Dissection, Aortic

Introduction

Aortic dissection is defined as separation of the layers within the aortic wall. Tears in the intimal layer result in the propagation of dissection (proximally or distally) secondary to blood entering the intima-media space.

This disease was first described long ago (>200 y), but new challenges have arisen since the advent of advanced diagnostic and therapeutic modalities. The clinical manifestations are diverse, making the diagnosis difficult and requiring a high clinical index of suspicion.1,2,3

Aortic dissection can be diagnosed premortem or postmortem because many patients die before presentation to the emergency department (ED) or before diagnosis is made in the ED.

Aortic dissection is more common in males than in females, with a male-to-female ratio of 2:1. The condition commonly occurs in persons in the sixth and seventh decades of life.3 Patients with Marfan syndrome present earlier, usually in the third and fourth decades of life.


Aortic dissection. CT scan showing a flap (right side of image).



Aortic dissection. CT scan showing a flap (center of image).


Aortic dissection. CT scan showing a flap (center of image).

For more examples of aortic dissection visible on CT scans, see the Multimedia section.


History of the Procedure

Morgagni first described aortic dissection more than 200 years ago. The condition was associated with a high mortality rate before the introduction of the cardiopulmonary bypass in the 1950s, which led to aortic arch repair and construction.

Recent advancements in the field of stent placements and percutaneous aortic fenestrations have further reduced mortality rates. However, despite recent advancements, the mortality rate associated with aortic dissection remains high.1,3
Problem

An aortic dissection is a split or partition in the media of the aorta; this split is frequently horizontal or diagonal. An intimal tear connects the media with the aortic lumen, and an exit tear creates a true lumen and a false lumen. The true lumen is lined by intima, and the false lumen is within the media.


Aortic dissection. True lumen and false lumen separated by an intimal flap.


Typically, flow in the false lumen is slower than in the true lumen, and the false lumen often becomes aneurysmal when subjected to systemic pressure. The dissection usually stops at an aortic branch vessel or at the level of an atherosclerotic plaque.

An acute aortic dissection (<2 wk) is associated with high morbidity and mortality rates (highest mortality in the first 7 d) compared with chronic aortic dissection (>2 wk), which has a better prognosis.

Frequency

In the United States, aortic dissection is an uncommon disease. The true prevalence of aortic dissection is difficult to estimate, and most estimates are based on autopsy studies. Evidence of aortic dissection is found in 1-3% of all autopsies (1 in 350 cadavers). The incidence of aortic dissection is estimated to be 5-30 cases per 1 million people per year. Aortic dissection occurs once per 10,000 patients admitted to the hospital; approximately 2,000 new cases are reported each year in the United States.4
Etiology
Arterial hypertension:3 Of patients with aortic dissection, 70% have elevated blood pressure.
Aortic dilatation and wall thinning: Aortic aneurysm is defined as a pathologic dilatation of a segment of a blood vessel. A true aneurysm involves all 3 layers of the aortic wall.
Iatrogenic: Aortic dissection can be caused by cardiac surgery, including aortic and mitral valve replacements, coronary artery bypass graft surgery, or percutaneous catheter placement (eg, cardiac catheterization, percutaneous transluminal coronary angioplasty). Aortic dissection occurs when the layers are split in the process of cannulation or aortotomy.
Aortic atherosclerosis: Factors include cystic medial necrosis and aortic medial disease.
Congenital aortic valve anomalies: These may include unicommissural or bicuspid aortic valves or aortic coarctation.
Marfan syndrome
Advanced age
Pregnancy
Ehlers-Danlos syndrome
Syphilitic aortitis
Deceleration injury possibly with related chest trauma
Aortic arch hypoplasia
Cocaine use
Pathophysiology

The aortic wall is continuous and is exposed to high pulsatile pressure and shear stress (the steep slope of the pressure curve, ie, the water hammer effect), making the aorta particularly prone to injury and disease from mechanical trauma. The aorta is more prone to rupture than any other vessel, especially with the development of aneurysmal dilatation, because its wall tension, as governed by the Laplace law (proportional to the product of pressure and radius), is intrinsically high.

An intimal tear connects the media with the aortic lumen, and an exit tear creates a true lumen and a false lumen. The true lumen is lined by intima, and the false lumen is lined by media. The true lumen is frequently smaller than the false lumen, but not invariably. The false lumen is indeed within the media, but suggesting that it is "lined" with it is misleading; if the aortic dissection becomes chronic, the lining becomes a serosal pseudointima. Typically, flow in the false lumen is slower than flow in the true lumen, and the false lumen often becomes aneurysmal when subjected to systemic pressure.


Aortic dissection. True lumen versus false lumen in an intimal flap.



The dissection usually stops at an aortic branch vessel or at the level of an atherosclerotic plaque. Most classic aortic dissections begin at 1 of 3 distinct anatomic locations, including (1) the aortic arch, (2) approximately 2.2 cm above the aortic root, or (3) distal to the left subclavian artery.

