The hematoma in the arterial wall can lead to arterial occlusion, or death due to exsanguinating hemorrhage if it extends into a body cavity. Patients with Marfan's syndrome, a disease of collagen crosslinking, get cystic medial necrosis as well as true aneurysms of the ascending aorta. There are two ways to classify dissecting aneurysms according to their location. They are saccular out-pouchings of the arterial wall which may fill with thrombus and which may bleed to produce subarachnoid hemorrhage. Syphilitic aneurysms were the most common cause of ascending aortic aneurysm forty years ago, but are now extremely rare.
Syphilitic vasculitis is discussed further below.
They occur when injury to an arterial wall allows escape of blood to form a hematoma which is confined by the arterial adventitia. This may be perceived clinically as a pulsatile mass, but it differs from a true aneurysm in that the actual diameter of the vessel is not increased. Pseudoaneurysms typically occur after penetrating trauma, but sometimes after closed injury.