Pathophysiology Of Aneurysm or Aneurism
The occurrence and expansion of an aneurysm depends on the local hemodynamic factors and factors intrinsic to the arterial segment itself.
The aorta has a relatively low-resistance circuit for circulating blood and the lower extremities have higher arterial resistance. Repeated trauma of a reflected arterial wave on the distal aorta may injure an already weakened aortic wall and lead to aneurysmal degeneration. Systemic hypertension further increases the injury, accelerates the expansion of existing aneurysms, and also contributes to their formation.
Aneurysm formation is due to multiple factors affecting the arterial segment and its local environment. Many aneurysms are atherosclerotic in nature. As the aneurysmal dilatation increases, the arterial wall tension also increases and this can result in rupture of the aneurysm. Increase in blood pressure (systemic hypertension) leads to increase in aneurysm size, which in turn increases the arterial wall tension and therefore increases the risk of rupture.
The vessel wall is supplied by the blood within its lumen although the aorta has vasa vasorum, therefore as the aneurysm develops, the mostischemic portion of the aneurysm is at the distal end, which results in weakening of the vessel wall there and aids in further expansion of the aneurysm. Thus eventually, all aneurysms if left to their natural course rupture without intervention.
Aneurysms that result from an infectious process involving the arterial wall are called mycotic aneurysms. The term “mycotic” is misleading implying a fungal cause. In fact, the main pathogens of mycotic aneurysms are gram-positive cocci, e.g. bacterial endocarditis can cause mycotic aneurysm by blocking the small arteries with infected emboli.
While most aneurysms occur in an isolated form, the occurrence of berry aneurysms is always confined to the base of the brain. Berry aneurysms are the smallest aneurysmal dilatations and are congenital in nature.
Syphilis is the most common cause for inflammatory aneurysm, involving aorta and resulting in saccular aneurysm.