Acute aortic dissection

Background

Aortic dissection follows an intimal tear occurring in the wall of the aorta leading to a dissection plane within the media that separates the intima from the overlying adventitia of the aorta. Consequently blood flow in the aorta passes through the aortic (true) lumen and the dissection plane within the media (false lumen). The site of intimal disruption is termed primary (intimal) tear.Aortic dissection is a medical and surgical emergency and is the most common life-threatening pathology of the aorta. 

Two systems are employed to classify the type of aortic dissection according to location and extent of the dissection for the De Bakey system and according to functional criteria for the Stanford system (Figure 1). Accordingly, aortic dissections De Bakey type I and Stanford type A include those that involve the proximal aorta, arch, and descending thoracic aorta, and generally necessitate urgent surgical management. On the other hand, the De Bakey type II and Stanford type B include dissections that originate beyond the left subclavain artery and can be initially treated conservatively.

Etiology

Several theories have tried to explain intimal rupture as the starting point for aortic dissection. A weakened aortic wall from connective tissue diseases or cystic medial necrosis /degeneration, the presence of an intramural hematoma and the role of penetrating atherosclerotic ulcers have all been considered predisposing factors. Aortic aneurysms carry a risk of acute aortic dissection and the guidelines for indications of replacement of the ascending aorta in patients with aneurysmal dilatation of the aorta have been discussed in this journal. In addition to aortic wall pathologies, several risk factors have been known to be strongly associated with aortic dissection. Systemic arterial hypertension is the medical risk factor that is most often associated with dissection and is found in the majority of patients. Other risk factors include smoking, hyperlipidemia, cocaine use and pregnancy. Finally, aortic dissection due to iatrogenic causes has been described following endovascular procedures, aortic root cannulation, aortic cross-clamping, and surgical procedures on the aorta.

Clinical Presentation

Aortic dissections form part of the acute aortic syndrome. A high clinical suspicion is important to diagnose aortic dissection and should be considered in patients presenting with the following symptoms :

• Sudden severe chest pain is the most frequent symptom, often described as ripping or tearing in the intrascapular area but the topography varies according to the part of the aorta affected and may change as the dissection evolves.
• Patients may also present with signs or symptoms related to complications of the dissection and malperfusion of the brain, limbs, or visceral organs (neurologic symptoms, limb paresthesia, pain, or weakness, flank pain from renal artery involvement).
• Blood pressure may be high from a catecholamine surge or underlying essential hypertension or low from cardiac tamponade, or hypovolemia from rupture of the dissection.
• A blood pressure differential of >20 mm Hg is an important sign of aortic dissection.
• Superior vena cava syndrome.
• Dyspnea from congestive heart failure or bronchial compression.
• Dysphagia from esophageal compression.

However the presentation can be limited to only mild pain, often mistaken for musculoskeletal conditions.

Investigations

Besides routine exams, the diagnosis of acute aortic dissection is confirmed by diagnostic imaging which defines the anatomical extent and type of dissection.  Currently, computed tomography with injection of contrast material and echocardiography (transesophageal-TEE) are the most appropriate tests (figures 2 and 3). They provide information regarding the site of the intimal tear,  patency of the false lumen, reentry points, status of the aortic valve, and brachiocephalic vessels involvement.

Management

Acute type A (type I) aortic dissection is an emergency and carries a 50% mortality rate within 48 hours of onset.  Thus rapid diagnosis followed by immediate referral to a cardiac surgical center are crucial for survival.  High clinical suspicion should be confirmed with either CT scanning or TEE. The latter can be undertaken in the operating room to allow for subsequent immediate surgical management.
For type B dissection, an initial conservative management is advocated followed by surgical or endovascular management if necessary. 

The goal of surgical management is to prevent rupture of the aorta and to avoid extension of the dissection to the coronary ostia or aortic valve which is obtained by resecting the ascending aorta and arch if affected by the intimal tear and replacing it with a prosthetic graft. Several techniques can be employed to replace the ascending aorta with or without reimplantation of the coronary ostia and replacement of the aortic valve or aortic arch (figure 3).

Figure 1: Classification of aortic dissection (the stars denote the false lumen)
The DeBakey classification :
Type I dissection involving the ascending aorta, arch, and descending aorta.
Type II is limited to the ascending aorta.
Type III is limited to the descending aorta distal to the left subclavian artery.
The Stanford classification:
Type A involves the ascending aorta (DeBakey types I and II);  type B does not (DeBakey type III).



Figure 2: Computed tomography with injection of contrast showing intimal flap (arrow) in the aortic arch.
 

Figure 3: TEE showing intimal flap (arrow) in the ascending aorta.