Discussion Author: Michael D Wirt
Aneurysmal dilatation of the thoracic aorta can occur in a symmetric/longitudinal (fusiform) fashion or in a more eccentric-focal (saccular) configuration. Fusiform dilatation of the aorta when the diameter is larger than the mean, but less than two standard deviations above the mean for that segment (where mean values for the aortic root, ascending and descending aorta are 3.7 cm, 3.2 cm and 2.5 cm respectively (1)).
The generally accepted definition of aneurysm is an aortic diameter greater than 1.5 times normal. Aneurysmal dilatation of the thoracic aorta occurs when the diameter of the ascending aorta is > 5cm or of the descending aorta is >4cm (2). Aneurysms can be classified as true, involving all three layers of the aortic wall (tend to be fusiform) or false, where the intima and media are involved (tend to be saccular). Most thoracic aortic aneurysms are asymptomatic, fusiform in configuration and secondary to atherosclerosis (2).
Additional etiologies include:
Connective tissue diseases (Marfan's and Ehler-Danlos)
Post-traumatic (contained rupture)
Aortitis (Takayasu disease, Giant cell aortitis, Collagen vascular disease (Rheumatoid arthritis, Ankylosing spondylitis).
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