Intramural hematoma (IMH) of the aorta is an acute clinical entity that represents a hemorrhage confined to the aortic media in which there is no intimal tear (Fig. 4D). It is thought to arise from rupture of the vasa vasorum within the aortic media and as in aortic dissection tends to affect hypertensive patients, although it can result from trauma or develop as a complication of a penetrating atherosclerotic ulcer. Since only first recognized as a distinct radiographic entity in 1985 , it is likely that a significant percentage of cases of IMH have been traditionally misclassified as ''atypical'' aortic dissections. In support of this concept is the finding that at one institution a retrospective review of 214 patients originally classified as aortic dissection showed 17 (8%) that met the imaging criteria for IMH .
Patients with IMH tend to be older at age of presentation than those with aortic dissection, particularly when compared to Type A dissection associated with Marfan's syndrome. As in classic dissection, pain radiating to the back is the most common presenting complaint. Unlike classic dissection, most patients with IMH (70%) have involvement limited to the descending aorta, and branch vessel occlusion is very uncommon. Based on a similar mode of presentation to classic dissection and severity of this disorder, most experts recommend classifying IMH in a manner analogous to that of aortic dissection (i.e., Type A with ascending aortic involvement and Type B limited to the descending aorta).
Transesophageal echocardiography can depict IMH but this diagnosis is often difficult to distinguish sonographically from mural thrombus within a dilated aorta. Computed tomography has a 96% sensitivity for the detection of intramural hematoma . Noncontrast-enhanced CT demonstrates a high-attenuation crescent-shaped thickening of the aortic wall that extends in the cephalocaudal direction without significant compromise of the vascular lumen (i.e., there is a concave interface between the hematoma and the aortic lumen) (Fig. 7A). There may be medial displacement of intimal calcifications and the aorta is often dilated or aneurysmal. Following intravenous contrast administration, there is lack of enhancement of the hematoma, no intimal flap is identified, and the hematoma appears hypodense relative to the enhancing aortic lumen (Fig. 7B). It may be difficult to distinguish IMH from atherosclerotic
Figure 7 Intramural hematoma (Type A) of the aorta. (A) Unenhanced CT just below the aortic arch shows crescentic high attenuation material within the ascending and descending aorta. (B) Following contrast administration, there is no enhancement of the crescentic material.
thickening of the aorta or thrombus within an aortic aneurysm. The presence of a high-attenuation smooth crescentic intramural density on precontrast scans that does not enhance and the absence of an intimal flap usually allows for accurate diagnosis. While the distinction from an acutely thrombosed aortic dissection is more difficult, an IMH, unlike a dissection, does not typically spiral around the circumference of the aorta, as it extends longitudinally. MRI, particularly with the use of dynamic cine-gradient-echo (GRE) sequences, is particularly accurate for assessing IMH and is used as the primary method of diagnosis in some institutions [17,18]. Aortography is insensitive to the detection of intramural hematoma unassociated with penetrating atherosclerotic ulcer as the thickened aortic wall is difficult to appreciate and distinguish from atherosclerotic disease.
The prognosis for patients with IMH depends on the type or extent of hematoma formation, the age of the patient, and the presence of comorbid conditions. Most centers with experience managing IMH recommend an operative approach to Type A IMH, since there is a high rate of rupture with resultant pericardial tamponade or mediastinal hemorrhage or the development of intimal disruption with dissection in these patients [15,17,19]. Most patients with Type B IMH have good short-term outcome with aggressive medical control of hypertension. However, close clinical and cross-sectional imaging follow-up within the first 10-14 days of presentation is recommended to detect progressive aortic involvement or dilatation (Fig. 8) or progression to typical dissection, conditions that might warrant a more aggressive approach.
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