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Findings: An oblong region of contrast is seen to extend posteriorly from the contrast-filled lumen of the aorta somewhat cephalad to the graft anastomosis.
Diagnosis: Aortic dissection.
Discussion: Aortic dissection represents the entrance of blood into the vascular layer of the aortic wall causing a tear along the length of the vessel. It is the most common catastrophe involving the aorta. There are varied causes of aortic dissection of which the common underlying mechanism is weakening of the aortic media. The most common cause is hypertension. Other causes include: collagen diseases such as Marfan's and Ehlers-Danlos syndrome, congenital cardiovascular abnormality such as bicuspid aortic valve and aortic coarctation and pregnancy. Iatrogenic causes include: angiographic catheterization, angioplasty, and any surgical procedure where incision or a clamping of a friable aorta is required.
There are two classification schemes based on the location of the dissection. In the DeBakey classification, a Type I dissection involves the ascending and descending aorta, Type II involves only the ascending aorta, and Type III is limited to the descending aorta. In the more commonly used Stanford classification, Type A dissection involves at least the ascending aorta and Type B involves only the descending aorta.
The most common clinical presentation is sudden chest or backpain. Other signs and symptoms include: aortic insufficiency, neurologic deficits, and pulse deficits. Dissections are more common in men in a ratio of three to one. They are also most common in the 50 to 70 year age group.
Various imaging modalities are used to investigate patients suspected of having an aortic dissection. Initial chest radiographs are often obtained which although may be useful in supporting a clinical diagnosis of dissection, cannot be used to exclude the diagnosis. Findings suggestive of a dissection on chest radiography includes widening of the superior mediastinum and displacement of calcified plaques. Aortography has traditionally been considered the procedure of choice for evaluating aortic dissections. Direct signs which are pathognomonic for dissection include visualization of a double lumen or an intimal flap. Indirect signs include compression of the true lumen, thickening of the aortic wall, aortic insufficiency, or extravasation of contrast from the true lumen into the false lumen. CT and MR are being used more frequently to evaluate dissection both as a screening tool and to make a definitive diagnosis. In many institutions CT has replaced angiography for both screening and to make a definitive diagnosis. An unenhanced scan is of value to look for displaced calcifications, and recently thrombosed blood both in the false lumen or in the retroperitoneum if a leak has occured. An enhanced dynamic scan can readily identify the true and false lumen or define the intimal flap. It is felt that in experienced hands, CT accuracy exceed aortography and approach 100%. In any case, if one modality is negative and clinical suspicion remains high, multiple modalities including CT, MRA and angiography can be used to exclude the diagnosis. CT is the primary screening modality in our institution. Judicious use of contrast will not preclude angiography as a follow-up examination. A specific diagnosis of aortic dissection requires the visualization of a double lumen and an intimal flap. Transthoracic and transesophageal echocardiography are also used to evaluate dissections. Transthoracic echocardiography is most successful in evaluating the ascending aorta and transesophageal echo is used to evaluate the aortic arch and descending aorta.
The treatment of dissection depends on the location of the dissection. Surgical therapy is indicated in any dissection involving the ascending aorta or in complicated Type B dissection, i.e., rupture, ischemia to the gut or kidneys, or enlargement of a false lumen. Noncomplicated Type B dissections are treated medically which includes control of hypertension.
References:
Petasnick JP. Radiologic Evaluation of Aortic Dissection. Radiology.
1991;180:297-305.
Ponraj, Pepper J. Aortic Dissection. BJCP. 1992; 46(2):127-131.
Haaga JR, Alfidi RJ. Computed Tomography of the Whole Body.
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