Ascending aortic involvement may result in death from wall rupture, hemopericardium and tamponade, occlusion of the coronary ostia with myocardial infarction, or severe aortic insufficiency. The nervi vascularis (ie, bundles of nerve fibers found in the aortic adventitia) are involved in the production of pain.

DeBakey and coworkers classify aortic dissection into 3 types, as follows:
Type I: The intimal tear occurs in the ascending aorta, but the descending aorta is also involved.
Type II: Only the ascending aorta is involved.
Type III: Only the descending aorta is involved.
Type IIIA involves the descending aorta that originates distal to the left subclavian artery and extends as far as the diaphragm.
Type IIIB involves the descending aorta below the diaphragm.

The Stanford classification has 2 types, as follows:
Type A: The ascending aorta is involved (DeBakey types I and II).
Type B: The descending aorta is involved (DeBakey type III).

This system also helps delineate treatment. Type A dissections usually require surgery, whereas type B dissections are managed medically under most conditions.5


Image A represents a Stanford A or a DeBakey type 1 dissection. Image B represents a Stanford A or DeBakey type II dissection. Image C represents a Stanford type B or a DeBakey type III dissection. Image D is classified in a manner similar to A but contains an additional entry tear in the descending thoracic aorta. Note that a primary arch dissection does not fit neatly into either classification.


Presentation

Patients with acute aortic dissection present with the sudden onset of severe and tearing chest pain, although this description is not universal. Some patients present with only mild pain, often mistaken for a symptom of musculoskeletal conditions located in the thorax, groin, or back. Some patients present with no pain.6

Consider thoracic aortic dissection in the differential diagnosis of all patients presenting with chest pain. The pain is usually localized to the front or back of the chest, often the interscapular region, and typically migrates with propagation of the dissection.

The pain of aortic dissection is typically distinguished from the pain of acute myocardial infarction by its abrupt onset, although the presentations of the two conditions overlap to some degree and are easily confused. Aortic dissection can be presumed in patients with symptoms and signs suggestive of myocardial infarction but without classic ECG findings.

Presenting signs and symptoms in acute thoracic aortic dissection include the following:

Anterior chest pain is a manifestation of ascending aortic dissection. Neck or jaw pain is a manifestation of aortic arch dissection. Interscapular tearing or ripping pain is a manifestation of descending aortic dissection.

Neurologic deficits are a presenting sign in up to 20% of cases. Syncope is part of the early course of aortic dissection in approximately 5% of patients and may be the result of increased vagal tone, hypovolemia, or dysrhythmia.6 Cerebrovascular accident (CVA) symptoms include hemianesthesia and hemiparesis or hemiplegia.6 Altered mental status is also reported. Other causes of syncope or altered mental status include (1) CVA from compromised blood flow to the brain or spinal cord or (2) ischemia from interruption of blood flow to the spinal arteries.

Patients with peripheral nerve ischemia can present with numbness and tingling in the extremities, limb paresthesias, pain, or weakness.

Horner syndrome is caused by interruption in the cervical sympathetic ganglia and manifests as ptosis, miosis, and anhidrosis.

Hoarseness from recurrent laryngeal nerve compression has also been described.

Cardiovascular manifestations involve symptoms and signs suggestive of congestive heart failure6 secondary to acute severe aortic regurgitation or dyspnea, orthopnea, bibasilar crackles, or elevated jugular venous pressure. Signs of aortic regurgitation include bounding pulses, wide pulse pressure, and diastolic murmurs. Hypertension may result from a catecholamine surge or underlying essential hypertension.7,6 Hypotension is an ominous finding and may be the result of excessive vagal tone, cardiac tamponade, or hypovolemia from rupture of the dissection.

Other cardiovascular manifestations include findings suggestive of cardiac tamponade (eg, muffled heart sounds, hypotension, pulsus paradoxus, jugular venous distension); these may be present and must be recognized quickly. Superior vena cava syndrome can result from compression of the superior vena cava from a large, distorted aorta. Wide pulse pressure and pulse deficit or asymmetry of peripheral pulses is reported. Patients with right coronary artery ostial dissection may present with acute myocardial infarction, commonly inferior myocardial infarction. Pericardial friction rub may occur secondary to pericarditis.

Respiratory symptoms can include dyspnea and hemoptysis if dissection ruptures into the pleura or if tracheal or bronchial obstruction has occurred. Physical findings of a hemothorax may be found if the dissection ruptures into the pleura.

GI symptoms include dysphagia, flank pain, and/or abdominal pain. Dysphagia may occur from compression of the esophagus. Flank pain may be present if the renal artery is involved. Abdominal pain may be present if the dissection involves the abdominal aorta.

