ANEURYSM

a bulge or enlargement of some point in the wall of a blood vessel (usually an artery) resulting from disease of the vessel wall. Disease or injury can weaken an artery or cause thinning of its walls, which tend to balloon outward from the pressure of the circulating blood, forming a sac. In a typical aneurysm, the two innermost layers of the arterial wall, the tunica intima and tunica media, have ruptured and a blood-filled bulge or sac is formed by the vessel's outermost layer, the tunica adventitia. In a false aneurysm, all three layers have ruptured, and the arterial blood is held in the vicinity only by the surrounding tissues. Aneurysms may form as a result of arteriosclerosis (thickening of arterial walls), embolism (a blood clot or foreign object that travels through the bloodstream and eventually becomes lodged in an artery), syphilis, physical injury, or congenital weakness of the artery walls. A small aneurysm can exist for many years without causing any symptoms. A popliteal artery aneurysm is easily detected by the affected person because it causes a noticeable, pulsating bulge behind the knee. An aneurysm in this location may lead to a blood clot and a resultant cutoff of circulation to the lower leg (with danger of gangrene) unless circulation is restored by surgery. The principal artery prone to aneurysms, however, is the aorta, particularly as it descends through the chest and abdomen. The symptoms of aortic aneurysm vary with the size of the defect and its location. If an aortic aneurysm presses against the windpipe and the bronchi, it may interfere with breathing and lead to coughing. Since aneurysms tend to enlarge over time and blood vessel walls tend to weaken with age, there is risk that an aneurysm will eventually burst, or rupture, an event marked by serious, even massive, internal bleeding. The rupture of an aortic aneurysm causes severe pain and results in immediate collapse. The cerebral hemorrhage that accompanies a ruptured aneurysm in the brain is one of the chief causes of strokes. An aneurysm on a small artery can be treated simply by tying off the affected vessel, whose blood flow is redirected to adjacent ones. The treatment of more serious aneurysms involves the surgical removal of the diseased section of artery and its replacement with a plastic graft.