<Li> Other causes of the development of AAA include: infection, trauma, arteritis, and cystic medial necrosis . </Li> <P> The most striking histopathological changes of the aneurysmatic aorta are seen in the tunica media and intima layers . These changes include the accumulation of lipids in foam cells, extracellular free cholesterol crystals, calcifications, thrombosis, and ulcerations and ruptures of the layers . Adventitial inflammatory infiltrate is present . However, the degradation of the tunica media by means of a proteolytic process seems to be the basic pathophysiologic mechanism of AAA development . Some researchers report increased expression and activity of matrix metalloproteinases in individuals with AAA . This leads to elimination of elastin from the media, rendering the aortic wall more susceptible to the influence of blood pressure . Other reports have suggested the serine protease granzyme B may contribute to aortic aneurysm rupture through the cleavage of decorin, leading to disrupted collagen organization and reduced tensile strength of the adventitia . There is also a reduced amount of vasa vasorum in the abdominal aorta (compared to the thoracic aorta); consequently, the tunica media must rely mostly on diffusion for nutrition, which makes it more susceptible to damage . </P> <P> Hemodynamics affect the development of AAA, which has a predilection for the infrarenal aorta . The histological structure and mechanical characteristics of the infrarenal aorta differ from those of the thoracic aorta . The diameter decreases from the root to the aortic bifurcation, and the wall of the infrarenal aorta also contains a lesser proportion of elastin . The mechanical tension in the abdominal aortic wall is therefore higher than in the thoracic aortic wall . The elasticity and distensibility also decline with age, which can result in gradual dilatation of the segment . Higher intraluminal pressure in patients with arterial hypertension markedly contributes to the progression of the pathological process . Suitable hemodynamic conditions may be linked to specific intraluminal thrombus (ILT) patterns along the aortic lumen, which in turn may affect AAA's development . </P> <P> An abdominal aortic aneurysm is usually diagnosed by physical exam, abdominal ultrasound, or CT scan . Plain abdominal radiographs may show the outline of an aneurysm when its walls are calcified . However, this is the case in less than half of all aneurysms . Ultrasonography is used to screen for aneurysms and to determine the size of any present . Additionally, free peritoneal fluid can be detected . It is noninvasive and sensitive, but the presence of bowel gas or obesity may limit its usefulness . CT scan has a nearly 100% sensitivity for an aneurysm and is also useful in preoperative planning, detailing the anatomy and possibility for endovascular repair . In the case of suspected rupture, it can also reliably detect retroperitoneal fluid . Alternative less often used methods for visualization of an aneurysm include MRI and angiography . </P>

Dilatation of the infrarenal abdominal aorta icd 10