What is the diagnostic approach for a patient suspected of having an acute aortic dissection, particularly one with a history of hypertension, atherosclerosis, or previous cardiovascular surgery?

Diagnosis of Acute Aortic Dissection

In stable patients with suspected acute aortic dissection, CT angiography (CTA) of the chest, abdomen, and pelvis is the diagnostic test of choice, while in hemodynamically unstable patients, transesophageal echocardiography (TEE) should be performed at the bedside or in the operating room as the sole diagnostic procedure before emergency surgery. 1

Clinical Presentation and Initial Assessment

Acute aortic dissection presents with sudden, severe chest or back pain in 80-90% of patients, often described as tearing or ripping in character. 1 Key clinical features to identify include:

• Pulse deficits between extremities or between upper and lower extremities
• Blood pressure differentials >20 mmHg between arms
• Neurological deficits from malperfusion
• Diastolic murmur indicating acute aortic regurgitation 2

Risk factors conferring the greatest population attributable risk include advanced age, male gender, long-term hypertension, atherosclerosis, and presence of aortic aneurysm. 1, 3 Patients with genetic connective tissue disorders (Marfan syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome) or bicuspid aortic valves develop dissection at much younger ages. 3

Diagnostic Imaging Algorithm

For Hemodynamically Stable Patients

CT angiography is the preferred first-line imaging modality with sensitivity of 93-100% and specificity of 98-99%. 1 The imaging protocol should include:

• Non-contrast CT first to detect intramural hematoma appearing as high-attenuation aortic wall thickening 1
• Contrast-enhanced CT to delineate the dissection flap, identify true and false lumens, localize intimal tears, and detect contrast extravasation 1
• Imaging from thoracic inlet to pelvis including iliac and femoral arteries to plan surgical or endovascular treatment 1

CTA provides the additional benefit of detecting alternative diagnoses in 13% of cases without aortic disorders. 4

For Hemodynamically Unstable Patients

Perform bedside transthoracic echocardiography (TTE) immediately, though image quality is rarely adequate for definitive decision-making. 1 If cardiac tamponade is identified on TTE, proceed directly to sternotomy and exploratory surgery without further imaging. 1

TEE should be performed as the sole diagnostic procedure in the intensive care unit or operating room before emergency surgery, with sensitivity of 98-100% and specificity of 95-100%. 1, 4 TEE can be completed rapidly at the bedside and provides comprehensive evaluation in unstable patients who cannot be transported to CT. 1, 5

Alternative Imaging Modalities

MRI demonstrates the highest sensitivity (100%) for aortic dissection but is rarely used acutely due to patient instability, longer acquisition times, and limited availability. 1 MRI should be reserved for stable patients when CT is contraindicated (renal insufficiency, contrast allergy) or for follow-up imaging. 2

Angiography should only be considered when noninvasive imaging cannot establish the diagnosis or when the differential diagnosis includes acute coronary syndrome requiring coronary anatomy delineation before surgery. 1

Diagnostic Goals and Key Imaging Features

Every imaging study must address these critical questions: 1

• Confirm the diagnosis of dissection versus other acute aortic syndromes (intramural hematoma, penetrating ulcer)
• Classify the dissection as Stanford Type A (involving ascending aorta) or Type B (descending aorta only)
• Differentiate true from false lumen: True lumen is typically smaller, shows systolic expansion, has antegrade systolic flow, and follows the inner contour of the aortic arch 1
• Localize intimal tears (entry and re-entry sites) for surgical planning 1
• Assess branch vessel involvement including coronary arteries, carotid arteries, and visceral vessels 1
• Detect aortic regurgitation and quantify severity 1
• Identify extravasation including periaortic hematoma, pleural effusion, or pericardial effusion 1

Common Pitfalls and Caveats

Avoid using multiple imaging techniques sequentially, as this causes unnecessary time loss and increases mortality risk (1-2% per hour untreated). 1, 3 The International Registry of Acute Aortic Dissection found an average of 1.8 imaging methods were used per patient, which is excessive. 1

TTE has significant limitations with sensitivity of only 60-80% for Type A dissection and 70% for descending aorta dissections. 1, 4 TTE is particularly inadequate in patients with obesity, pulmonary emphysema, abnormal chest wall configuration, or mechanical ventilation. 4

Beware of motion artifacts on non-ECG-gated CT that can create pseudodissection appearance in the ascending aorta and aortic root. 1 ECG-gated techniques eliminate this artifact and provide motion-free images. 1

TEE has a "blind spot" preventing visualization of the distal ascending aorta and anterior aortic arch due to interposition of the trachea and left main bronchus. 4 Reverberation artifacts within the ascending aorta can lead to false diagnoses, particularly by inexperienced observers. 4

Never perform pericardiocentesis as an initial therapeutic step before surgery in patients with cardiac tamponade from aortic dissection, as reducing intrapericardial pressure may cause recurrent bleeding. 1

Chest radiograph is inadequate to exclude dissection even though abnormal in 60-90% of cases, showing mediastinal widening. 2 Do not delay definitive imaging to obtain chest X-ray in unstable patients. 2

Institutional Protocol Development

Each institution should establish guidelines for the most rapid and accurate diagnostic pathway based on: 1

• Availability of imaging modalities in emergency situations
• Experience of emergency room and imaging staff with each technique
• Proximity to surgical and interventional capabilities

Patients presenting to regional hospitals without comprehensive imaging or surgical capabilities should be transferred immediately to a center with interventional and surgical backup once aortic dissection is suspected. 1