Classic aortic dissection is a longitudinal split or partition in the media of the aorta. An intimal tear connects the media with the aortic lumen, and an exit tear creates a true and a false lumen. The smaller true lumen is lined by intima, and the false lumen is lined by media. Typically, flow in the false lumen is slower than in the true lumen, and the false lumen often becomes aneurysmal when subjected to systemic pressure. An acute aortic dissection is considered chronic at 2 weeks. The dissection usually stops at an aortic branch vessel or at the level of an atherosclerotic plaque.
See the images of aortic dissection below.
Image A represents a Stanford A or a DeBakey type 1 dissection. Image B represents a Stanford A or DeBakey type II dissection. Image C represents a Stanford type B or a DeBakey type III dissection. Image D is classified in a manner similar to A but contains an additional entry tear in the descending thoracic aorta. Note that a primary arch dissection does not fit neatly into either classification.
Contrast-enhanced axial CT image demonstrates an intimal flap that separates the 2 channels in the ascending and descending aorta diagnostic of a Stanford type A dissection. Courtesy of Joel L Fishman, MD.
Contrast-enhanced CT scan obtained at the level of the aortic arch demonstrates an aortic dissection with almost complete separation of the aortic intima. The slight prolapse may be the beginning of a configuration at risk for intimo-intimo intussusception, a potentially fatal event. Courtesy of Joel L Fishman, MD.
3-D color reconstruction of CTA in descending thoracic aortic dissection shows severance of the left renal artery due to dissection (white arrow). Note the few spidery collaterals at the point of left renal artery severance and the atheromatous calcified plaques (yellow arrow).
3-D color reconstruction of CTA in descending thoracic aortic dissection shows severance of the left renal artery due to dissection (yellow arrow).
3-D color reconstruction of CTA in descending thoracic aortic dissection shows resumption/re-entry of normal distal aortic circulation, past the point of the false lumen (yellow arrow).
3-D color reconstruction of CTA in descending thoracic aortic dissection shows the opening up of collateral circulation proximal to the point of dissection (yellow arrow). The dissection and the false lumen are demonstrated by the red arrow.
3-D color reconstruction of CTA in descending thoracic aortic dissection shows the opening up of collateral circulation proximal to the point of dissection (white arrow). The point of dissection is not shown. Note the aortic calcified plaque (black arrow).
Most classic aortic dissections begin at 3 distinct anatomic locations: the aortic root; 2 cm above the aortic root; and just distal to the left subclavian artery. Ascending aortic involvement may result in death from wall rupture, hemopericardium and tamponade, occlusion of the coronary ostia with myocardial infarction, or severe aortic insufficiency.
3-D color reconstruction of CTA in descending thoracic aortic dissection shows the opening up of collateral circulation proximal to the point of dissection (yellow arrow). The point of dissection is not shown.
Aortic intramural hematoma (AIH) is a more recently described entity in which no intimal flap is present. It results in a spontaneous medial hematoma that may be secondary to an infarction of the vasa vasorum of the adventitia. Aortic intramural hematoma accounts for approximately 25% of aortic dissections. Involvement of the ascending aorta, especially if the overall aortic diameter is greater than 5 cm, should be treated surgically to prevent rupture or progression to a classic dissection with intimal tear. Conservative management is indicated for AIH of the descending aorta.
Preferred examinations for aortic dissection include contrast-enhanced spiral CT transesophageal echocardiography (TEE) in the emergency setting and MRI for hemodynamically stable patients. TEE has an advantage over CT and MRI in its ability to evaluate the status of the aortic valve and the ostia of the coronary arteries. CT and MR angiography have largely replaced conventional diagnostic angiography in the assessment of aortic dissection.
Several factors determine the best modality for the initial evaluation and postoperative follow-up. These factors include the following: stability of the patient’s condition, the patient’s renal function, suspected postoperative complication, and the availability of each imaging modality.
Maffei et al performed a randomized, controlled trial in which 44 patients (252 evaluations) were examined with TEE and CT.
The authors concluded that both TEE and CT are atraumatic, safe, and accurate techniques for serial follow-up studies of patients treated for aortic dissection.
Three noninvasive studies are associated with high specificity and sensitivity for aortic dissection. CT and MRI are associated with a sensitivity and a specificity of 94-100% and 95-100%, respectively. TEE is less sensitive and specific than spiral CT or MR, and TEE is operator-dependent. In addition, because of tracheal interposition, there is a 2 cm “blind spot” for TEE just proximal to the innominate arteries. Also, TEE is contraindicated in approximately 1% of patients (eg, TEE is contraindicated in patients with esophageal varices).