Aortic dissection pathophysiology: Difference between revisions
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==Overview== | ==Overview== | ||
Aortic dissection begins as a tear in the aortic wall in > 95% of patients. The tear is usually transverse, extends through the [[intima]] and halfway through the [[media]] and involves ~50% of the aortic circumference. Tow thirds of dissections originate in the ascending aorta, and 20% are in the proximal [[descending aorta]]. | |||
==Normal Anatomy of the Aorta== | ==Normal Anatomy of the Aorta== | ||
Revision as of 23:10, 28 October 2012
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]
Overview
Aortic dissection begins as a tear in the aortic wall in > 95% of patients. The tear is usually transverse, extends through the intima and halfway through the media and involves ~50% of the aortic circumference. Tow thirds of dissections originate in the ascending aorta, and 20% are in the proximal descending aorta.
Normal Anatomy of the Aorta
As with all other arteries, the aorta is made up of three layers. The layer that is in direct contact with the flow of blood is the tunica intima, commonly called the intima. This layer is made up of mainly endothelial cells. Just deep to this layer is the tunica media, known as the media. This "middle layer" is made up of smooth muscle cells and elastic tissue. The outermost layer (furthest from the flow of blood) is known as the tunica adventitia or the adventitia. This layer is composed of connective tissue.
Location of Aortic Dissections
- Aortic dissection begins as a tear in the aortic wall in > 95% of patients.
- It is usually transverse, extends through the intima and halfway through the media and involves ~50% of the aortic circumference.
- Location of dissections:
- The initial tear is usually within 100 mm of the aortic valve.
- 65% of dissections originate in the ascending aorta, distal to the aortic valve and coronary ostia
- 10% arise in the transverse aortic arch
- 20% in the proximal descending aorta
- 5% in the more distal descending aorta
Propagation of the Tear
In an aortic dissection, blood penetrates the intima and enters the media layer. The high pressure rips the tissue of the media apart, allowing more blood to enter. This can propagate along the length of the aorta for a variable distance, dissecting either towards or away from the heart or both. Once a tear develops, blood then passes into the media, and a false lumen is dissected in the outer layer of aortic media involving ~50% of the aortic circumference. This false lumen can enlarge, and compress the true lumen, as well as extend proximally or distally and occlude aortic branches. For some unknown reason, the right lateral wall of the ascending aorta is the most common site for dissection. The right coronary artery can become occluded as a result of this propagation.
Separating the false lumen from the true lumen is a layer of intimal tissue. This tissue is known as the intimal flap. As blood flows down the false lumen, it may cause secondary tears in the intima. Through these secondary tears, the blood can re-enter the true lumen.
Risk Factors for an Intimal Tear
While it is not always clear why an intimal tear may occur, quite often it involves degeneration of the collagen and elastin that make up the media. This is known as cystic medial necrosis and is most commonly associated with Marfan syndrome and is also associated with Ehlers-Danlos syndrome. Aging, atherosclerosis, diabetes, hypertension and trauma are common risk factors for aortic dissection. Uncommon risk factors include Bicuspid aortic valve, cocaine, Coarctation of the aorta, Cystic medial necrosis, Ehlers-Danlos syndrome, Giant cell arteritis, Heart surgery, Marfan’s syndrome, Pseudoxanthoma elasticum, Turner's syndrome, Tertiary syphilis and the third trimester of pregnancy.
Aortic Dissection in the Absence of an Intimal Tear
In about 13% of aortic dissections, there is no evidence of an intimal tear. It is believed that in these cases the inciting event is an intramural hematoma (caused by hemorrhage within the media). Since there is no direct connection between the true lumen and the false lumen in these cases, it is difficult to diagnose an aortic dissection by aortography if the etiology is an intramural hematoma. An aortic dissection secondary to an intramural hematoma should be treated the same as one caused by an intimal tear. An aortic intramural hematoma can form when the vaso vasorum ruptures into the aortic wall. This is distinguished from dissection by the lack of an intimal tear. This disorder parallels aortic dissection in terms of prognosis when the ascending aorta or aortic arch is involved, and rapid surgical intervention is indicated. Involvement of the descending aorta, however, carries a better prognosis than dissection, and outcome tends to be similar with medical or surgical therapy.
Pathological Findings
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Dissecting Aneurysm: Gross fixed tissue external view of heart aortic arch and descending aorta showing dilated first and second portion of arch from anterior projection.
Image courtesy of Professor Peter Anderson DVM PhD and published with permission © PEIR, University of Alabama at Birmingham, Department of Pathology
