What is the pathophysiology of thoracic aortic aneurysms (TAA)?

Updated: Apr 02, 2021
  • Author: Elaine Tseng, MD; Chief Editor: Mary C Mancini, MD, PhD, MMM  more...
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Answer

Aneurysms are usually defined as a localized dilation of an arterial segment greater that 50% its normal diameter. Most aortic aneurysms occur in the infrarenal segment (95%). The average size for an infrarenal aorta is 2 cm; therefore, AAAs are usually defined by diameters greater than 3 cm. The normal size for the thoracic and thoracoabdominal aorta is larger than that of the infrarenal aorta, and aneurysmal degeneration in these areas is defined accordingly. The average diameter of the mid-descending thoracic aorta is 26-28 mm, compared with 20-23 mm at the level of the celiac axis.

The occurrence and expansion of an aneurysm in a given segment of the arterial tree probably involves local hemodynamic factors and factors intrinsic to the arterial segment itself.

The medial layer of the aorta is responsible for much of its tensile strength and elasticity. Multiple structural proteins make up the normal medial layer of the human aorta. Of these, collagen and elastin are probably the most important. The elastin content of the ascending aorta is high and diminishes progressively in the descending thoracic and abdominal aorta. The infrarenal aorta has a relative paucity of elastin fibers in relation to collagen and compared with the thoracic aorta, possibly accounting for the increased frequency of aneurysms in this area.

In addition, the activity and amount of specific enzymes is increased, which leads to the degradation of these structural proteins. Elastic fiber fragmentation and loss with degeneration of the media result in weakening of the aortic wall, loss of elasticity, and consequent dilation.


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