Classification

Anatomic Classification

Two distinct classification systems are currently used to classify aortic dissection based on the location of the intimal tear and the degree of distal extension (Figure 38-1). The DeBakey classification, first developed in 1965, classifies aortic dissection into four types:

• Type I: The entry tear originates in the ascending aorta and extends from the aortic arch into the descending or abdominal aorta for varying distances.

• Type II: The dissection originates and is confined to the ascending aorta.

• Type IIIa: The entry tear is just distal to the left subclavian artery, and the extent is limited to the descending thoracic aorta.

• Type IIIb: The dissection originates just distal to the left subclavian artery and extends for a variable distance of the abdominal aorta.

FIGURE 38-1 The DeBakey and Stanford classification systems for aortic dissection.

(With permission from Erbel R, Alfonso F, Boileau C, et al: Diagnosis and management of aortic dissection. Eur Heart J 22:1642–1681, 2001.)

The Stanford classification categorizes the dissection primarily based on the location of the intimal tear. It is the more clinically relevant and referenced system for classification of aortic dissection.

• A Stanford type A dissection originates in the ascending aorta and encompasses DeBakey types I and II.

• A Stanford type B dissection originates in the descending aorta and includes DeBakey types IIIa and IIIb.

The majority of patients with Stanford type A dissections require urgent surgical repair because this disease entity is associated with high rates of mortality if left untreated. Common complications associated with untreated type A dissections include: intrapericardial rupture with cardiac tamponade, extrapericardial rupture, and occlusion of the arch branches resulting in end-organ ischemia. Conversely, patients with Stanford type B dissections do not usually require urgent surgical treatment and are treated with medical therapy. Certain indications for acute surgical treatment exist for these patients, which will be discussed later in this chapter.

Incidence and Survival Rates of Aortic Dissection

The annual age- and sex-adjusted incidence of acute aortic dissection has been estimated at 2.9 to 3.5 per 100,000 persons in modern series.^1,2 It occurs more commonly in men (4 : 1 male-to-female ratio) and type A dissections are more common than type B dissections.¹ Dissections are seen most frequently in the 40- to 70-year age range; however, patients with collagen vascular disorders (e.g., Ehlers-Danlos syndrome, Marfan syndrome) are usually seen at a much younger age.^3,4

The mortality associated with acute aortic dissection is high and was estimated at 30% in the first 24 hours, 50% by 48 hours, and 90% at 1 year in a classic study by Hirst and colleagues.⁴ Surgical mortality for acute type A dissections has been found to be 25.1% in a recent review of a population-based International Registry of Acute Aortic Dissection (IRAD) registry.⁵ Operative mortality in patients with acute, complicated type B dissection approach 25% to 50%.⁶ Despite the development of improved medical therapy regimens, early mortality for patients with acute type B aortic dissection ranges from 10% to 12%.⁷

Risk Factors

The most common risk factors for aortic dissection include advanced age, hypertension, and structural abnormalities of the aortic wall. More than 70% of patients are hypertensive. In a study by Juvonen and colleagues, older age, chronic obstructive pulmonary disease, and elevated mean blood pressures were unequivocally associated with aortic rupture following type B dissection.⁸ Associated structural abnormalities of the aortic wall include: bicuspid aortic valve, aortic coarctation, and chromosomal abnormalities including Turner syndrome and Noonan syndrome. Hereditary conditions associated with collagen vascular disease are also risk factors (e.g., Ehlers-Danlos syndrome, Marfan syndrome). Marfan syndrome is the most common associated disorder in patients younger than 40 years with acute aortic dissection. In patients with Marfan syndrome, a mutation in the fibrillin gene leads to an alteration of the extracellular matrix in connective tissue, resulting in aortic dilatation and dissection.⁹ Pregnancy, especially with accompanying preeclampsia and hypertension, has also been shown to be an independent risk factor for this condition.¹⁰ Cocaine and methamphetamine abuse are also associated with aortic dissection, especially in hypertensive male smokers.^11–13

