Aortic dissection

Degeneration of the aortic media, cystic medial necrosis, is the pathological substrate for aortic dissection. This may be induced by chronic stress against the wall such as occurs with systemic hypertension, aortic coarctation, aortic stenosis and bicuspid aortic valve. Cystic medial necrosis is a feature of hereditary defects of connective tissue, especially Marfans syndrome and Ehlers Danlos syndrome. The risk of dissection is increased during pregnancy.

There are two classifications of aortic dissection (Fig.1). The Stanford classification recognizes Type A (involvement of ascending aorta alone or involvement of ascending and descending aorta) and Type B (involvement of descending aorta alone). The De Bakey classification describes Type I (ascending and descending aorta), Type II (ascending aorta alone) and Type III (descending aorta only). Stanford A and De Bakey I and II are treated by emergency surgery.

Imaging

The chest X-ray is neither sensitive nor specific for establishing the diagnosis of aortic dissection. The chest radiograph frequently demonstrates an enlarged thoracic aorta as a consequence of underlying predisposing diseases such as hypertension or aortic valvular disease. The chest X-ray may display features indicative of Marfans syndrome such as sternal deformity, scoliosis and elongated thorax. Infrequently, the chest X-ray may demonstrate inward displacement of intimal calcification in the aortic arch on the frontal view or an apical pleural cap.

The definitive diagnosis of aortic dissection can be established by thoracic aortography, computed tomography, magnetic resonance imaging and echocardiography. The goals of imaging studies in the typical dissection are to identify: the intimal flap; extent of the dissection; involvement of aortic branches; patency of the false channel; periaortic haematoma or haemorrhagic pericardial effusion; and aortic regurgitation.

Aortography demonstrates an intimal flap separating the true and false channel of the aortic lumen. Aortography is the most accurate method for depicting extension of the flap into the coronary arteries, arch arterial branches, intercostal, visceral and renal arteries. The most frequently involved aortic branches are the innominate artery and left renal artery. The aortogram usually displays delayed opacification and/or clearance of contrast media from the false channel. In the presence of intramural haematoma (dissection without intimal rupture) or thrombosis of the false channel, there is no opacification of the false channel and no visualization of an intimal flap. However, the presence of dissection may be inferred from compression and distorsion of the contour of the true channel.

Computed tomography (CT), MRI and magnetic resonance angiography MRA are highly sensitive and specific for the diagnosis of aortic dissection. They display the presence and extent of the intimal flap and the sizes and configurations of the true and false channels (Fig 2).

These techniques readily demonstrate the extension of the flap into or occlusion of aortic branches. They may also display arteries perfused from the false channel.

Intramural haematoma may be recognized on nonenhanced CT as high density asymmetric thickening of the aortic wall. It is recognized as high intensity asymmetric thickening on T1-weighted MR images. Depending on the age of the haematoma, it may have very low intensity on GRE images.

After repair of Type A dissection, the intimal flap is still present in most patients. Some years after surgery, complications may evolve in patients with dissection. These include aneurysm of the false channel; graft disruption; and ischaemia due to compromise of aortic branches. Consequently, postoperative patients require monitoring of aortic morphology in the early postoperative period (baseline status after surgery) and at intervals over their lifetime. The noninvasive nature of MRI makes it the optimal technique for monitoring of these patients. Contrast enhanced CT is also effective for this purpose. Transthoracic and transoesophageal echocardiography are also used for the early evaluation of patients with suspected aortic dissection. Because of limitations in acquiring diagnostic images in some subjects and lower diagnostic accuracy, transthoracic echocardiography has been replaced by transoesophageal echocardiography. The portability of transoesophageal echocardiography is a highly attractive attribute since it can be taken to the patient in the emergency department or intensive care unit.

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