Epidemiology

Aortic dissection occurs more often in men and increases with age.5,6 The incidence and prevalence are difficult to determine because of under-reporting of this condition. Mortality is 1 to 5 per 100,000 population per year. Hypertension is the most common risk factor associated with aortic dissection and is seen in most patients.^5–7 A history of cardiac surgery is present in 18% and a bicuspid aortic valve in 14% of all patients with aortic dissections but more often in proximal dissections.^5,7 Atherosclerosis is rarely involved at the site of dissection. Patients with aortic dissection may have a positive family history.⁸

Aortic dissection is uncommon before age 40 except in association with congenital heart disease, connective tissue disease, or inflammatory vasculitides. As many as 44% of patients with Marfan syndrome develop aortic dissection and account for about 5% of cases.5,7,9 Women with Marfan syndrome are at particular risk during pregnancy.¹⁰ In patients without connective tissue disease and with an aortic root size of less than 40 mm, pregnancy does not appear to be an independent risk factor.^11,12 Loeys-Dietz syndrome is an autosomal dominant genetic syndrome associated with aortic aneurysms and skeletal features similar to Marfan syndrome. The vascular disease in these patients is aggressive, and the mean age of death is 26 years.¹³ Inflammatory vasculitides associated with thoracic aortic disease include Takayasu’s arteritis, giant cell arteritis, and Behçet’s disease.¹ Acute aortic dissection also occurs with stimulant use, exertion, cardiac surgery, or intra-aortic balloon pump insertion.⁵

Blunt trauma from a high-speed deceleration injury usually causes traumatic aortic rupture, which is an entity distinctly different from aortic dissection (see Chapter 45).

Anatomy and Physiology

With each contraction the heart swings from side to side, resulting in flexion of both the ascending aorta and the descending aorta. The descending aorta flexes just distal to the left subclavian artery, where the mobile aorta is tethered. At an average of 70 heartbeats per minute, this sequence occurs about 37 million times a year, causing repetitive stress on the aorta.

The aortic wall has three distinct layers: the intima, the media, and the adventitia. The media is composed of elastic tissue and smooth muscle. Dissection occurs through a degeneration of the media characterized by loss of smooth muscle cells and elastic tissue, accompanied by scarring, fibrosis, and hyalin-like changes. Pathologic studies show that this process is neither cystic nor necrotic; therefore the old term cystic medial necrosis is no longer used.

Pathophysiology

Medial degeneration, previously thought to be specific for aortic dissection, is now considered to be part of normal aging, although it is augmented by hypertension and with aortic dissection. The anatomic differences between the “normal” aorta and a dissection are quantitative rather than qualitative. Although medial degeneration was initially thought to be noninflammatory, more recent evidence suggests that inflammatory cell infiltration results from medial degeneration.¹⁴

The repetitive hydrodynamic forces produced by the ejection of blood into the aorta with each cardiac cycle contribute to weakening of the aortic intima and to medial degeneration. These hydrodynamic forces primarily affect the ascending aorta. Sustained hypertension intensifies these forces and results in an increase in medial degeneration. A bicuspid valve, present in 1 to 2% of the population, is the most common congenital abnormality affecting the aortic valve and proximal aorta.15,16 A bicuspid aortic valve may disrupt laminar flow and reorient the flow of blood toward the aortic wall, producing local injury. In Marfan and Ehlers-Danlos syndromes, normal hydrodynamic forces act on an aortic media that is already weakened.

As a result of medial degeneration and repeated flexion of the aorta, hydrodynamic stress tears the aortic intima, and a column of blood gains access into the aortic media. An alternative theory suggests that these forces damage the vasa vasorum of the aorta, which rupture and hemorrhage into the aortic media, which may explain the absence of an intimal tear in some cases of dissection. Regardless of which of these theories is correct, the depth of penetration into the media and the distance and direction of dissection are at least partially determined by the degree of medial degeneration.

Once a dissecting hematoma is established in the media, migration of the hematoma occurs in an antegrade or retrograde fashion, or both, forming a “false lumen.” The false lumen forms in the outer half of the media and propagates until it ruptures back into the “true lumen” of the aorta, resulting in a rare “spontaneous cure,” or through the adventitia into the pericardial sac or pleural cavity. Because the outer wall of the aorta that contains the hematoma is thin, rupture is much more likely to occur to the outside. The most important factors favoring continued dissection of the aorta are (1) the degree of elevation of blood pressure and (2) the steepness (slope) of the pulse wave (upstroke pattern on apex cardiogram, dP/dt).

Classification

Anatomic classification is important for diagnosis and therapy. The Stanford classification is based on the involvement of the ascending aorta. Type A dissections involve the ascending aorta; type B dissections do not (Fig. 85-1). Dissections that involve the ascending aorta are more often lethal than those limited to the distal aorta and call for a different therapeutic approach. In the International Registry of Acute Aortic Dissection (IRAD), 62% of dissections are type A and 38% are type B.^3–5 Patients with distal dissections tend to be older, heavy smokers with chronic lung disease and more often have generalized atherosclerosis and hypertension compared with patients who have proximal aortic dissections.

