Acute aortic dissection is defined as the rapid development of a false, blood-filled channel within the tunica media of the aorta.¹ It has an estimated incidence of 3 per 100,000 persons per year.

Three AADs are ultimately diagnosed out of every 1,000 emergency department patients presenting with acute back, chest, or abdominal pain.^2,3 Mortality in untreated AAD is estimated at more than 1% per hour after onset of symptoms, whereas 30-day survival for appropriately treated patients is greater than 80%. Therefore, timely diagnosis and rapid management of AAD are of paramount importance for the emergency physician.^3,4 Diagnosis is delayed more than 24 hours after initial presentation in almost half of all cases, highlighting the need for emergency physicians to maintain appropriate clinical suspicion for AAD in patients with chest, back, or abdominal pain.⁵

Failure to diagnose AAD carries a significant risk for poor outcomes because of the consequences of progressive disease (e.g., aortic rupture) and the possibility of treating a falsely diagnosed myocardial infarction (MI) or pulmonary embolism (PE) with anticoagulation, a potentially catastrophic error.⁶

Pathophysiology and Risk Factors^2,5

The most common location of a tear is the right lateral ascending aorta.⁷ Tears of the descending aorta usually originate just distal to the left subclavian artery.⁷ Blood may enter the tunica media from the lumen of the aorta via a breech of the intima or from the vasa vasorum, the small vessels that supply nutrients to the aortic wall. Mechanical forces compromise the integrity of the intima by physiologic flexion of the descending aorta with every heartbeat and nonlaminar blood ejection from the left ventricle.⁸

Once blood dissects the tunica media, it forms a false lumen and may stagnate; extend anterograde or retrograde; or rupture into the lumen of the vessel or into the surrounding tissues. Retrograde extension can involve the aortic valve, causing aortic insufficiency and heart failure, and continue into the right coronary artery ostium, causing myocardial infarction.

The mean age of AAD is 62 years, and incidence is extremely low in patients younger than age 40 years in the absence of sympathomimetic drug use, Marfan syndrome, tertiary syphilis, or Ehlers-Danlos syndrome.^6,7,9 Intimal damage is a consequence of normal aging, hypertension, bicuspid aortic valve, cardiac surgery, stimulant use, and insertion of an intra-aortic balloon pump. Atherosclerosis is a common comorbid condition but not often found at the site of dissection.⁸

Definition, Classification

The emergency physician should not use the antiquated term “dissecting aortic aneurysm,” as more than 80% of dissections have no pre-existing aneurysm. Furthermore, the pathophysiology and resultant treatment of AAD is fundamentally different from that of aortic aneurysms.²

The Stanford system of classification for AAD is the most widely used. Stanford Type A dissections represent 62% of AADs, according to the International Registry of Acute Aortic Dissection (IRAD), and involve the ascending aorta. They are usually managed with emergent surgery. Stanford Type B dissections have lower mortality, do not involve the ascending aorta, and are often managed medically.¹⁰

Signs and Symptoms

The IRAD enrolls patients from 12 large referral centers in six countries with confirmed nontraumatic AAD and collects data regarding demographics, history, physical findings, management, imaging results, and outcomes.¹⁰ The results (see chart) are compiled from two analyses of IRAD data.^9,10

The classic history of pain in AAD is excruciating, abrupt, and most severe at onset. Physical exam should include pulses in bilateral upper and lower extremities, auscultation for diastolic murmur, and assessment for gross motor and sensory deficitis.⁷ In one prospective study with 51% prevalence of AAD, inter-arm systolic blood pressure difference greater than 20 mm Hg was an independent predictor of AAD, with a positive predictive value of 98%.³

Several studies have examined the accuracy of history and physical in detecting AAD. ^3,4,10-23 Most, however, are retrospective and lack a control group. Therefore, inclusion bias and a lack of independence between the confirmatory test and initial history overestimate sensitivity, and no study can accurately assess specificity.⁷ The emergency physician should not rely on any individual sign or symptom to rule out AAD. In four studies that included patients based on an overall clinical picture suggestive of AAD after patient history and physical exam, chest x-ray (CXR), and laboratory studies, the pooled incidence of AAD was 52%, arguing for the role of combinations of signs.^3,11,23,24 None of these studies outlined an explicit algorithm for identifying such patients. A lack of classic pain features (severe, sudden-onset, tearing), inter-arm blood pressure/pulse differential, and wide mediastinum had an LR of 0.1, but decreased incidence only to 4%.³ Therefore, available evidence suggests that no combination of patient history, physical exam, CXR, or laboratory findings can obviate the need for advanced imaging in patients suspected of having AAD.

