Aortic dissection is the most common of the three acute aortic syndromes with an estimated U.S. annual incidence of 3.5/100,000 (1). The exact incidence is likely much higher because sudden death is often the only manifestation. Aortic dissection was described as early as 1761 during the autopsy of King George II of England (2), but the actual term is credited to Laennec’s description in 1826 (3). The dissection of the aortic wall is caused by a disruption or tear of the intimal layer through which the underlying weakened media is exposed to pulsatile systemic pressure and propagation of the tear. The resultant intimal flap divides the aorta into true and false lumens. The location of the tear usually occurs at the site of highest stress, the right lateral wall of the proximal aorta in type A dissection and the ligamentum arteriosum in type B dissection. Any process weakening the integrity of the medial layer will predispose to AD (Table 17-1). Hereditary connective tissue disorders such as Marfan syndrome or Ehlers-Danlos syndrome weaken the aortic media by inducing cystic medial degeneration. Chronic hypertension, smoking, dyslipidemia, and cocaine use degrade the elastic components of the media, leaving the stiffer wall vulnerable to the pulsatile forces of the cardiac cycle. Inflammatory vasculitides and iatrogenic aortic injury (4) also weaken the wall and increase the risk of AD. Aortic dissection can also complicate the course of patients with coarctation or bicuspid aortic valve disease. It has also been reported in pregnancy.

More recently, IMH and penetrating aortic ulcer (PAU) have received increasing recognition. These syndromes frequently coexist, raising the question whether IMH and PAU are indeed independent lesions. The preponderance of data supports an independent etiology for these syndromes (5). Aortic IMH was first described in 1920 (6), but its prevalence has been under-recognized due to the limitations of traditional contrast angiography. The recognition of IMH has been advanced by the newer, more precise tomographic imaging techniques. In the largest registry of acute AD, IMH was
diagnosed in about 6% of the patients (7), but other, more inclusive, series suggest that IMH may represent as many as 20% to 30% of the acute aortic syndromes (8,9,10,11). Aortic IMH is caused by rupture of the vasovasorum in the aortic wall or by extension of a PAU into the media. A hematoma within an intact aortic wall has no communication with the lumen and no outlet for decompression. Hypertension and older age are the most common risk factors for IMH (7).

Penetrating aortic ulcer was first recognized in 1934 (12). It accounts, in some series, for 8% of acute aortic syndromes (13). PAU results from erosion of an atherosclerotic lesion through the intima and beyond the elastic lamina of the arterial wall. It is a disease of older patients with established systemic arteriosclerosis and arterial calcification (14,15). It is often associated with abdominal aortic aneurysms, which are present in up to 40% of PAU patients (5).

The natural history and prognosis of AAS are directly related to the segment of aorta involved. The Stanford classification categorizes the AAS into those involving the ascending aorta and arch (type A) and those involving exclusively the descending aorta distal to the ligamentum arteriosum (type B). Aortic syndromes identified within 2 weeks of onset of symptoms are defined as acute, whereas those identified later are considered chronic.

When untreated, AD carries a high mortality rate, which increases by 1% per hour in the first 24 hours, rises to 50% at 1 week, and reaches 90% after 3 months (16). Aggressive surgical and medical treatment of AD can reduce the early mortality rates to 15% to 25% (17,18,19) and increase 5-year survival rates to between 50% to 70% (18,20,21). Type A dissection accounts for 65% of ADs. It remains a particularly lethal disorder with a 35% in-hospital mortality rate, particularly in patients presenting with shock, tamponade, renal failure, abnormal electrocardiogram (ECG), peripheral pulse deficits, or advanced age (22,23). Patients with type B AD fare better, with a 13% in-hospital mortality rate, which increases in patients presenting with the triad of hypotension, branch vessel involvement, and absence of pain (24). The risk of early rupture is relatively low, but in the chronic phase the injured aorta undergoes remodeling and aneurysmal transformation and rupture accounts for 30% of late deaths (25). Persistent patency of the false lumen and pressurization of this compartment is associated with progressive aortic dilatation and worse long-term survival (26). Patients with Marfan syndrome constitute 5% of AD patients, are more likely to suffer from type A dissection (27), and have a higher rate of subsequent complications and late death (28).

Data on the natural history of IMH and PAU come from observational studies with heterogeneous patient populations and variable stages of presentation. IMH primarily affects the descending aorta in older patients (7).The natural history of the aortic IMH is variable and characterized by dynamic changes leading to regression and healing, extension of the hematoma, progression to classic dissection, frank rupture, or aneurysmal remodeling. The site of injury in IMH is closer to the adventitial layer and may explain the observed higher risk of rupture for IMH compared with AD (29). In general, type A IMH is associated with worse outcomes compared to type B IMH (9,30). The hospital mortality associated with type A IMH exceeds that of type B, reported to be 34% versus 14%, with most deaths occurring within the first 72 hours (7,10,31). Type A IMH is associated with higher rates of rupture and progression to frank dissection than type B IMH (32) and carries a prognosis similar to type A AD (31,33,34,35). Type A IMH may progress to AD in 25% of patients and to aortic rupture in 28% (14,15). Several series suggest a more benign outcome for some type A IMH patients, with mortality rates below 10% with medical therapy alone (8,32,36,37,38). However early surgical intervention remains the recommended treatment for type A IMH. Medical therapy may be an alternative in elderly patients with multiple comorbidities and high perioperative risk.

Type B IMH follows a more benign course (14,15,39). Evangelista et al followed 68 patients with predominantly type B IMH over a mean period of 45 months (40). A third of the lesions regressed completely, 54% developed various aneurysmal changes, and only 12% progressed to classic AD. Most of the regression occurred in the first 6 months after presentation. In another cohort, 57% of patients showed regression, 40% showed progressive aortic dilatation, 13% progressed to frank dissection, and 7% developed rupture (41). Other series suggest progression to dissection in up to 33% of patients (20,11,31,33). The prognosis of medically treated type B IMH appears to be no different from that for type B dissection (7). The actuarial 10-year survival rate for type B IMH can reach 85% (42).

The morphological characteristic most predictive of IMH evolution is the initial aortic diameter. A smaller aortic caliber, specifically less than 40 to 50 mm, and aortic wall thickness below 10 mm predict regression and healing (40,41,43,44,45). Progression of IMH into aneurysmal disease may be predicted by signs of intrinsic weakness in the wall, such as larger diameter on presentation, absence of intramural echolucency, and the presence of an ulcerated atherosclerotic plaque (40). The coexistence of IMH with an aortic ulceration, a finding more common in the descending aorta, carries a worse prognosis (46).