Epidemiology

Aortic dissection is longitudinal cleavage of the aortic wall by blood, which creates a false lumen that may propagate. The disease was first described in detail by Morgagni in 1761. In 1955, with the advent of cardiopulmonary bypass, DeBakey successfully repaired a dissected descending aorta, thereby providing the first cure. The proximal aorta was first repaired and medical management was established in the 1960s, thus setting the stage for the modern approach to this disease.

A patient in the emergency department (ED) whose proximal aorta has dissected has about a 2% chance of dying every hour during the first 12 hours and almost a 50% chance of dying within 48 hours without surgical treatment.^1,2 Unfortunately, this disease is often difficult to diagnose. A study involving three academic EDs found that the diagnosis was suspected during the initial encounter in only 43% of cases.³

The incidence is 3 per 100,000 people per year.^4,5 A typical large urban ED sees several cases per year.³ About 1 in 350 patients evaluated in an ED for chest pain has aortic dissection.^4–6

Sixty percent of aortic dissections involve the ascending aorta, either alone or with other parts of the aorta; they are referred to as type A in the now standard Stanford classification. The 40% that do not involve the ascending aorta are Stanford type B (Fig. 65.1). This distinction has important prognostic and therapeutic implications.

Fig. 65.1 Classification of aortic dissection.

Types A and type B represent the two subtypes of dissection under the Stanford classification system. Subtypes of the older DeBakey classification system are types I, II, and III.

(From Zipes DP, Libby P, Bonow RO, et al, editors. Braunwald’s heart disease: a textbook of cardiovascular medicine. 7th ed. Philadelphia: Saunders; 2005.)

The Young Patient

About 7% of patients with aortic dissection are young (<40 years) and usually have no history of hypertension or other known medical problems.⁷ These patients nearly always have occult structural cardiovascular disease.

The most important structural vascular disease is Marfan syndrome, which has a prevalence of 1 in 10,000 and occurs in all races. Marfan syndrome accounts for half of aortic dissections in patients younger than 40 years.⁷ It is caused by an autosomal dominant mutation in the gene for a type of fibrillin, a protein that makes up part of the elastic fibers in connective tissue of the aorta, lens, and periosteum. Patients are typically tall with long digits, scoliosis, pectus excavatum, and visual problems because of lens dislocation (Fig. 65.2, A to C). Most important, however, thoracic aneurysms invariably develop early in life, usually in the ascending aorta. If their aneurysms are not repaired, most patients will die of aortic dissection or rupture.

Fig. 65.2 These patients with Marfan syndrome have long extremities and digits, tall stature, and pectus carinatum.

(Photographs courtesy National Marfan Foundation.)

Other young patients without a history of severe hypertension who are nevertheless at risk for dissection are those with bicuspid aortic valves and those experiencing acute insults such as cocaine use and trauma. A family history of aortic aneurysms and dissection, beyond syndromes usually associated with aortic pathology, is a newly recognized risk factor seen in 10% to 20% of patients with dissection.⁸

Pathophysiology

The aorta dissects by two possible mechanisms (Fig. 65.3). The classic mechanism is an intimal tear, which is generally transverse and extends through the very thin tunica intima into the tunica media. Under pulsatile force, blood enters a layer of the media and dissects longitudinally and usually in a distal direction. The other major mechanism is rupture of the vasa vasorum, usually of a penetrating branch within the tunica media, with consequent bleeding into the media. Progression then occurs in the same fashion, either with or without secondary tearing through the intima into the aortic lumen. The aorta generally tears near tethering points, where the vessel undergoes the greatest flexion stress during cardiac contractions. Thus the most common location for initiation of dissection is the first few centimeters of the ascending aorta, the next most common being the origin of the descending aorta just distal to the left subclavian artery.

Fig. 65.3 Two mechanisms of aortic dissection.

A, Intimal tear. B, Rupture of the vasa vasorum. A, Adventitia; I, intima; M, media.

(From Zipes DP, Libby P, Bonow RO, et al, editors. Braunwald’s heart disease: a textbook of cardiovascular medicine. 7th ed. Philadelphia: Saunders; 2005.)

Major complications of aortic dissection, in order of risk for mortality, are (1) rupture through the thin remaining outer wall; (2) proximal propagation, which can cause coronary occlusion, acute aortic regurgitation, or cardiac tamponade (or any combination of such complications); and (3) occlusion or dissection of branch arteries.

Those who suffer dissection are predisposed to it by a weakened aorta, specifically, degeneration of the tunica media, or suffer a hemodynamic or traumatic insult, or both (Table 65.1).^7–9 Medial degeneration can be secondary to chronic hypertension, hereditary diseases of elastin (Marfan syndrome) or collagen (Ehlers-Danlos syndrome), hereditary structural abnormalities (bicuspid aortic valve and aortic coarctation), chronic inflammation, and aneurysm of any cause, which increases wall tension according to Laplace’s law.

Table 65.1 Risk Factors for Aortic Dissection

Data from references 7–9.

Hemodynamic stress on the aorta is produced by hypertension and shear force (dP/dt)*, or the force of blood ejected from the left ventricle. A hypercontractile, tachycardic heart produces the greatest shear force. These factors may contribute to the initiation of dissection and strongly determine whether a dissection propagates. Exacerbations of hypertension, cocaine intoxication, and third-trimester pregnancy are examples of states that can cause hemodynamic stress on the aorta and result in dissection.

*

Traumatic causes include medical procedures, such as aortic catheterization and cardiac surgery, and deceleration events (e.g., falls, motor vehicle collisions), although in blunt trauma, aortic rupture is far more common than dissection.

Presenting Signs and Symptoms

Dissection of the aorta is usually extremely painful, so acute pain is a chief complaint in nearly 95% of cases (Table 65.2).^7,10–14 Aortic pain is sudden and maximal at onset, with its intensity often proportional to the length of dissection. The location of pain is midline and, classically, correlates with the location of dissection: dissection of the ascending aorta results in chest pain, dissection of the arch results in neck or jaw pain, and dissection of the descending aorta results in back and sometimes abdominal pain. Thus the pain may migrate as the dissection propagates. However, there is considerable variability in symptoms and major overlapping of symptoms in type A and type B dissections.^3,9

Table 65.2 Symptoms, Signs, and Findings in Patients with Aortic Dissection