Aneurysm - Pathophysiology

Pathophysiology

Aneurysm formation is probably the result of multiple factors affecting that arterial segment and its local environment.Many aneurysms are atherosclerotic in nature. The occurrence and expansion of an aneurysm in a given segment of the arterial tree involves local hemodynamic factors and factors intrinsic to the arterial segment itself.

The aorta is a relatively low-resistance circuit for circulating blood. The lower extremities have higher arterial resistance, and the repeated trauma of a reflected arterial wave on the distal aorta may injure a weakened aortic wall and contribute to aneurysmal degeneration. Systemic hypertension compounds the injury, accelerates the expansion of known aneurysms, and may contribute to their formation.

Increasing aneurysmal dilatation leads to increasing arterial wall tension or stress. In hemodynamic terms, the coupling of aneurysmal dilatation and increased wall stress is approximated by the law of Laplace. The Law of Laplace applied to a cylinder states that the (arterial) wall tension is equal to the pressure times the radius of the arterial conduit divided by wall thickness (T = /t). As diameter increases, wall tension increases, which contributes to more increase in diameter and risk of rupture. Increased blood pressure (systemic hypertension) and increased aneurysm size increase arterial wall tension and therefore increase the risk of rupture. Wall thickness is decreased in aneurysms and further adds to the increase in wall tension.

In addition, the vessel wall is supplied by the blood within its lumen in humans in a developing aneurysm, the most ischemic portion of the aneurysm is at the farthest end, resulting in weakening of the vessel wall there and aiding further expansion of the aneurysm. Thus eventually all aneurysms will, if left to complete their evolution, rupture without intervention.

A mycotic aneurysm is an aneurysm that results from an infectious process that involves the arterial wall. A person with a mycotic aneurysm has a bacterial infection in the wall of an artery, resulting in the formation of an aneurysm. The most common locations include arteries in the abdomen, thigh, neck, and arm. A mycotic aneurysm can result in sepsis, or life threatening bleeding if the aneurysm ruptures. Less than 3% of abdominal aortic aneurysms are mycotic aneurysms.

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). This type of aneurysm places pressure on the surrounding brain tissue, causing it to malfunction. A rupture of this cerebral brain tissue, would cause excessive bleeding around the brain, called a subarachnoid hemorrhage. Severe bleeding can cause brain damage and lead to permanent disability.

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.