condition resulting when blood passes from the true lumen of an artery into a false lumen within the arterial wall; layers of the wall are effectively split; most often due to necrosis of the medial layer, as in Marfan syndrome and with the tear or tears originating in the ascending (type A) or descending (type B) thoracic aorta or occasionally in smaller arteries such as the carotids. The false lumen may thrombose, rupture, reenter the true lumen downstream, and/or shear off vital arterial branches; more properly termed aortic dissection rather than aneurysm because the process is not transmural.aortic dissection.