Excerpt from Dissection, Aortic

Synonyms, Key Words, and Related Terms: aortic dissection, dissection of the thoracic aorta, aortic aneurysm, aortic tear, tear in the aortic wall, dissecting, Stanford classification, DeBakey classification, cystic medial necrosis, atherosclerosis, Marfan syndrome, Ehlers-Danlos syndrome, aortopathy, annuloaortic ectasia, adult polycystic kidney disease, Turner syndrome, Noonan syndrome, osteogenesis imperfecta, bicuspid aortic valve, coarctation of the aorta, connective-tissue disorders, homocystinuria, familial hypercholesterolemia, syphilis, crack cocaine use, cardiac catheterization, myocardial infarction, syncope, cerebrovascular accident, hemiparesis, hemiplegia, Horner syndrome, anxiety, orthopnea, dysphagia, dyspnea, hemoptysis, superior vena cava syndrome, congestive heart failure, cardiac tamponade, hemothorax, hypertension

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Background: Much has been written on the subject of aortic dissections, from the first well-documented case of aortic dissection, when King George II of England died while straining on the commode, to the first successful operative repairs by DeBakey in 1955, to modern techniques of diagnosing and repairing thoracic aortic dissections.

Aortic dissection is the most common catastrophe of the aorta, 2-3 times more common than rupture of the abdominal aorta. When left untreated, about 33% of patients die within the first 24 hours, and 50% die within 48 hours. The 2-week mortality rate approaches 75% in patients with undiagnosed ascending aortic dissection.

Dissections of the thoracic aorta have been classified anatomically by 2 different methods. The more commonly used system is the Stanford classification, which is based on involvement of the ascending aorta and simplifies the DeBakey classification.

The Stanford classification divides dissections into 2 types, type A and type B.

Type A involves the ascending aorta (DeBakey types I and II); type B does not (DeBakey type III).
This system also helps delineate treatment. Usually, type A dissections require surgery, while type B dissections may be managed medically under most conditions.

The DeBakey classification divides dissections into 3 types.

Type I involves the ascending aorta, aortic arch, and descending aorta.
Type II is confined to the ascending aorta.
Type III is confined to the descending aorta distal to the left subclavian artery.
- Type III dissections are further divided into IIIa and IIIb.
- Type IIIa refers to dissections that originate distal to the left subclavian artery but extend both proximally and distally, mostly above the diaphragm.
- Type IIIb refers to dissections that originate distal to the left subclavian artery, extend only distally and may extend below the diaphragm.

Thoracic aortic dissections should be distinguished from aneurysms (ie, localized abnormal dilation of the aorta) and transections, which are caused most commonly by high-energy trauma.

Pathophysiology: The essential feature of aortic dissection is a tear in the intimal layer, followed by formation and propagation of a subintimal hematoma. The dissecting hematoma commonly occupies about half and occasionally the entire circumference of the aorta. This produces a false lumen or double-barreled aorta, which can reduce blood flow to the major arteries arising from the aorta. If the dissection involves the pericardial space, cardiac tamponade may result.

Cystic medial necrosis

The normal aorta contains collagen, elastin, and smooth muscle cells that contribute the intima, media, and adventitia, which are the layers of the aorta. With aging, degenerative changes lead to breakdown of the collagen, elastin, and smooth muscle and an increase in basophilic ground substance. This condition is termed cystic medial necrosis. Atherosclerosis that causes occlusion of the vasa vasorum also produces this disorder. Cystic medial necrosis is the hallmark histologic change associated with dissection in those with Marfan syndrome.

Cystic medial necrosis was first described by Erdheim in 1929. Sources disagree over the accuracy of this term in elderly patients because the true histopathologic changes are neither cystic nor necrotic. Researchers have used the term cystic medial degeneration.

Early on, cystic medial necrosis described an accumulation of basophilic ground substance in the media with the formation of cystlike pools. The media in these focal areas may show loss of cells (ie, necrosis). This term still is used commonly to describe the histopathologic changes that occur.

Dissection sites

The most common site of dissection is the first few centimeters of the ascending aorta, with 90% o .....