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This patient's chest CT scan reveals a widened, ascending thoracic aorta with a nonenhancing septum dividing the lumen of the ascending and descending aorta.  The "septum" is actually the tunica intima of the aorta, which has been torn from the remainder of the aortic wall.  A tear in the tunica intima is thought to be the primary event in the process leading to aortic dissection.  As more and more blood is forced through the tear, an intramural hematoma begins to develop between the intimal flap and medial wall, creating a false lumen.  The dissection can extend both proximally toward the heart and distally, sometimes extending all the way to the iliac and femoral arteries.  Hypertension is the single most important risk factor for the development of intimal tears leading to aortic dissection.  Cystic medial degeneration, which may be seen in connective tissue diseases such as Marfan syndrome, also predisposes patients (especially younger ones) to aortic dissection.

(Choice A)  Diabetes mellitus is an important risk factor for the development of atherosclerosis and hypertension.  It can increase the relative risk of aortic dissection by promoting hypertension, which is the single most important risk factor for aortic dissection.

(Choice B)  High cholesterol is a risk factor for atherosclerosis.  Atherosclerosis predisposes more to aortic aneurysm formation (especially abdominal aortic aneurysms) than to aortic dissection.

(Choice D)  A sedentary lifestyle is a risk factor for cardiovascular disease (eg, stroke, myocardial infarction) but does not appear to be directly linked to aortic dissection.

(Choice E)  Smoking contributes significantly to the risk of atherosclerosis and thromboangiitis obliterans (Buerger disease).  Thromboangiitis obliterans is a vasculitis of medium and small arteries, principally the tibial and radial arteries.

(Choice F)  Tertiary syphilis can result in spirochetal endarteritis that affects the vasa vasorum within the adventitia of the ascending thoracic aorta.  This weakens the aortic wall and predisposes infected patients to ascending aortic aneurysms.  Aortic dissection is only rarely caused by syphilitic aneurysms.

Educational objective:
Hypertension is the single most important risk factor for the development of intimal tears leading to aortic dissection.  Hypertension, smoking, diabetes mellitus, and hypercholesterolemia are all major risk factors for atherosclerosis, which predisposes more to aortic aneurysm formation than aortic dissection.