Two other aortic conditions are closely related to aortic dissection: intramural hemorrhage (IMH) and penetrating aortic ulcer (PAU).17,18 Both groups of patients have clinical symptoms and management recommendations similar to those for patients with aortic dissection. An IMH is a contained hematoma within the aortic wall and occurs in about 10% of aortic dissections.⁵ Rupture of the vasa vasorum is believed to be the initial event. Penetrating atherosclerotic ulcers of the aorta occur in older hypertensive patients with evidence of coronary artery disease. Computed tomography (CT) shows a focal ulceration without dissection, most commonly in the distal descending aorta. The progression of penetrating ulcers results in progressive aortic enlargement with saccular and fusiform aneurysm formation. Patients can have both an intramural hematoma and a penetrating atherosclerotic ulcer.¹⁹

A dissection is acute if it is of less than 2 weeks’ duration and chronic if has been present for more than 2 weeks.

History

Pain is by far the most common presenting complaint, affecting more than 90% of patients.5,7,9 Most cases of painless aortic dissection are chronic in nature.⁷ The pain is usually excruciating, occurs abruptly, is most severe at onset, and is typically described as “sharp” more often than “tearing” or “ripping.”⁴ A family history of thoracic aortic disease may be reported.⁸

The location of the pain may help localize the dissection. Anterior chest pain is associated with the ascending aorta, neck and jaw pain with the aortic arch, pain in the interscapular area with the descending thoracic aorta, and pain in the lumbar area or abdomen with involvement below the diaphragm. Migration of the pain consistent with propagation of the dissection suggests aortic dissection but occurs in only 17% of cases.⁵ The onset of aortic dissection is often accompanied by visceral pain symptoms, such as diaphoresis, nausea, vomiting, lightheadedness, and severe apprehension.

Syncope occurs early in aortic dissection in approximately 9% of cases and may be the sole presentation in some patients.5,9 It most often heralds dissection into the pericardium, causing pericardial tamponade, but may occur from transient interruption of blood flow to the cerebral vasculature. Other causes of syncope from aortic dissection are hypovolemia, excessive vagal tone, and cardiac conduction abnormalities. Patients with aortic dissection and syncope have a higher mortality.²⁰ Neurologic symptoms such as focal weakness or change in mental status occur in up to 17% of cases.^5,7,9

Physical Examination

The presentation varies greatly, depending on the patient and the location and extent of the dissection. In general, the patient appears apprehensive. Most of the patients have a history of chronic hypertension that may be exacerbated by a catecholamine release related to the acute event. Severe hypertension refractory to medical therapy may occur if the dissection involves the renal arteries with subsequent renin release. If hypotension is present, either the dissection has progressed back into the pericardium, with resulting pericardial tamponade, or hypovolemia has occurred from rupture through the adventitia. Pseudohypotension, a condition in which the blood pressure in the arms is low or unobtainable and the central arterial pressure is normal or high, may be present. This results from the interruption of blood flow to the subclavian arteries.

Aortic regurgitation occurs in up to 32% of patients and is more common with type A dissections.5,7 The murmur of aortic insufficiency may have a musical, vibrating quality with variable intensity, and congestive heart failure may develop. The patient with presumed aortic dissection should be examined carefully for findings that suggest hemorrhage into the pericardium or tamponade, such as jugular venous distention, muffled heart sounds, tachycardia, and hypotension.

When the integrity of one of the branches of the aorta is compromised, the expected ischemic findings occur. Pulse deficits and discrepancies in blood pressure between limbs can be helpful if present but have a sensitivity of only around 30%.5,7,9,21 Usually these are present in the upper extremities and result from involvement of one or both of the subclavian arteries. Obstruction of one or both common iliac or superficial femoral arteries may produce pulse deficits in the lower extremities. Arterial obstruction may occur by either of two mechanisms. An intimal flap produced by the dissection may cover the true lumen of a branch vessel, or the dissecting hematoma may compress an adjacent true lumen. Frequent reexamination may detect transient pulse deficits.

Neurologic findings are related to the site of blood flow interruption. Proximal dissections are a more frequent cause of strokes or coma. Stroke treatment with a fibrinolytic agent in the patient with aortic dissection can be fatal. Distal dissections occluding the anterior spinal artery commonly cause ischemic paraparesis or ischemic peripheral neuropathy.⁷

In up to 3% of cases, a proximal dissection can dissect into the ostium of a coronary artery, most frequently the right coronary artery, and cause an acute myocardial infarction (MI), usually inferior to posterior.⁵ Failure to identify the inciting aortic dissection with incorrect administration of a fibrinolytic agent occurs in about 0.1 to 0.2% of MIs.²² Distal extension of aortic dissections into the abdomen can cause mesenteric ischemia, renal failure, femoral pulse deficits, and lower extremity ischemia.²³

Electrocardiography

The electrocardiogram (ECG) is often useful in excluding MI; however, 15% of patients with aortic dissection may have ECG abnormalities suggesting ischemia.⁵ Proximal dissections that involve the right coronary artery may show an inferior wall MI, and the constellation of symptoms and signs (pain, diaphoresis, hypotension) may be difficult to distinguish from those associated with primary acute MI. The ECG typically shows left ventricular hypertrophy in 26%, reflecting long-standing hypertension. Other findings include nonspecific ST-T wave changes and prior Q wave infarction. No abnormalities are noted on the ECG in 31% of cases (Table 85-1).⁵