Imaging in AAD

Sixteen percent of patients with aortic dissection have a normal CXR.10 Even in patients with an abnormal CXR, there are no validated standards for radiographic findings in AAD, leading to low intra-observer and inter-observer agreement.⁷ Every patient with suspected AAD should undergo advanced imaging to confirm the diagnosis, establish Stanford classification, and detect valvular or branch involvement.^2,5 The European Society of Cardiology guidelines recommend CT aortography or transesophageal echocardiogram (TEE) for diagnosis of AAD.⁵

CT aortography has the advantage of imaging adjacent chest structures for alternative diagnoses.⁵ It can delineate extent of dissection and branch compromise and is helpful in planning definitive surgical management.² Sensitivity and specificity are 94% and 77%, respectively.²⁶ New EKG gating can eliminate motion artifact in structures close to the heart and may allow simultaneous imaging of the coronary and pulmonary arteries. This creates potential for a novel, single “triple rule-out” test for AAD, PE, and coronary artery disease, which is currently being studied.²⁶

TEE has a sensitivity of 98% and specificity of 83%, and can image the entire descending thoracic aorta, unlike transthoracic echocardiogram (TTE).²⁶ The advantage of TTE is ease of performance from lack of sedation and airway monitoring. The major drawback of echocardiography is dependence on an experienced operator and limited availability in small institutions and at off hours.

Conventional aortography and magnetic resonance imaging (MRI) are not currently recommended as first-line diagnostic modalities. Aortography has limited availability, sensitivity less than 80% when compared to CT or TEE, and important technical limitations in diagnosing an intimal flap or thrombosis in the false lumen.²⁶ Although MRI is highly accurate (98% sensitive and specific) and lacks radiation exposure, it requires potentially unstable patients to stay in undermonitored settings for long durations. Accordingly, MRI is best used for postoperative follow-up and assessment of chronic dissection.²⁶

Management

No randomized clinical trials exist to guide initial management of AAD. The European Society of Cardiology provided a consensus statement in 2001: Once AAD is highly likely or confirmed, therapy includes analgesia, heart rate control, blood pressure control, and surgical evaluation.⁵

Morphine is the preferred analgesic, as it decreases sympathetic output as well. Reductions in heart rate and blood pressure will reduce overall aortic wall tension and limit the extent of dissection. Short-acting IV beta-blockers such as labetalol or esmolol are ideal, with esmolol preferred if the subject is potentially intolerant of beta-blockers (chronic obstructive pulmonary disease, bradycardia, congestive heart failure). The goal heart rate is 60-80 bpm. No data for calcium-channel blockers exists, but IV dihydropyridines such as nicardipine may be used to reduce blood pressure. Vasodilators such as nitroprusside may be necessary to decrease systolic blood pressure to 100-120 mm Hg once heart rate is controlled.⁵

The emergency physician should consult cardiothoracic and vascular surgery early in patients with confirmed AAD. Establishment of Stanford classification is extremely important to direct management and prognosis. Stanford Type A dissections should be treated surgically, and Stanford Type B dissections initially should be treated medically. Most patients with Stanford Type A dissections managed surgically will return to independent living. Because most patients with Stanford Type A dissections managed nonsurgically will die, medical management should be pursued only in the presence of severe comorbidities or patient refusal.²

Stanford Type B dissections are usually managed medically unless there is aortic rupture, ischemic bowel/extremities, or uncontrollable hypertension. Mortality for Stanford Type B dissections with medical management is 13%. Endovascular stenting is increasingly popular and used in patients previously not considered for open surgery. Preliminary studies with endovascular stenting of Stanford Type B dissections show 5% inhospital mortality, with 11% experiencing life-threatening complications, including aorto-esophageal fistula and transformation into Stanford Type A dissection. Ninety percent survive at 1 year, and 12% require reintervention.²

Controversies

Undifferentiated patients with chest pain presenting to the emergency department ultimately can be diagnosed with MI, PE, or AAD (among many other less serious diagnoses). Although treatment of PE and MI both involve anticoagulation, this therapy given to patients with a missed diagnosis of AAD carries the risk of potentially fatal hemorrhage. Antiplatelet therapy theoretically carries a similar risk, but no studies have demonstrated poor outcomes in AAD patients. While empiric antiplatelet therapy is still indicated in chest pain patients with suspicion for acute coronary syndrome, emergency physicians should withhold thrombolytic and fibrinolytic agents until they sufficiently evaluate the risk of AAD with clinical assessment and, if necessary, advanced imaging.²⁷

d-dimer has been evaluated in several small trials as a specific laboratory marker to rule out aortic dissection without the need for advanced imaging. Limitations of these studies include small size, variable cutoffs, and many false negatives (completely thrombosed dissection lumens, shorter dissection lengths, young subject age). Pooled sensitivity in 11 studies was 94%, with a negative likelihood ratio of 0.15.28 Until larger prospective trials with fixed threshold values have been evaluated, d-dimer cannot be recommended for use in evaluation of AAD.²⁹

Summary

Acute aortic dissection is a rapidly fatal disease, necessitating emergent evaluation and management. Patients often present with sudden onset of severe chest pain radiating to the back, but atypical symptoms are common. Clinical suspicion based on history, asymmetric pulses/blood pressure or abnormal CXR warrants advanced imaging with CT or TEE. No single laboratory test, historical feature, or physical finding can safely rule out AAD. Optimal management hinges on aggressive blood pressure and heart rate control, avoidance of anticoagulation, and timely surgical consultation.

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