Other nonspecific clinical presentations include fever or anxiety and premonitions of death.8

Indications

Emergent surgical correction is the preferred treatment for the following classifications of aortic dissection:

Stanford type A (DeBakey type I and II) ascending aortic dissection
Complicated Stanford type B (DeBakey type III) aortic dissections with clinical or radiological evidence of the following conditions:
Propagation (increasing aortic diameter)
Increasing size of hematoma
Compromise of major branches of the aorta
Impending rupture
Persistent pain despite adequate pain management
Bleeding into the pleural cavity
Development of saccular aneurysm
Relevant Anatomy

From outside to inside, the aorta is composed of the intima, media, and adventitia. The intima, the innermost layer, is thin, delicate, lined by endothelium, and easily traumatized.

The media is responsible for imparting strength to the aorta and is composed of laminated but intertwining sheets of elastic tissue. The arrangement of these sheets in a spiral provides the aorta with its maximum allowable tensile strength. The aortic media contains very little smooth muscle and collagen between the elastic layers and thus has increased distensibility, elasticity, and tensile strength. This contrasts with peripheral arteries, which, in comparison, have more smooth muscle and collagen between the elastic layers.

The outermost layer of the aorta is adventitia. This largely consists of collagen. The vasa vasorum, which supplies blood to the outer half of the aortic wall, lies within the adventitia. The aorta does not have a serosal layer.

The aorta plays an integral role in the forward circulation of the blood in diastole. During left ventricular contraction, the aorta is distended by blood flowing from the left ventricle, and kinetic energy from the ventricle is transformed into potential energy stored in the aortic wall. During recoil of the aortic wall, this potential energy is converted to kinetic energy, propelling aortic lumen blood into the periphery.

The volume of blood ejected into the aorta, the compliance of the aorta, and resistance to blood flow are responsible for the systolic pressures within the aorta. Resistance is mainly due to the tone of the peripheral vessels, although the inertia exerted by the column of blood during ventricular systole also plays a small part.

The aorta has thoracic and abdominal regions. The thoracic aorta is divided into the ascending, arch, and descending segments; the abdominal aorta is divided into suprarenal and infrarenal segments.

The ascending aorta is the anterior tubular portion of the thoracic aorta from the aortic root proximally to the innominate artery distally. The ascending aorta is 5 cm long and is made up of the aortic root and an upper tubular segment. The aortic root consists of the aortic valve, sinuses of Valsalva, and left and right coronary arteries. The aortic root extends from the aortic valve to the sinotubular junction. The aortic root supports the base of the aortic leaflets and allows the 3 sinuses of Valsalva to bulge outward, facilitating the full excursion of the leaflets in systole. The left and right coronary arteries arise from these sinuses.

The upper tubular segment of the ascending aorta starts at the sinotubular junction and ends at the beginning of the aortic arch. The ascending aorta lies slightly to the right of the midline, with its proximal portion in the pericardial cavity. Structures around the aorta include the pulmonary artery anteriorly; the left atrium, right pulmonary artery, and right mainstem bronchus posteriorly; and the right atrium and superior vena cava to the right.

The arch of the aorta curves upward between the ascending and descending aorta. The brachiocephalic arteries originate from the aortic arch. Arteries that arise from the aortic arch carry blood to the brain via the left common carotid, innominate, and left subclavian arteries. Initially, the aortic arch lies slightly left and in front of the trachea and ends posteriorly to the left of the trachea and esophagus. Inferior to the arch is the pulmonary artery bifurcation, the right pulmonary artery, and the left lung. The recurrent laryngeal nerve passes beneath the distal arch, and the phrenic and vagus nerves lie to the left. The junction between the aortic arch and the descending aorta is called the aortic isthmus. The isthmus is a common site for coarctations and trauma.

The descending aorta extends from distal to the left subclavian artery to the 12th intercostal space. Initially, the descending aorta lies in the posterior mediastinum to the left of the course of the vertebral column. It passes in front of the vertebral column in its descent and ends behind the esophagus before passing through the diaphragm at the level of the 12th thoracic vertebra. The abdominal aorta extends from the descending aorta at the level of the 12th thoracic vertebra to the level of bifurcation at the fourth lumbar vertebra. The splanchnic arteries branch from the abdominal aorta. The thoracoabdominal aorta is the combination of the descending thoracic and abdominal aorta.

With increasing age, the elasticity and distensibility of the aorta decline, thus inducing the increase in pulse pressure observed in elderly individuals. The progression of this process is exacerbated in patients with hypertension, coronary artery disease, or hypercholesterolemia. The loss of physiologic distensibility is observed anatomically by fragmentation of elastin and the resultant increase in collagen. This results in an increased collagen-to-elastin ratio. This, along with impairment in flow in the vasa vasorum, may be responsible for the age-related changes. These factors cumulatively lead to increased left ventricular systolic pressure and wall tension with associated increases in end-diastolic pressure and volume.

Contraindications

Cautions and relative contraindications to surgery include the following:

Cerebrovascular accident
Severe left ventricular dysfunction
Coagulopathy
Pregnancy
Postmyocardial infarction ( <6 mo)
Significant arrhythmias
Advanced age
Severe valvular disease

http://emedicine.medscape.com/article/425118-overview