Coloured 3D computed tomography (CT) scan of a patient with an aortic dissection. The aorta (yellow), the body's main artery, is at centre right. An aneurysm is a blood-filled dilation in a blood vessel caused by a weakening of the vessel wall. In a dissecting aortic aneurysm, a tear in the inner wall of the artery causes blood to flow between the layers of the wall. Without surgical repair the condition is usually fatal.