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.
The establishment of the International Registry of Acute Aortic Dissection in 1996, which gathers information from 24 centers in 11 countries, has helped in the development of an understanding of the complexity of 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.
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Pathophysiology & Risk Factors
Aortic dissection essentially features a tear in the intimal layer, followed by the formation and propagation of a subintimal hematoma. The dissecting hematoma commonly occupies up to 50% and, occasionally, 100% of the aortic circumference, leading to the development of a false lumen or double-barreled aorta. This can reduce the flow of blood to any of the major arteries arising from the aorta. If the dissection involves the pericardial space, cardiac tamponade can result.
The normal aorta contains collagen, elastin, and smooth muscle cells, which help contribute to the layers of the aorta, the intima, the media, and the adventitia. Degenerative changes associated with aging lead to the breakdown of the collagen, elastin, and smooth muscle, as well as to an increase in basophilic ground substance. This is termed cystic medial necrosis, the hallmark histologic change associated with dissection and with Marfan syndrome.
Any disease that weakens the strength of the aortic wall will predispose one to aortic dissection. Shearing forces give rise to the separation of the layers in the media of the aorta. Intimal rupture occurs at points of fixation along the aorta where the hydraulic stress is maximal. The most common site is the first few centimeters of the ascending aorta, with 90% found within 10 cm of the aortic valve. The second most common site is just distal to the left subclavian artery.
Risk factors for aortic dissection include the following:
- Aging: Approximately 75% of dissections occur in those aged 40-70 years, with a peak age of 50-65 years.
- Aortopathy: Aortopathy can be present in heritable diseases such as Marfan, Ehlers-Danlos, anuloaortic ectasia, familial aortic dissections, adult polycystic kidney disease, Turner syndrome, Noonan syndrome, osteogenesis imperfecta, bicuspid aortic valve, and coarctation of the aorta. It is also seen in metabolic disorders such as homocystinuria and familial hypercholesterolemia.
- Atherosclerosis: The formation of atherosclerotic lesions can weaken and cause tears within the intima layer, serving as a risk factor for aortic dissection.
- Bicuspid aortic valve: In one study, the probability of type A dissection increased gradually at a sinus diameter of 5.0 cm (from 4.1% to 13% at 7.2 cm) and then increased steeply at an ascending aortic diameter of 5.3 cm (from 3.8% to 35% at 8.4 cm).  Thus, early prophylactic ascending aortic replacement is recommended in patients with bicuspid aortic valve with aortas larger than approximately 5.0 cm.
- Blunt trauma: The proximal descending aorta is the area most commonly involved in blunt trauma, due to its relative mobility over the fixed abdominal aorta, which is held in place by the ligamentum arteriosum.  A tearing or shearing occurs in sudden deceleration leading to a traumatic aortic dissection.
- Coarctation of the aorta is an area within the aorta that is focally narrowed. The most common site of congenital coarctation of the aorta is at the site of the ductus arteriosum. Affected patients typically have a longer aortic segment or section that is underdeveloped, unusually small, and hypoplasic. This affects the layers of the aorta and thus increases risk for dissection.
- Cocaine: Cocaine affects the connective tissue and has the ability to produce abrupt and severe hypertension. The differential of “cocaine chest pain” should always include aortic dissection.
- Connective tissue disorders: As noted earlier, certain diseases, such as Marfan, and Ehlers- Danlos, affect the media of the aorta and make it prone to dissection. Pulsatile flow and high blood pressure contribute to the propagation of the dissection.
- Infectious or inflammatory conditions: Infectious conditions can lead to a vasculitis that affects the vaso vasorum or the small arteries that supply blood to the layers of the aortic wall. If these arteries become compromised, a lack of blood flow can cause ischemic injury to the aorta and predispose it to dissection. Inflammatory involvement of tertiary syphilis begins at the adventitia of the aortic arch ,which progressively causes obliterative endarteritis of the vasa vasorum. This leads to lumenal narrowing of the vasa vasorum, causing ischemic injury of the medial aortic arch and then, finally, loss of elastic support and dilation of the vessel.
- Previous heart surgery, including aortic valve replacement surgery: Prior surgical interventions on the aorta weaken the blood vessel wall in some cases, resulting in an abnormal dilatation of the aorta with the risk of dissection. These conditions are usually addressed at the dilation stage in order to prevent the development of aortic dissection. Aortic dilatation is surgically addressed through a variety of procedures that help to strengthen the blood vessel wall and to prevent progression of the dilating process. In most cases, these procedures are done without having to remove or disrupt the aortic valve.
- Pregnancy: Aortic dissection in pregnancy occurs most commonly in the third trimester due to the hyperdynamic state and hormonal effects on the vasculature.
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 helps to 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, as follows:
- 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 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.