problems even without rupturing.
What the Artery Wall Looks Like Up Close
A healthy brain artery has neatly organized layers: a smooth inner lining, an elastic layer that lets the vessel stretch, and a muscular layer that helps control blood flow. In an aneurysm, this structure progressively breaks down.
Early on, the wall thickens as muscle cells and supporting cells scramble to reinforce the weakening spot. But over time, those muscle cells die off. The elastic fibers degrade. What remains is a thin wall made mostly of collagen, the body’s structural protein, with fewer and fewer living cells to maintain it. The dome of the aneurysm is where this thinning is most severe, sometimes leaving only a few layers of collagen between the blood inside and the brain outside.
Some aneurysm walls develop calcium deposits or pockets of fat. Aneurysms that have ruptured look different under a microscope: the collagen layers are disorganized and infiltrated with immune cells, and the wall itself may be replaced by a glassy, fragile material that lacks normal structural proteins entirely.
What a Ruptured Aneurysm Looks Like on a CT Scan
When a brain aneurysm ruptures, blood spills into the space surrounding the brain. On a CT scan performed shortly after, this blood appears as bright white streaks or pools against the darker gray of brain tissue. The blood often collects in the natural grooves and fissures around the brain, particularly the sylvian fissures along the sides. Sometimes the only visible sign is subtle asymmetry, like one side of the brain looking slightly different from the other, or enlarged fluid-filled spaces inside the brain.
Timing matters. CT is most sensitive in the first hours after a rupture. Several days or weeks later, the blood breaks down and a scan may look completely normal, even if bleeding did occur. In one study comparing imaging methods, CT detected signs of bleeding in only 46% of cases when the hemorrhage wasn’t acute.
Warning Signs Before a Rupture
Some people experience a sudden, intensely severe headache days before a full rupture. This is called a sentinel headache, and it likely results from a small leak or sudden expansion of the aneurysm. These headaches typically occur within two weeks of the major bleeding event, with a median interval of about five days. The pain comes on abruptly and is unusually severe, distinct from a typical headache or migraine. It’s often described as the worst headache of a person’s life.
Most unruptured aneurysms, however, produce no symptoms at all. They’re frequently discovered incidentally during brain imaging performed for other reasons. When a large or growing aneurysm does cause symptoms without rupturing, it’s usually because the bulge is pressing on a nearby nerve, leading to a drooping eyelid, double vision, or pain behind one eye.