Residency · Residency · Diagnostic Radiology

Thoracic Aortic Emergencies

Acute Aortic Syndrome

Definition

Acute aortic syndrome is an umbrella term encompassing three related emergent conditions: classic aortic dissection, intramural hematoma (IMH), and penetrating atherosclerotic ulcer (PAU). All three present acutely with severe chest or back pain and share similar risk factors. Traumatic aortic injury is a separate entity but is also included among thoracic aortic emergencies.

Risk Factors

Hypertension is the most common predisposing factor, present in 70 to 80% of cases. Connective tissue disorders, including Marfan syndrome, Ehlers-Danlos type IV, and Loeys-Dietz syndrome, predispose to aortic disease at younger ages. Additional risk factors include bicuspid aortic valve, prior cardiac or aortic surgery, cocaine or amphetamine use, coarctation of the aorta, giant cell arteritis (inflammatory aortitis), third-trimester pregnancy, and iatrogenic injury from catheterization or cardiac surgery.

Classic Aortic Dissection

Pathophysiology

In classic aortic dissection, a tear in the intima allows blood to enter the media, creating a false lumen that propagates along the length of the aorta. The true lumen is the continuation of the native aortic lumen and is typically smaller and often compressed. The false lumen is frequently larger and may thrombose partially or completely. An intimal flap, the thin tissue layer separating the two lumens, is the hallmark finding.

Stanford Classification (Most Clinically Useful)

The Stanford classification is the most important system for guiding management. Type A dissection involves the ascending aorta regardless of where the primary tear originates and is a surgical emergency, with mortality increasing by 1 to 2% per hour without intervention. Complications of Type A dissection include aortic regurgitation, coronary malperfusion leading to myocardial infarction, pericardial tamponade, and stroke. Type B dissection involves only the descending aorta, distal to the left subclavian artery origin. Most Type B dissections are managed medically with anti-impulse therapy using IV beta-blockers to target a heart rate below 60 and systolic blood pressure below 120 mmHg. Complicated Type B dissection, defined by malperfusion, rupture, or rapid expansion, requires endovascular or surgical intervention.

DeBakey Classification

The DeBakey classification provides more anatomic detail. Type I originates in the ascending aorta and extends to at least the aortic arch and often beyond. Type II is confined to the ascending aorta. Type III originates in the descending aorta, with IIIa limited to the descending thoracic aorta and IIIb extending below the diaphragm.

CT Angiography Findings

The intimal flap appears as a thin linear low-attenuation structure dividing the aortic lumen into true and false lumens. Several features help identify the true lumen: it is usually smaller and compressed, is continuous with the undissected aorta proximally, and may show the beak sign (an acute angle of the false lumen at the margins of the dissection) and the cobweb sign (thin strands within the false lumen representing residual medial tissue). The false lumen is often larger, may enhance later on delayed images, and may be partially or completely thrombosed. The entry tear, the site of intimal disruption, is often visible as a defect in the flap. Branch vessel involvement must be carefully evaluated, including the celiac axis, SMA, renal arteries, and iliac arteries, assessing whether each is supplied by the true or false lumen and whether malperfusion is present. Complications to report include hemopericardium, hemothorax, mediastinal hemorrhage, and end-organ ischemia.

Reporting Essentials

The report should include the Stanford classification (A or B), the extent of dissection from origin to termination, the location of the entry tear if identifiable, branch vessel involvement and perfusion status, aortic diameter measurements, the presence of complications (pericardial effusion, pleural effusion, malperfusion), and comparison with prior imaging.

Intramural Hematoma (IMH)

Pathophysiology

Intramural hematoma is hemorrhage within the aortic media without an identifiable intimal tear, thought to result from rupture of the vasa vasorum (the small vessels that supply the aortic wall). IMH can progress to classic dissection, rupture, or resolve spontaneously. It is classified as Stanford A or B based on location, with similar management implications as classic dissection.

CT Findings

The key finding is crescentic or circumferential high-attenuation thickening of the aortic wall on non-contrast CT, with the wall appearing hyperdense relative to flowing blood at typically 50 to 70 HU. There is no intimal flap and no false lumen flow on CTA. The wall thickening does not enhance on post-contrast images. IMH may be subtle or invisible on CTA alone because the high-attenuation wall thickening can be obscured by intraluminal contrast. Associated findings include pleural effusion, periaortic hematoma, and penetrating atherosclerotic ulcer.

Protocol Implications

A non-contrast CT of the chest must be obtained before CTA in any aortic emergency protocol to identify the high-attenuation wall thickening of IMH. On CTA alone, a thrombosed false lumen of a classic dissection can mimic IMH, and the non-contrast phase helps differentiate the two.

Penetrating Atherosclerotic Ulcer (PAU)

Pathophysiology

A penetrating atherosclerotic ulcer forms when ulceration of an atherosclerotic plaque penetrates through the internal elastic lamina into the media. PAU is most common in elderly patients with extensive atherosclerosis and usually occurs in the descending thoracic aorta. It can lead to IMH, dissection, pseudoaneurysm, or rupture.

CT Findings

PAU appears as a focal ulcer crater extending beyond the expected aortic lumen contour into the thickened aortic wall. Surrounding IMH (wall thickening) is often present. Extensive aortic atherosclerosis and calcification are typically seen. A focal outpouching or pseudoaneurysm may develop. The key distinction from the irregular contour of an atheromatous aorta is that a PAU is deeper and more focal.

