Aneurysms

The word “aneurysm” comes from the Greek word aneurysma, meaning “dilation.” This makes sense because aneurysms are defined as abnormal dilations in a blood vessel. A bulge in a blood vessel is officially labelled as an aneurysm when the diameter of the bulge is approximately one and a half times larger than the normal diameter of the blood vessel.

Aneurysms can happen to any blood vessel in your body, including the aorta, the femoral artery, the iliac artery, the popliteal artery, and the cerebral arteries. They can also happen in your veins too, but those are less common as blood pressure in veins is much, much lower than in the arteries.

There are two major categories of aneurysms: true aneurysms and pseudoaneurysms. In true aneurysms, all the layers of the blood vessel wall dilate together. True aneurysms that balloon out symmetrically on all sides of the blood vessel are called fusiform aneurysms, whereas asymmetrically shaped aneurysms balloon out on one side of the blood vessel. This asymmetrical shape usually happens because for some reason one side of the blood vessel wall experiences higher blood pressure than the rest of the vessel wall, or because the wall was weaker on one side to begin with. Asymmetrical true aneurysms can be called either “saccular” or “berry” aneurysms.

Pseudoaneurysms, on the other hand, are like false aneurysms because they are not actually aneurysms at all. They are caused by a small hole in your blood vessel which allows blood to leak out of the vessel and form a pool of blood that looks like a fusiform or berry aneurysm depending on where the hole is and its size. The blood pools because the surrounding tissues act as walls that contain the blood in one spot.

Arterial aneurysms occur most commonly in the aorta with about 60% of true aortic aneurysms happening in the abdominal section of the aorta, and the other 40% happening in the thoracic section. Of all the abdominal aortic aneurysms, you can find about 95% of them just below the point where the renal arteries branch off from the abdominal aorta but above the aortic bifurcation. This is because there is naturally less elastin[a] in the walls of this section of the aorta.