The outpouching of the wall of the aorta occurs when the musculoelastic middle layer or media of the artery becomes weak (often caused by plaque and cholesterol deposits) and degenerative changes occur. The inner and outer layers of the arterial wall are stretched, and as the pulsatile force of the blood rushes through the aorta, the vessel wall becomes increasingly weak, and the aneurysm enlarges. Abdominal aneurysms can be fatal. More than half of people with untreated aneurysms die of aneurysm rupture within 2 years.