Many patients with acute aortic dissection die before arriving at the hospital or before having their condition diagnosed. The most common predisposing factors are chronic hypertension and weakening of the aortic media. Sudden onset of severe pain is the classic presentation, but aortic dissection is often associated with a wide array of manifestations, especially in women and the elderly. Suspecting the condition is key to recognition. Imaging studies of choice include transesophageal echocardiography, computed tomography, and magnetic resonance imaging. Tight blood pressure control is the most important measure to prevent recurrence.

Degeneration of the aortic media is felt to be a prerequisite for the development of nontraumatic aortic dissection.¹ Medial degeneration tends to be more extensive in older individuals and in persons with hypertension, Marfan syndrome, and bicuspid aortic valves (Table 1).²

The primary event in aortic dissection is a tear in the aortic intima, forming the intimal flap (Figure 1) that is the most characteristic feature. Blood passes through the tear into the aortic media, separating the intima from the surrounding media and creating a false aortic lumen (Figure 2) or coming out through the aortic adventitia and causing death. Propagation of the dissection can occur distal and proximal to the initial tear (Figure 3) and can involve branch vessels and the aortic valve and enter the pericardial space. Aortic dissection can also be the result of intramural hemorrhage and hematoma formation in the media, followed by perforation of the intima. About 8% to 15% of aortic dissections are caused by rupture of the vasa vasorum in the aortic wall and the development of an intramural hematoma.³

The etiologic role of atherosclerosis is controversial. Some authors suggest that aortic dissection may develop around the crater of a penetrating atherosclerotic aortic ulcer and propagate into the media.⁴ Penetrating atherosclerotic aortic ulcers typically occur in elderly individuals with a history of hypertension, hyperlipidemia, and/or severe aortic atherosclerosis. These ulcers, which most frequently occur in the descending thoracic aorta, are characterized by a discrete ulcer crater and a thickened underlying aortic wall. Progressive penetration deep into the aortic wall may result in an intramural hematoma and weakening of the aortic wall, which, in turn, can lead to aortic enlargement and aneurysm formation.⁴ Spontaneous healing of the ulcer and resolution of the associated intramural hematoma may cause remodeling in the aortic wall, which can also result in aortic dilatation. Intramural hematoma formation is more common in the descending than in the ascending thoracic aorta. It is uncertain whether the initiating event is a primary rupture of the intima with secondary dissection of the media or hemorrhage within the media and subsequent rupture of the overlying intima.

Aortic dissection is a dynamic process that may occur anywhere within the aorta. The dissection can be classified anatomically and chronologically as outlined in Table 2. The International Registry of Acute Aortic Dissection (IRAD), a multicenter, multinational cohort study, classifies acute aortic dissections into proximal and distal types.⁵ The former type involves the aorta proximal to the origin of the left subclavian artery; in the latter, dissection is limited to the aortic segments distal to the origin of the left subclavian artery. In this article, we follow the IRAD classification.

Clinical Manifestations
Aortic dissection can present with a wide array of clinical signs and symptoms. Misdiagnosis can occur if the examining physician relies on history and physical examination findings alone (Table 3). Nonetheless, some evidence shows that improved history taking may increase the diagnostic yield, especially when risk factors for aortic dissection are identified. One clinical prediction model showed that history, physical examination, and chest x-ray findings allowed identification of 96% of cases (Table 4).⁶

The most common presentation of proximal aortic dissection is sudden-onset, severe, sharp, and frightening chest pain that is maximal at the beginning. Sudden-onset chest pain has been shown to have a sensitivity of approximately 84% for aortic dissection,⁷ but it may not be a typical presenting feature in the elderly and in women.^8,9 Associated mortality is much higher in women than in men (30% versus 21%), with delayed diagnosis being one of the most important contributors.⁹ Physical examination findings can be nonspecific, although several abnormalities suggest aortic dissection (Table 4), especially in patients with risk factors.

One of the most useful signs is the presence of pulse deficits (defined as absent or decreased carotid or peripheral pulses) in the arms or legs?the most specific physical sign of aortic dissection?which has been reported in 31% of patients.⁷

Blood pressure (BP) differences in the arms may not be helpful for diagnosis. A convenience sample of 610 emergency department patients without thoracic aortic dissection showed that 53% had interarm BP differences of more than 10 mm Hg, and 19% had differences of more than 20 mm Hg.¹⁰ Nonetheless, a good-quality, prospective, observational study did identify a BP differential of more than 20 mm Hg as an independent predictor of dissection.⁶

Retrograde extension of the tear into the aortic valve can result in aortic regurgitation. If the tear communicates with the pericardial space, patients can present with symptoms of acute pericardial tamponade. Cerebral ischemia/stroke is the most common neurologic manifestation of proximal aortic dissection. Altered cerebral perfusion may also produce symptoms of transient cerebral hypoperfusion, ranging from altered mental status to syncope; up to 12.7% of patients with proximal aortic dissection present with syncope.⁵

Back pain and abdominal pain are significantly more common with distal dissections.⁵ The initial aortic tear and subsequent extension of a false lumen along the aorta can occlude blood flow from the true lumen of the aorta into any of the arteries originating from the aorta. Depending on which arteries are occluded, patients can present with a variety of corresponding syndromes. Distal aortic dissection may present with splanchnic ischemia, renal insufficiency, leg ischemia, or focal neurologic deficits caused by spinal artery involvement and spinal cord ischemia (Table 5).