Traumatic Aortic Injury

Mechanism

Traumatic aortic injury results from rapid deceleration forces, most commonly in motor vehicle collisions and falls from height. Shear forces are greatest at points of aortic fixation. The vast majority of injuries (approximately 90%) occur at the aortic isthmus, just distal to the left subclavian artery origin at the ligamentum arteriosum. Less common locations include the ascending aorta near the aortic root and the descending aorta at the diaphragm.

Grading (Society for Vascular Surgery)

Traumatic aortic injuries are graded by severity. Grade I is an intimal tear with intimal irregularity or a small intimal flap. Grade II is an intramural hematoma with wall thickening but no extravasation. Grade III is a pseudoaneurysm, representing a contained rupture with a focal outpouching beyond the normal aortic contour. Grade IV is free rupture with contrast extravasation and mediastinal hemorrhage, which is usually fatal before the patient reaches the hospital.

CT Angiography Findings

The most common surviving injury pattern is a pseudoaneurysm, a focal outpouching of the aortic contour at the isthmus. An intimal flap may appear as a linear defect within the aortic lumen. Irregular aortic contour with abnormal caliber change or luminal irregularity at the isthmus is another finding. Periaortic hematoma, representing mediastinal hemorrhage surrounding the aorta, is an important associated finding. Pseudocoarctation, or luminal narrowing from intimal injury, may also be seen.

A critical distinction must be made between a traumatic pseudoaneurysm and the ductus diverticulum, a normal variant consisting of a smooth bulge on the anteroinferior aortic isthmus. The ductus diverticulum has smooth margins and obtuse angles with the aortic wall, while a pseudoaneurysm has irregular margins and acute angles.

Chest Radiograph Signs (Low Sensitivity, High Suspicion)

Chest radiograph findings in traumatic aortic injury include a widened mediastinum (greater than 8 cm on supine AP), abnormal aortic contour or obscured aortic knob, left apical pleural cap (from extrapleural blood), depression of the left mainstem bronchus, deviation of the trachea or esophagus to the right, and left pleural effusion (hemothorax). However, a normal chest radiograph does not exclude aortic injury, and CTA is required for definitive evaluation in all patients with an appropriate mechanism of injury.

<image>A set of three axial CTA diagrams showing the three components of acute aortic syndrome. Diagram 1 (Classic Dissection): an aortic cross-section with a clearly visible intimal flap dividing the lumen into a smaller true lumen and a larger false lumen, with arrows indicating flow in both lumens and the beak sign at the margins. Diagram 2 (Intramural Hematoma): two paired images showing the non-contrast CT with crescentic high-attenuation (bright) thickening of the aortic wall, and the corresponding CTA showing the thickened wall that does not enhance and has no intimal flap. Diagram 3 (Penetrating Atherosclerotic Ulcer): an aortic cross-section with a focal ulcer crater extending beyond the intimal calcification into the thickened aortic wall, with surrounding intramural hematoma and extensive atherosclerotic calcification.</image>

<image>A sagittal CTA diagram of the thoracic aorta showing the Stanford classification of aortic dissection. The aorta is depicted from the root through the descending thoracic aorta. Type A dissection is illustrated with the intimal flap originating in the ascending aorta and extending into the arch and descending aorta, with the ascending aorta region shaded red and labeled "Type A -- surgical emergency." Type B dissection is illustrated with the intimal flap beginning distal to the left subclavian artery, with the descending aorta region shaded blue and labeled "Type B -- usually medical management." The origins of the great vessels (brachiocephalic, left common carotid, left subclavian) are labeled. A dashed line at the left subclavian artery origin marks the division between Type A and Type B territories.</image>

<image>A sagittal CTA diagram of the aortic isthmus region comparing a normal ductus diverticulum with a traumatic aortic pseudoaneurysm. The left panel shows a smooth, gently sloping focal bulge on the anteroinferior surface of the isthmus with obtuse angles and no periaortic hematoma, labeled "Ductus diverticulum (normal variant)." The right panel shows an irregular focal outpouching with acute angles at its margins, an adjacent intimal flap, and surrounding periaortic soft tissue density representing mediastinal hematoma, labeled "Traumatic pseudoaneurysm." Key differentiating features are listed in a comparison box between the two panels.</image>

Clinical Pearls

Every aortic dissection report must include the Stanford classification (A or B), because Type A is a surgical emergency with mortality increasing 1 to 2% per hour. A non-contrast CT phase is essential in the aortic emergency protocol, because intramural hematoma may be invisible on CTA alone when the high-attenuation wall thickening is obscured by intraluminal contrast. The ductus diverticulum is a normal variant at the aortic isthmus that can mimic a traumatic pseudoaneurysm; smooth margins and obtuse angles favor the normal variant, while irregular margins and acute angles favor injury. A normal chest radiograph does not exclude traumatic aortic injury, and CTA is required in all patients with an appropriate mechanism. When reporting aortic dissection, branch vessel involvement and malperfusion signs must always be documented because mesenteric, renal, and limb ischemia require urgent intervention. Intramural hematoma can progress to dissection or rupture, and Stanford A IMH is managed similarly to Type A dissection with surgical intervention.

References

• Hiratzka LF, et al. "2010 ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/SIR/STS/SVM Guidelines for the Diagnosis and Management of Patients with Thoracic Aortic Disease." Circulation, 2010
• McMahon MA, Squirrell CA. "Multidetector CT of Aortic Dissection: A Pictorial Review." RadioGraphics, 2010
• Squillaci E, et al. "CT Angiography of the Acute Aortic Syndrome." RadioGraphics, 2006
• Lee WK, et al. "Traumatic Aortic Injury: Imaging Diagnosis." Emergency Radiology, 2011
• Steenburg SD, et al. "Acute Traumatic Aortic Injury: Imaging Evaluation and Management." Radiology, 2008

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