Seasonal variation has been observed in the development of aortic dissection with more patients being seen in the winter months.¹⁴ It is also more likely to occur during the hours of 6:00 am and 12:00 pm.¹⁵ As with acute myocardial infarction or myocardial ischemia, physical and mental activities can also be triggers for acute aortic dissection.¹⁶

Pathophysiology of Aortic Dissection

Aortic dissection is believed to occur when intraluminal wall stress exceeds aortic wall strength.¹⁷ Therefore known risk factors such as hypertension and vessel diameter increase aortic wall stress, and collagen vascular disorders contribute to a reduction of wall strength, both contributing to the pathophysiology of the disease. Acute type A dissections tend to occur at the sinotubular junction, and type B dissections occur distal to the origin of the left subclavian artery. Nathan and colleagues¹⁷ recently demonstrated, in a study using electrocardiogram-gated computed tomography angiography in 47 patients, that peaks of aortic wall stress exist in the sinotubular junction and distal to the left subclavian artery ostium, likely contributing to the development of type A and type B dissections in these locations. They also found that the wall stress proximal to the sinotubular junction exceeded that of the area distal to the left subclavian artery, thus supporting the increased prevalence of type A dissections compared with type B (Figure 38-2).

FIGURE 38-2 Enlarged false lumen relative to the true lumen in a chronic type B aortic dissection.

Once an intimal tear occurs, propagation of the dissection is a dynamic process that can result in both antegrade and retrograde (less common) propagation of blood flow through the false lumen. Fenestrations between the two channels connect the true and false lumen and occur downstream, usually at the origin of branch vessels. The most common location of the false lumen in type B dissections is in the posterolateral aorta (Figure 38-3, see color plate). Although significant variation occurs, the celiac trunk, the superior mesenteric artery, and the right renal artery most commonly arise from the true lumen, and the left renal artery arises from the false lumen.

FIGURE 38-3 Three-dimensional wall stress distribution for the normal ascending aorta. The black arrows indicate maxima of stress on the convex (A) and concave (B) sides of the ascending aorta.

(From Nathan DP, Xu C, Gorman III JH, et al: Pathogenesis of acute aortic dissection: a finite element stress analysis. Ann Thorac Surg 91:458–464, 2011.)

The aortic wall surrounding the false lumen is significantly weaker than that of the true lumen. In the majority of patients, the false lumen has the larger diameter. A large false lumen diameter relative to the size of the true lumen has been shown to be a strong predictor for secondary dilatation in chronic dissections.¹⁸ Sueyoshi and colleagues¹⁹ followed 62 patients with chronic type B dissections and demonstrated that 83.9% of patients had one or more aortic segments increase in size during the follow-up period (mean, 49.1 months). Patients with blood flow in the false lumen had a significantly higher mean growth rate (3.3 mm/yr) than the group without blood flow (–1.4 mm/yr). The presence of blood flow in the false lumen was also the only significant risk factor for increase in the diameter in the univariate and multivariate analysis.

In a model of chronic type B dissection, Tsai and colleagues²⁰ demonstrated that diastolic false lumen pressures were highest in the setting of smaller proximal tear size and the lack of a distal tear. Therefore, the lack of an outflow tear may lead to false lumen expansion and dissection-related morbidity and mortality. Significant atherosclerosis is relatively infrequent in patients with acute aortic dissection, and it has been hypothesized that the presence of atheromatous plaque may serve to terminate or limit the extent of the dissection. One potential theory is enhanced fusion of the aortic wall layers secondary to inflammatory aortic wall plaques.

Weakening of medial collagen and elastin fibers within the aortic wall is a pathologic process referred to as cystic medial necrosis and has been implicated in the development of aortic dissection.²¹ Patients with hereditary connective tissue disorders such as Marfans or Ehlers-Danlos syndromes universally demonstrate cystic medial necrosis in the aortic wall. Patients without these disorders but with aortic dissection also tend to have a higher degree of cystic medial necrosis than the general population.