Diagnosis: Imaging Studies
The diagnosis of aortic dissection begins with clinical suspicion and continues with the use of safe and readily available confirmatory tests, which provide rapid and correct recognition of the condition and also guide therapy by identifying associated structural abnormalities. Electrocardiographic (ECG) changes are nonspecific, chiefly consisting of nonspecific ST-segment/T-wave changes. When the dissection flap extends into a coronary ostium, acute myocardial infarction (MI) with ST-segment elevation may occur. Thrombolytic therapy is absolutely contraindicated when aortic dissection is responsible for cardiac injury in ST-elevation MI. A normal ECG in a patient with severe chest pain should alert the physician to the possibility of dissection. In the IRAD study, 31% of patients had no abnormal findings on ECG.⁵

Imaging modalities important to diagnosis are transesophageal echocardiography (TEE), helical computed tomography (CT), and magnetic resonance imaging (MRI). Transthoracic echocardiogram is not useful. Aortography is an invasive technique that has been largely replaced by noninvasive methods,¹¹ although it may be used when ascending aortic dissection is strongly suspected and noninvasive modalities are unavailable or provide inconclusive results. The choice of the initial imaging study often depends on availability.

Although chest x-rays lack specificity, they are routinely obtained initially. Radiographs could be of value when combined with the history and physical examination findings. The classic radiographic signs that suggest, but are not diagnostic of, aortic dissection are listed in Table 4. Figure 4 demonstrates widening of the distal aortic knob.

The sensitivities, specificities, benefits, and limitations of CT, TEE, and MRI modalities are discussed in Table 6.

Contrast-enhanced CT
The most practical test for emergency evaluation of a suspected dissection is contrast-enhanced CT, which is usually available in most hospitals. In CT, it is important to ensure that the entire aorta is scanned when dissection is suspected, not just the site of the pain. Helical CT usually shows 2 distinct lumens with a visible intimal flap (Figures 1, 2). CT can visualize the course of the dissection in the aortic arch relative to the origin of the left subclavian artery. In distal dissections, this information is especially important for ruling out retrograde dissection into the aortic arch, which is associated with a substantially higher risk of death.¹² CT scanning is quick, easy to perform, and is probably the least operator-dependent imaging modality available for the detection of aortic dissection.¹³ Limitations include motion artifacts that may obscure the intimal flap and the inability to detect aortic regurgitation.^11,14

TEE
The greatest values of TEE may be the information it provides on the entry site of dissection; the presence of a thrombus in a false lumen; abnormal flow characteristics; involvement of coronary and arch vessels; the presence, extent, and hemodynamic significance of pericardial effusion; and the presence and severity of aortic valve regurgitation?all of which are crucial findings to the cardiothoracic surgeon.¹⁴ In addition, TEE can be performed at the bedside of a hemodynamically unstable patient. The main limitation is a strong dependence on the operator?s skills. Also, it cannot evaluate the distal ascending aorta and proximal transverse arch.¹¹ Thus, in patients with descending aortic dissection, angiography, CT, or MRI is preferable.¹¹

MRI
MRI is a very accurate, noninvasive technique for evaluating the thoracic aorta. It can delineate the extent of the dissection, demonstrate the site of the entry tear, identify arch vessels involved, and assess for renal artery involvement. MRI has the unique capabilities of diagnosing intramural hematoma and of detecting serial pathologic changes within the hematoma, which can help identify the progression or regression of a hematoma on follow-up studies. MRI is also well suited for the evaluation of preexisting aortic disease, valvular involvement, and previous surgical repair. The gadolinium-based contrast agents used in MRI have been proven safe in patients with renal failure.¹⁵ This makes gadolinium an excellent contrast agent when assessing severely ill patients, including those with contraindications to iodinated contrast media. The limitations of MRI include a lack of immediate availability, long examination time, and limited access for patient monitoring. The advent of the newer short-bore MRI units has lessened the impact of all these limitations.

Studies comparing MRI, TEE, and CT have concluded that MRI is preferred in stable patients and TEE in unstable patients suspected of having thoracic aortic dissection.^11,14 Although considered safe, TEE and CT are semi-invasive, and TEE can cause stress and BP fluctuations.¹⁴ A recent meta-analysis, however, showed that TEE, helical CT, and MRI all yield equally reliable clinical diagnostic values for confirming or ruling out thoracic aortic dissection.¹⁶ Therefore, the physician must consider the availability of each test and the levels of expertise and resources at the specific hospital.

