Aneurysms may involve arteries or veins and have various causes. They are commonly further classified by shape, structure and location.
A saccular aneurysm resembles a small bubble that appears off the side of a blood vessel. The innermost layer of an artery, in direct contact with the flowing blood, is the tunica intima, commonly called the intima. Adjacent to this layer is the tunica media, known as the media and composed of smooth muscle cells and elastic tissue. The outermost layer is the tunica adventitia or tunica externa. This layer is composed of tougher connective tissue. A saccular aneurysm develops when fibers in the outer layer separate allowing the pressure of the blood to force the two inner layers to balloon through.
A fusiform aneurysm is a bulging around the entire circumference of the vessel without protrusion of the inner layers. It is shaped like a football or spindle.
These aneurysms can result from hypertension in conjunction with atherosclerosis that weakens the tunica adventitia, from congenital weakness of the adventitial layer (as in Marfan syndrome) or from infection.
In a true aneurysm the inner layers of a vessel have bulged outside the outer layer that normally confines them. The aneurysm is surrounded by these inner layers.
A false- or pseudoaneurysm does not primarily involve such distortion of the vessel. It is a collection of blood leaking completely out of an artery or vein, but confined next to the vessel by the surrounding tissue. This blood-filled cavity will eventually either thrombose (clot) enough to seal the leak or it will rupture out of the tougher tissue enclosing it and flow freely between layers of other tissues or into looser tissues. Pseudoaneurysms can be caused by trauma that punctures the artery and are a known complication of percutaneous arterial procedures such as arteriography or of arterial grafting or of use of an artery for injection, such as by drug abusers unable to find a usable vein. Like true aneurysms they may be felt as an abnormal pulsatile mass on palpation.
Most non-intracranial aneurysms (94%) arise distal to the origin of the renal arteries at the infrarenal abdominal aorta, a condition mostly caused by atherosclerosis. The thoracic aorta can also be involved. One common form of thoracic aortic aneurysm involves widening of the proximal aorta and the aortic root, which leads to aortic insufficiency. Aneurysms occur in the legs also, particularly in the deep vessels (e.g., the popliteal vessels in the knee). Arterial aneurysms are much more common, but venous aneurysms do happen (for example, the popliteal venous aneurysm).
- While most aneurysms occur in an isolated form, the occurrence of berry aneurysms of the anterior communicating artery of the circle of Willis is associated with autosomal dominant polycystic kidney disease (ADPKD).
- The third stage of syphilis also manifests as aneurysm of the aorta, which is due to loss of the vasa vasorum in the tunica adventitia.
Aneurysms can also form in the left ventricle (left ventricular aneurysm), often following ST elevation myocardial infarction. In addition, coronary artery aneurysm can occur in the coronary arteries.