Clinical Presentation

Pain in the chest, back, and abdomen is the most common symptom of acute aortic dissection. It is usually described as abrupt in onset and typically associated with a “ripping and tearing” substernal chest pain when type A dissection is present. Cardiac tamponade and aortic valve dysfunction can result in syncope, especially in the presence of a type A dissection. Horner syndrome from compression of the adjacent sympathetic ganglia is less common but has been reported. Type B dissection is associated more with back and flank pain that may be migratory in nature. Paresthesias to the lower extremity can occur from lumbar plexopathy, and severe limb pain can occur in the setting of acute arterial ischemia from malperfusion. Pulse deficit with or without extremity ischemia in a patient with severe chest or back pain should raise the suggestion of an acute aortic dissection. Spinal cord ischemia can occur in up to 10% of patients with dissection in the descending thoracic aorta.

Diagnostic Pitfalls

The diagnosis of aortic dissection may often be misleading because the common clinical signs and symptoms in these patients are often attributed to various other clinical syndromes. As many as 65% of aortic dissections are misdiagnosed upon initial presentation in the emergency department.²⁷ Acute chest pain is commonly associated with myocardial infarction and acute coronary syndrome, and delayed or incorrect diagnosis can occur if a high index of suspicion for dissection is not present. In addition, positive electrocardiogram and serologic markers for acute myocardial infarction do not rule out associated aortic dissection. Peripheral neurologic symptoms may also be mistakenly attributed to musculoskeletal pain, neuropathy, or radiculopathy.

The following clinical scenarios should prompt immediate imaging studies to confirm the diagnosis of aortic dissection as well as urgent surgical consultation²⁸:

With these scenarios, initiation of intravenous antihypertensive therapy is imperative to prevent propagation of the dissection.

Diagnostic Imaging

Computed Tomography

Computed tomography angiography (CTA) has become the noninvasive imaging study of choice to evaluate aortic pathology. Continued advances in CT scanning in regard to high spatial and temporal resolution have allowed comprehensive analysis of detailed elements for even the most complex dissections (Figure 38-4, see color plate).³⁰ Several recent studies have demonstrated a sensitivity and specificity for detection of aortic dissection and aortic intramural hematoma to be near 100%.^31–34 Reliable imaging of both the true and false lumens, location of the intimal flap, fenestrations and branch vessel involvement can be achieved with this diagnostic modality. Multidetector CTA with electrocardiograph-gated synchronization improves diagnosis of type A aortic dissections by decreasing motion-related artifacts in the ascending aorta and aortic valve.³⁵ Three-dimensional reconstructions can be rendered and used for endovascular or surgical planning.

FIGURE 38-4 Three-dimensional reconstruction following computed tomography for aortic dissection delineates the complex anatomy of this disease process.

Magnetic Resonance Angiography

Because of long acquisition times, magnetic resonance angiography (MRA) is not as useful for immediate diagnosis of suspected acute aortic dissection. However, contrast-enhanced three-dimensional MRA has an excellent reported sensitivity and specificity (95% to 100%) for the diagnosis of this disorder.³⁸ Similar to CTA, MRA can accurately determine the presence or absence of intimal flaps and branch vessel involvement. Information regarding false lumen perfusion can also be readily obtained and may be helpful in the evaluation of visceral ischemia and impaired branch vessel perfusion. Recently newer techniques, such as three-dimensional velocity-encoded cine magnetic resonance imaging, are being used increasingly for superior visualization of pathophysiologic hemodynamics and geometrically triggered changes in blood-flow characteristics within the dissected aortic lumens (true and false; Figure 38-5, see color plate).³⁹ Limitations of MRA include inability to perform the study in patients with pacemakers or other metallic implants, long examination times, poor tolerance in claustrophobic patients, and the association with nephrogenic systemic fibrosis in patients with advanced chronic kidney disease.

FIGURE 38-5 Streamline visualization shows (A) parasagittal view of blood flow in the true and false lumen and (B) a double-oblique view of blood flow in the false lumen distal to the primary entry.

(From Müller-Eschner M, Rengier F, Partovi S, et al: Tridirectional phase-contrast magnetic resonance velocity mapping depicts severe hemodynamic alterations in a patient with aortic dissection type Stanford B. J Vasc Surg 54:559–562, 2011.)