Management
Patients with suspected aortic dissection should be admitted to an intensive care unit as quickly as possible and administered medications to control pain (eg, morphine sulfate) and reduce systolic BP to 100 to 120 mm Hg or to the lowest level tolerated (preferably, using intravenous beta-blockers)¹⁷ (Table 7). Table 8 summarizes management recommendations and the strength of each recommendation.

Patients who are hemodynamically unstable should be intubated. Nitroprusside sodium (Nitropress) can be added if systolic BP remains elevated, but it should not be administered without concomitant beta-blockade, since vasodilation induces reflex activation of the sympathetic nervous system that can lead to increased ventricular contraction and aortic shear stress. Calcium channel blockers or angiotensin-converting-enzyme inhibitors can also be used to lower BP when beta-blockers are contraindicated.

Other direct vasodilators, such as hydralazine HCl (Apresoline), should be avoided, since they increase shear stress and offer less accurate and less reversible BP control. Similarly, inotropic agents are not recommended, since they increase aortic shear stress and can worsen the dissection.

Diagnostic evaluation should be accomplished as promptly as possible. In severely hemodynamically unstable patients, bedside TEE is the procedure of choice, especially for diagnosing proximal dissection. In more stable patients, CT, MRI, TEE, or angiography may be appropriate.

Surgical intervention is indicated for all patients with proximal dissection. Despite advances in diagnostic imaging, preoperative mortality is up to 1.4% per hour within the first 48 hours.¹⁸ In the IRAD study, mortality was more than twice as high in patients with proximal dissection who did not undergo surgery compared with those treated surgically (58% versus 26%, respectively).⁵ The best surgical technique has yet to be determined, and the approach is often based on the surgeon?s experience. The goal of emergent operative intervention is to prevent catastrophic consequences (eg, rupture, cardiac tamponade), restore aortic valve competency, protect the patency of the coronary arteries, and remove or reimplant the diseased aortic segments.

In recent years, endovascular repair using stent grafts has made great advances as a minimally invasive alternative to conventional open surgery. Several studies suggest that endovascular stent graft repair of the thoracic aorta, including the aortic root, lowers morbidity and mortality compared with open surgical repair.^19,20 Aortic valve resuspension is a valve-sparing technique that does not require postoperative anticoagulation. Valve-sparing aortic root reconstruction using the reimplantation technique in patients with acute proximal dissection produces favorable results, even in patients with a pathologically dilated aortic root (eg, as in Marfan syndrome or annuloaortic ectasia).²¹

Medical treatment of acute distal aortic dissection carries a lower risk of death than immediate surgical intervention (10.7% versus 31.4%).⁵ Indications for early surgery in patients with distal dissections are a rapidly expanding aortic diameter, blood leakage, development of periaortic or mediastinal hematoma as signs of imminent aortic rupture, persistent and uncontrollable pain, and/or impairment of blood flow to an organ or limb. Endovascular stent grafting is becoming an increasingly important treatment for patients with acute complex distal dissection who have complications, such as leg or visceral ischemia or renal failure. A recent meta-analysis of 39 trials involving 609 patients concluded that endovascular stent graft placement produces success rates of at least 96%.²⁰ Although 14% to 18% of patients experienced major procedure-related complications, the incidence of paraplegia was very low (0.8%).

Conclusion
In most cases of acute aortic dissection, an underlying chronic and generalized disease of the aortic wall media is a predisposing factor. This pathology persists even after radical surgical repair. The potential for aneurysm formation, progressive dissection, and redissection of the remainder of the aorta demands careful monitoring. Patients should be advised to avoid strenuous physical activity to minimize the risk of aortic shear stress. After hospital discharge, regular visits at 1, 3, 6, and 12 months and annually thereafter are recommended. Patients should undergo serial imaging evaluations during these visits. Imaging should not be confined to the region of initial involvement, because dissection and aneurysm can occur anywhere along the entire aorta.

Self-assessment test
1. All these statements about acute aortic dissection are true, except:
A. Elderly patients may present without a history of sudden-onset chest pain
B. The most common neurologic manifestation in proximal aortic dissection is cerebral ischemia or stroke
C. Back pain is more common with distal than with proximal dissections
D. Helical CT provides good visualization of aortic regurgitation

2. Which of these is the most unlikely presentation of acute aortic dissection?
A. Sudden-onset chest pain
B. Headache
C. Pulse differentials in the arms
D. Stroke

3. What is the least specific modality for the initial evaluation of suspected aortic dissection?
A. Chest x-ray
B. CT
C. TEE
D. MRI

4. All these factors are associated with proximal aortic dissection, except:
A. Bicuspid aortic valve
B. Pregnancy
C. Male gender
D. Age >60 years

5. All these options are appropriate initial therapy for BP control, except:
A. Propranolol
B. Verapamil in a patient with asthma
C. Hydralazine
D. Nitroprusside plus labetalol

(Answers at end of reference list)

References
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Answers: 1. D; 2. B; 3. A; 4. D; 5. C.