Aortic dissection is a lethal disease that can be difficult to diagnose. The majority of patients present with abrupt, severe pain of the chest or back, but a significant minority of patients present with different symptoms that can mimic a variety of other more common conditions. The average age of presentation is about 64 years and 65% are male. Approximately 5000 to 10,000 cases of aortic dissection are diagnosed annually in the United States. Consequently, an individual emergency physician can generally expect to see fewer than 1 case per year.

The aorta is composed of three layers: the inner endothelial intima, the smooth muscle media, and the connective tissue outer adventitia. In the media, elastic fibers composed of elastin and fibrillin intertwine with collagen and smooth muscle cells, providing the viscoelastic properties that enable the aorta to distend, storing a portion of the stroke volume and elastic potential energy during ventricular systole. The aorta then recoils during diastole so that blood continues to be propelled to peripheral end organs.

The aorta emanates from the left ventricular outflow tract of the heart, makes an approximate 180-degree curve, and travels posteriorly within the mediastinum of the chest until it crosses through the diaphragm. It assumes a retroperitoneal position within the abdomen and continues inferiorly until it bisects into the common iliac arteries at approximately the level of the umbilicus. Major arterial branches include the brachiocephalic, left common carotid, and left subclavian in the chest, and the celiac, superior mesenteric, bilateral renal and gonadal, and inferior mesenteric arteries in the abdomen. It provides the major supply to the anterior spinal artery and lumbar spinal cord via the artery of Adamkiewicz.

Aortic dissection is defined by separation of the media layer of the aortic wall, generally with formation of a hematoma or false lumen. This commonly occurs in the setting of degeneration of the medial layer with associated inflammation. Aortic dissection is classified as acute if it is diagnosed within 2 weeks of symptom onset, subacute if diagnosed with 2 weeks to 3 months of onset, or chronic if greater than 3 months from onset.

Aortic dissection is commonly described with the Stanford classification. Type A describes dissection involving the ascending aorta with or without descending involvement, and Type B dissection is limited to the descending aorta commencing distal to the left subclavian artery. The less commonly used DeBakey classification separates dissections into: type I, involving the ascending and descending aorta; type II, involving only the ascending aorta; and type III, limited to the descending aorta ( Fig. 71.1 ). These classification systems were proposed in part to differentiate patients who required surgical or pharmacologic therapy, though these delineations have blurred with modern therapeutic advances including endovascular treatments.

Acute aortic dissection classifications. The Stanford classification divides dissections into those involving the ascending aorta from dissection originating there or proximal dissection of a more distal origin (Type A), or those limited to the descending aorta (Type B). The DeBakey classification divides dissections into those arising in the ascending aorta and extending distally beyond the innominate artery (Type I), arising in the ascending aorta with extension limited to the level of the innominate artery take-off (Type II), and arising at or distal to the left subclavian artery take-off with extension distally (Type III).

Aortic dissection most commonly occurs due to a tear in the intimal layer subsequent to a process that has weakened the aortic media. Blood passes through the tear separating the intima from the media or adventitia, creating a false lumen that can propagate in an anterograde or retrograde fashion. Propagation of aortic dissection can cause complications such as visceral or neurologic malperfusion syndromes due to compromise of branch vessels, pericardial tamponade, or acute aortic insufficiency.

Intramural hematoma refers to hematoma formation within the wall of the aorta without evidence of intimal aortic tear. The hematoma may be localized or dissect along the plane of the aortic media. The risk factors, presentation, and natural course of this variant generally mirror those of typical aortic dissection due to intimal tear. However, over half of intramural hematomas occur in the descending aorta as a result of atherosclerotic disease or iatrogenic intravascular catheter manipulation trauma. Intramural hematoma is most easily diagnosed with CT angiography and may be missed with conventional angiography.

Penetrating atherosclerotic ulcer results from erosion of an intimal atherosclerotic lesion. It is an alternative mechanism to intimal tear, allowing blood to dissect into the media of the aortic wall or beyond. This process develops gradually in elderly patients with extensive atherosclerosis and often is heralded by chest or back pain and hypertension. Ulceration may lead to hematoma formation in the dissected media, or it can extend into the adventitia with pseudoaneurysm formation and potential rupture. It is usually a localized process most commonly occurring in the descending aorta without retrograde aortic valve, pericardial, or branch vessel involvement and associated diffuse symptoms and signs ( Fig. 71.2 ).

Pseudoaneurysm due to penetrating ulcer. A 66-year-old female presented with sharp chest pain radiating to the interscapular area and was found to have a penetrating ulcer of her descending thoracic aorta just beyond the left subclavian artery with a contained hematoma and blood in the mediastinum and left chest. The patient was treated with emergent endovascular repair. (A) Cross-sectional computed tomography (CT) with contained hematoma (pseudoaneurysm) and left hemothorax, and (B) three-dimensional CT angiogram rendering of aorta with pseudoaneurysm.

About two-thirds of aortic dissections are classified as type A, and the remainder as type B. Hypertension is the most common risk factor, present in a majority of patients. Both type A and B patients may have a history of cardiac surgery, including aortic valve replacement. Patients may have a history of aortic aneurysm or a prior aortic dissection. Marfan syndrome is the connective tissue disease most commonly associated with acute aortic dissection. Other associated conditions include: atherosclerosis, a family history of thoracic aortic disease with or without a defined genetic syndrome, bicuspid aortic valve, coarctation of the aorta, a bovine-type aortic arch, when the brachiocephalic artery shares a common origin with the left common carotid artery, and infectious disease such as syphilis. Activities and events associated with increased aortic sheer force including crack cocaine use, weight lifting, the peripartum period, and deceleration trauma can rarely cause aortic dissection. Traumatic aortic dissection most commonly occurs at the level of the left subclavian artery in the proximal descending aorta because the aorta is tethered at this point by the ligamentum arteriosum. Other causative genetic syndromes include Turner, type 4 Ehlers-Danlos, and Loeys-Dietz. Fluoroquinolones interfere with collagen synthesis and increase the risk of aortic dissection during treatment.