Clinical meaning
A thoracic aortic aneurysm (TAA) is an abnormal dilation of the thoracic aorta to greater than 1.5 times its normal diameter (normal ascending aorta approximately 3.5 cm; aneurysm defined as greater than 4.5-5.0 cm). The aortic wall has three layers: intima (endothelial lining), media (smooth muscle and elastic fibers providing strength and elasticity), and adventitia (connective tissue providing structural support). TAA develops from progressive weakening of the medial layer through cystic medial degeneration: loss of smooth muscle cells, fragmentation and loss of elastic fibers, and accumulation of mucoid material. This weakening allows the aortic wall to dilate under the constant stress of pulsatile blood pressure (following Laplace's law: wall tension = pressure x radius; as the radius increases, wall tension increases further, creating a self-perpetuating cycle of progressive dilation). Risk of rupture or dissection increases dramatically with size: less than 5 cm (1%/year risk), 5-6 cm (6-7%/year), greater than 6 cm (14%/year). Causes include degenerative atherosclerotic disease (most common in elderly), connective tissue disorders (Marfan syndrome, Ehlers-Danlos, Loeys-Dietz), bicuspid aortic valve (associated with ascending aortopathy), chronic hypertension, and familial thoracic...