Abnormal dilation (ballooning) of an artery caused by the pressure of blood flowing through a weakened area. Aneurysms most commonly affect the aorta and arteries supplying the brain. The most common cause of an aneurysm is atherosclerosis, a condition in which fatty deposits weaken the artery wall. The aorta is the usual site of atherosclerotic aneurysms. Less commonly, aneurysms may be due to a congenital weakness of the artery walls. Most cerebral aneurysms, known as berry aneurysms because of their appearance, are congenital. Marfan’s syndrome, an inherited disorder in which the muscular layer of the aorta is defective, is often associated with aneurysms just above the heart. The arterial wall can also be weakened by inflammation, as occurs in polyarteritis nodosa. Most aneurysms are symptomless and remain undetected, but if the aneurysm expands rapidly and causes pain, or it is very large, the symptoms are due to pressure on nearby structures. Aneurysms may eventually rupture, cause fatal blood loss, or, in the case of a cerebral aneurysm, loss of consciousness (see subarachnoid haemorrhage). In some cases, only the inner layer of the artery wall ruptures, which allows blood to track along the length of the artery and block any branching arteries. There is usually severe pain and high risk of rupture occurring. Aneurysms sometimes develop in the heart wall due to weakening of an area of heart muscle as a result of myocardial infarction. Such aneurysms seldom rupture but interfere with the pumping action of the heart. Aneurysms of the aorta may be detected by ultrasound scanning, and cerebral aneurysms by CT scanning or MRI. Angiography provides information on all types of aneurysm. Ruptured or enlarged aneurysms require immediate surgery (see arterial reconstructive surgery).


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