Introduction

Essentials of Diagnosis

• A history of hypertension or Marfan’s syndrome is often present.
• Sudden severe chest pain with radiation to the back, occasionally migrating to the abdomen and hips.
• Patient appears to be in shock, but blood pressure is normal or elevated; pulse discrepancy in many patients.
• Acute aortic regurgitation may develop.

General Considerations
Aortic dissection is the most common aortic catastrophe requiring admission to a hospital. It is caused by an intimal tear, which allows creation of a false lumen between the media and adventitia. Over 95% of intimal tears occur either in the ascending aorta just distal to the aortic valve (Stanford type A) or just distal to the left subclavian artery (Stanford type B). These are points where the aorta is fixed and allow for intimal injury during shear stress.

The false lumen can rupture into the pericardial sac, left pleural space, or retroperitoneum. More commonly, the dissection propagates distally to involve aortic branch vessels, producing acute spinal cord (3%), visceral (9%), renal (12%), or lower extremity (9%) ischemia. Proximal extension of a type A dissection can stretch the aortic annulus or occlude a coronary artery orifice, producing acute aortic regurgitation, myocardial infarction, and intrapericardial rupture with tamponade. Both blood pressure and the rate of acceleration of pulsatile flow (dP/dt) are important in propagation of dissection; 80% of patients with acute dissection are hypertensive. Other risk factors for dissection are Marfan’s syndrome, pregnancy, bicuspid aortic valve, and coarctation of the aorta.

When not appropriately diagnosed and treated, type A aortic dissection is a lethal disease. Untreated type A dissections are associated with 50% mortality at 48 hours and 90% mortality at 1 month -  due to free rupture, tamponade, or acute left ventricular failure. Mortality with untreated type B dissection is 10-20%, usually secondary to free rupture into the pleural space, acute mesenteric ischemia, or renal failure.

Clinical Findings
A. Symptoms and Signs
Eighty-five percent of patients report sudden excruciating (“ripping”) pain in the chest or upper back. The pain may radiate into the abdomen, neck, or groin. Many patients are hypertensive at presentation. Some present with syncope, hemiplegia, or lower extremity paralysis. On physical examination, peripheral pulses and blood pressures may be diminished or unequal. A diastolic murmur of aortic insufficiency may be heard.

B. Laboratory Findings
The ECG may be normal but often reveals left ventricular hypertrophy from long-standing hypertension. Acute ischemic changes suggest coronary artery involvement. Because dissections preferentially extend into the right coronary ostium, inferior wall abnormalities predominate.

C. Imaging
Chest radiographs often reveal -  in comparison with previous films -  an abnormal aortic contour or a widened superior mediastinum. Pleural or pericardial effusion may be present. Dynamic CT scanning, angiography, MRI, and transesophageal echocardiography (TEE) have all been used to diagnose acute dissection. TEE is favored because of its high sensitivity (98%) and specificity (99%) and because it can be performed rapidly and at the bedside. The best initial study is the one most readily available that can be interpreted accurately in a given hospital setting. MRI has not played a major role in the initial diagnosis but is useful for serial follow-up.

Differential Diagnosis
Acute myocardial infarction, pulmonary embolus, esophageal disruption, strangulated paraesophageal hernia, mesenteric ischemia, and symptomatic aortic aneurysm may all be considered in the differential diagnosis of the patient presenting with acute aortic dissection.

Treatment

A. Medical Treatment
Aggressive blood pressure control should be initiated immediately. Treatment is targeted to reduce aortic pressure and pulsatile flow (dP/dt). This is accomplished by lowering systemic vascular resistance and cardiac output (primarily heart rate).

Systemic vascular resistance is reduced by means of a rapid-acting antihypertensive agent titrated by intravenous infusion to maintain a systolic blood pressure of 100-120 mm Hg. Possible medications include nitroprusside (0.3-10 ug/kg/min), which causes direct vasodilation by its action on smooth muscle nitric oxide receptors, or fenoldopam (0.1-1.6 ug/kg/min), which acts as an agonist of D₁-dopamine receptors. When nitroprusside is continued for 48 hours or more, thiocyanate levels should be checked and the infusion stopped if the level is over 10 mg/dL (to avoid toxicity).

The heart rate is decreased by administering the selective ß₁-adrenergic antagonist esmolol (50-300 ug/kg/min intravenously) and by intermittent administration of metoprolol (5-10 mg intravenously every 15 minutes) or the a₁- and nonselective ß-antagonist labetalol (20-80 mg intravenously every 10 minutes or 1-2 mg/min).

Chronic medical management of aortic dissections involves use of a ß-antagonist (eg, metoprolol, 25-100 mg orally twice daily; atenolol, 50-100 mg orally daily), often in combination with another antihypertensive agent such as the centrally acting a₂-agonist clonidine (0.1-0.3 mg orally twice daily or 0.1-0.3 mg transdermal patch topically every 24 hours) or the direct vasodilator hydralazine (10-50 mg orally four times daily); the calcium channel blocker amlodipine (2.5-10 mg orally daily); or the angiotensin-converting enzyme inhibitor enalapril (2.5-20 mg orally daily).

B. Surgical Treatment
All patients with type A dissection should undergo emergent surgical repair. Most patients with type B dissection can be managed initially with aggressive drug therapy. Indications for surgical treatment of type B dissections are aortic rupture; severe intractable pain; mesenteric, renal, or limb ischemia; and progression of the dissection. For type A dissection, the ascending aorta and, if necessary, the aortic valve and arch are replaced with reimplantation of the coronary and brachiocephalic vessels. The mortality rate for such operations approaches 20%. Type B dissection with ischemic complications is treated by obliteration of the false lumen and/or secondary arterial bypass if this fails to restore blood flow to the ischemic organs. Several different techniques have been described for obliteration of the false lumen: resection of the entry point of the dissection and prosthetic tube graft interposition, open or endovascular fenestration of the dissection flap, or stent graft deployment to cover the entry point. Use of felt strips or instillation of tissue glue into the false lumen during open repair can strengthen the dissected aortic wall.

Surgical indications and risks for chronic type B dissections are the same as for degenerative thoracoabdominal aneurysms. Repair is considered in symptomatic patients or patients with aneurysms larger than 5 cm. Reported surgical mortality is 5-30%. For this reason, long-term medical management may be the preferred treatment for patients with significant comorbidities.

Prognosis
Because of comorbid illnesses, operative mortality of patients with type B dissection is twice that of patients with type A dissection. After hospital discharge, 5-year survival is 70-80% for repaired type A and 50-70% for repaired type B dissections. In some medically treated type B dissections, the false lumen thromboses and eventually heals with minimal dilation. In others, a chronic dissection results in a progressively enlarging aneurysm requiring eventual repair in up to 30% of patients. For this reason, all unoperated patients should be followed with annual CT scan or MRI.

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