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Introduction to Aortic Aneurysms and Aortic Dissection

By John W. Hallett, Jr., MD, Clinical Professor, Division of Vascular Surgery, Medical University of South Carolina

The aorta, which is about 1 inch (2.5 centimeters) in diameter, is the largest artery of the body. It receives oxygen-rich blood from the left ventricle and distributes it to all of the body except the lungs (which receive blood from the right ventricle). Just after the aorta leaves the heart, smaller arteries that carry blood to the head and arms branch off. The aorta then arches down, with additional smaller arteries branching off along its route from the left ventricle to the lower abdomen at the top of the hipbone (pelvis). At this point, the aorta divides into the two iliac arteries, which supply blood to the legs.

Disorders of the aorta include

  • Aneurysms: Bulges in weak areas of its walls

  • Dissection: Separation of the layers of its wall

These disorders can be immediately fatal, but they usually take years to develop.

Aneurysms can also develop in the arteries at the back of the knee (popliteal arteries), the main arteries of the thighs (femoral arteries), the arteries supplying the head (carotid arteries), the arteries supplying the brain (cerebral arteries), and the arteries supplying the heart muscle (coronary arteries). Aneurysms may also develop in other arteries (see Aneurysms of Arteries in the Arms, Legs, Heart, and Brain and see Subarachnoid Hemorrhage).

Aortic Aneurysms

  • The pressure of blood inside the artery forces any weak areas in the artery's wall to balloon outward.

  • If untreated, an aneurysm may rupture, resulting in pain and internal bleeding.

Consequences depend on the size of the rupture. A large rupture may be rapidly fatal, and a small one (sometimes termed a leak) may cause warning symptoms that allow people to seek medical care.

Aneurysms can develop anywhere along the aorta. Three fourths of aortic aneurysms develop in the part that passes through the abdomen (abdominal aorta), and the rest develop in the part that passes through the chest (thoracic aorta).

In older people, aneurysms are most likely to occur in areas where arteries branch (for example, where the abdominal aorta branches into the iliac arteries) or in areas of stress (for example, in the popliteal artery. Aneurysms may be round (saccular) or tubelike (fusiform). Most are fusiform.

The most common cause of aortic aneurysms is

  • Atherosclerosis, which weakens the wall of the aorta

Less common causes include

  • Injuries to the aorta, most often due to a motor vehicle crash

  • Inflammatory diseases of the aorta (aortitis)

  • Hereditary connective tissue disorders, such as Marfan syndrome

  • Some infectious diseases, such as syphilis

In people with Marfan syndrome, an aneurysm is most likely to develop in the first part of the aorta, where it emerges from the heart (the ascending aorta). In older people, almost all aneurysms are associated with atherosclerosis. High blood pressure, which is common among older people, and cigarette smoking increase the risk of an aneurysm.

A blood clot (thrombus) often develops in the aneurysm because blood flow inside the aneurysm is sluggish. The clot may extend along the entire wall of the aneurysm. A blood clot may break loose (becoming an embolus), travel through the bloodstream, and block arteries. Aneurysms in the popliteal arteries are more likely to produce emboli than aneurysms in other arteries. Occasionally, calcium is gradually deposited in the wall of an aneurysm, making it easier to see on x-rays.

Where Do Aortic Aneurysms Occur?

Aneurysms can develop anywhere along the aorta. Most develop in the abdominal aorta. The rest develop in the thoracic aorta, most commonly in the ascending aorta.

Aortic Dissection

When the inner lining of the aorta separates (tears) from the middle layer of the aorta, blood can push between these layers, separating (dissecting) the middle layer of the wall from the still intact outer layer. As a result, a new, false channel forms in the wall of the aorta. Virtually everyone who has an aortic dissection experiences pain—typically sudden, excruciating pain, often described as tearing or ripping. As the dissection advances further along the aorta, it can close off the points at which one or more arteries branch off from the aorta, blocking blood flow. The consequences vary depending on which arteries are blocked (see also Aortic Dissection).

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