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| Aortic dissection |
| A=Adventitia, M=Media, I=Intima, T=Intimal tear, FL=False lumen, TL=True lumen |
As with all other arteries, the aorta is made up of three layers. The layer that is in direct contect 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.
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. The initial tear is usually within 10cm of the aortic valve.
The risk in aortic dissection is that the aorta may rupture, leading to massive blood loss resulting in death. Actor John Ritter died in 2003 from this complication of aortic dissection.
Several different classification systems have been used to describe aortic dissections. The systems commonly in use are either based on the anatomy of the dissection or the duration of onset of symptoms prior to presentation.
The DeBakey system is an anatomical description of the aortic dissection. It categorizes the dissection based on where the original intimal tear is located and the extent of the dissection (localized to either the ascending aorta or descending aorta, or involves both the ascending and descending aorta.
| Aortic dissection | |
| An echocardiogram displaying the true lumen and false lumen of an aortic dissection. In the image to the left, the intimal flap can be seen separating the two lumens. In the image to the right, color flow during ventricular systole suggests that the upper lumen is the true lumen. | |
The initiating event in an aortic dissection is a tear in the intimal lining of the aorta. Due to the high pressuresBlood pressure or arterial blood pressure is the pressure (force per unit area) exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to the pressure in the large arteries, such as the brachial artery (i in the aorta, blood enters the media at the point of the tear. The force of the blood entering the media causes the tear to extend. It may extend proximally (closer to the heart) or distally (away from the heart) or both. The blood will travel through the media, creating a false lumen (the true lumen is the normal conduit of blood in the aorta). Separating the false lumen from the true lumen is a layer of intimal tissue. This tissue is known as the intimal flap.
The vast majority of aortic dissections originate with an intimal tear in either the ascending aorta (65%), the aortic arch (10%), or just distal to the ligamentum arteriosumThe ligamentum arteriosum is a small ligament between the pulmonary artery and aortic trunk. It is a nonfunctional vestige of the ductus arteriosum, and is formed within three weeks of birth. in the descending thoracic aorta (20%).
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.
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 syndromeMarfan syndrome is a connective tissue disorder, affecting many structures, including the skeleton, lungs, eyes, heart and blood vessels. It is named for the French pediatrician Antoine Marfan, who first described it in 1896. The disease is characterized and is also associated with Ehlers-Danlos syndromeEhlers-Danlos syndrome is a group of rare genetic disorders that diminish the body's ability to make connective tissues. It is caused by the inability of the body to synthesize different collagen types or a defect in synthesis. Six different variants of t.
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.
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