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.

Aneurysms are caused by a weakness in the blood vessel wall, so anything (really anything!), that causes the wall to weaken can cause an aneurysm. When a blood vessel wall weakens, it struggles to contain the pressure of the blood pushing against the walls so the diameter of the blood vessel lumen increases. On top of this, pressure on the blood vessel walls increases as the diameter of the lumen also increases, thanks to LaPlace’s law. This makes the aneurysm diameter even bigger, creating a cycle of continuous bulging. You’ve likely experienced this when you blow up a balloon. The first breath into the balloon is tough, but the next several breaths are pretty easy. This positive feedback loop is why aneurysms usually get bigger over time.

So what underlying conditions might weaken the wall of blood vessels? Well one way is if all the layers of those blood vessel walls don’t receive oxygen. Typically, the cells of the vessel walls are able to get oxygen from blood as it flows by. However, in the first section of the aorta, where the walls are super thick and muscular in order to handle all that blood pressure coming from the heart, the cells need to have their own set of blood vessels that supply the tunica media and the tunica externa. We call these special blood vessels vasa vasorum.

In hypertension, the vasa vasorum can develop hyaline arteriolosclerosis, narrowing their lumen, and causing ischemia. This leads to atrophy of the smooth muscle of the tunica media, and overall all, weakening in the aortic wall.

The same thing can happen in blood vessels without vasa vasorum if we have a bunch of plaque building up on the tunica intima thanks to our friendly neighbourhood atherosclerosis. Oxygen just can’t penetrate all the vessel’s walls. You see this type of aneurysm often in the abdominal aorta, where blood pressure is high and the aorta walls are nourished by passing blood. If you think about this it’s no wonder, then, why both aneurysms and atherosclerosis share similar risk factors: usually people who are male, who are over the age of 60, who have hypertension, and who smoke.

If we look at tertiary syphilis, it affects the vasa vasorum as syphilis causes inflammation in the tunica intima of the vasa vasorum (which again are those blood vessels that deliver oxygen to the thick walls of larger blood vessels). This ultimately leads to a narrower lumen and restricts blood supply to the vessel walls of the thoracic aorta, causing atrophy. By the way, the name for this vasa vasorum inflammation is endarteritis obliterans. If you were to look at the vasa vasorum in tertiary syphilis, you’d see the inflammation causes fibrosis and scarring on the walls creating this cool to look at, but less cool to have, tree-bark like texture.

Other bacterial infections can also cause aneurysms, which we label as mycotic aneurysms. The bacteria from an infection somewhere else in the body can break off from the main infection and travel around in the blood. Usually these embolic bacteria will get stuck in the intracranial arteries, visceral arteries, or the arteries feeding the arms and legs. There, the bacteria enter and weaken the blood vessel walls leading to, you guessed it, an aneurysm. Some of the bacteria that commonly cause mycotic aneurysms are, and these can be tricky to pronounce so bare with me, Bacteroides fragilis, Pseudomonas aeruginosa, and anything in the Salmonella species. You’ll also see mycotic aneurysms are a complication of infective endocarditis. However, infection leading to aneurysm is not limited to just bacteria. Infection from the fungi Aspergillus, Candida, and Mucor are also a common cause of mycotic aneurysms.