Clinical Presentation

A history of high-speed deceleration or blunt chest trauma should suggest thoracic aortic injury. Chest or upper back pain, dyspnea, dysphagia, absence of femoral pulses, ecchymoses of the chest wall, seat belt injuries, and fractured sternum act to increase suspicion but are frequently absent.

Imaging Indications and Algorithm

Indications for imaging are based on the mechanism of trauma and clinical signs detailed earlier. As part of a trauma series, most patients will undergo initial chest radiography but will progress immediately to CT regardless of the radiographic results, provided the patient is stable enough.

Imaging Techniques and Findings

Radiography

Plain film findings associated with aortic trauma include a widened mediastinal contour, obscuration of the aortic arch or descending aortic margin, displacement of the trachea or of a nasogastric tube (if present) to the right, and left apical cap (Fig. 94-3). Fracture of the first or second ribs may be seen as a result of the causative trauma.⁴ However, each of these findings has been shown to have poor sensitivity or poor specificity. For example, in the trauma setting, a widened mediastinum is more commonly explained by anteroposterior positioning, lung contusion, hemothorax, and venous mediastinal bleeding, decreasing the specificity of these findings. Furthermore, in the majority of aortic injuries, the adventitia remains intact, resulting in only mild radiographic mediastinal findings, decreasing the sensitivity of plain radiography. In one study of aortic trauma, 44% of patients with subsequently proven aortic injury had a normal chest radiograph.⁵ Therefore, plain radiography is not sufficiently sensitive or specific for the evaluation of aortic injury, and in the correct clinical setting, the patient should proceed directly to other modes of imaging.

FIGURE 94-3 Chest radiograph of a patient with acute traumatic aortic injury demonstrates a widened mediastinum, with deviation of the trachea to the right (arrow).

Computed Tomography

Multidetector computed tomography (MDCT) is the definitive modality for diagnosis of aortic trauma because of its speed, accuracy, postprocessing advantages, and ability to simultaneously evaluate the whole body for traumatic injury. CT angiography has been shown to have 100% sensitivity and 100% negative predictive value for traumatic aortic injury, at lower cost and with fewer complications than by catheter angiography.⁶

MDCT Technique

In most trauma centers, chest CT is performed as part of a trauma protocol that consists of an unenhanced head CT scan followed by an enhanced study of the chest, abdomen, and pelvis, with use of intravenous contrast material only. Evaluation of the neck can be included if cervical trauma is suspected. The chest CT scan is usually performed in the arterial phase, triggered by bolus timing from the mid-descending aorta. Images are obtained with 1-mm or thinner collimations; 3- to 5-mm axial sections are used for initial review. However, the data should be viewed in multiple planes, which can be achieved either by three-plane reconstructions generated by the CT technologist or by use of an independent workstation by the interpreting physician.

Aortic Pseudoaneurysm

Most patients with aortic trauma who survive to imaging will have a partial tear of the aortic wall. If the intima and media are torn but the adventitia remains intact, blood is forced between the layers, causing formation of a pseudoaneurysm. The imaging appearance is of a rounded bulge projecting from the aorta (Fig. 94-4). The margins are typically irregular, and there is often an intimal flap that forms a “collar” at the neck of the pseudoaneurysm. Note that the injury to the intima may extend several centimeters proximal or distal to the visualized pseudoaneurysm. The extent of the injury, especially the proximal extent, is crucial in the planning of repair because branch vessel involvement has a major influence on repair technique.

FIGURE 94-4 Axial CT image in a trauma patient demonstrates an intimal flap in the proximal descending thoracic aorta (A). Oblique sagittal maximum intensity projection (B) and oblique coronal (C) reformatted images depict a pseudoaneurysm (arrow). Note extensive mediastinal hematoma (asterisk) diffusely engulfing the aorta, great vessels, and remainder of the mediastinal structures.

Intimal Flap

An intimal flap arises from blood dissecting through an intimal tear as described earlier. It is commonly but not always seen in association with a pseudoaneurysm. To best visualize a flap and to avoid a false-negative diagnosis, thin-section images and an appropriate window must be employed. An intimal flap can be obscured by thick-slab maximum intensity projection images or rendered undetectable by suboptimal windowing during subsequent viewing. Care must be taken to evaluate source or single-voxel multiplanar reconstruction images to ensure that an intimal flap is not present.

A potential complication of an intimal flap is the formation of thrombus, which may subsequently embolize and cause distal organ infarction.

Contour Abnormality

Whereas a pseudoaneurysm presents an obvious contour abnormality, more subtle contour changes without obvious irregularity are sometimes seen. The aortic lumen is of relatively constant diameter from the distal arch through the aortic hiatus in most patients, with the occasional exception of congenital narrowing at the isthmus, which is typically mild and smooth. A change in contour of the aorta in the setting of suspected aortic injury may represent dissection of blood into the aortic wall without a visible intimal flap. The contour of the arch and descending thoracic aorta is usually better assessed on multiplanar reconstruction or maximum intensity projection images than on axial images.

Periaortic Mediastinal Hemorrhage

Mediastinal hemorrhage is an indirect sign of thoracic aortic trauma, being attributable to aortic injury in only 12.5% of cases.⁷ Arterial disruption of one of the great thoracic vessels with active contrast extravasation from the vessel into the hematoma represents a potentially catastrophic injury in which the patient would be highly unstable. Traumatic mediastinal hemorrhage, however, is most commonly caused by rupture of small mediastinal veins.

The location of mediastinal hematoma is a guide to its cause. Hematoma abutting the sternum or vertebral body should suggest fracture. Likewise, hematoma surrounding the aorta should provoke a thorough evaluation of the aorta for direct signs of injury (Fig. 94-5). Nevertheless, an isolated finding of mediastinal hemorrhage, even if it is periaortic, is unlikely to be accompanied by aortic injury if no direct sign of aortic injury is seen.⁸ In addition, mediastinal hematoma can track inferiorly. The presence of retrocrural hematoma on an abdominal CT image should prompt careful evaluation of the thorax for vascular injury.

FIGURE 94-5 Extensive mediastinal hematoma surrounds a traumatic aortic injury (A; intimal tear with pseudoaneurysm, asterisk). The hematoma tracks inferiorly in the mediastinum, surrounding the descending aorta (B). If hematoma is seen in a retrocrural location on the superior aspect of an abdominal CT image, aortic injury must be suspected.

Contrast Agent Extravasation

On CT, the extravasation of contrast agent from an aortic injury is diagnosed by the presence of high-density streaks or puddles within a mediastinal hematoma, but it is a very rare finding. Such patients are extremely unstable. In one series of four patients with contrast agent extravasation from thoracic aortic trauma, none survived.⁹

Magnetic Resonance

MR can be used to depict the findings of aortic trauma (Fig. 94-6). However, the typically longer duration of the examination (relative to CT), difficulty of monitoring the patient, and relative lack of 24-hour emergent availability combine to exclude MR in most cases from evaluation for acute traumatic aortic injury. In equivocal or unusual cases, MR can be an effective problem-solving tool. The diagnosis of very small pseudoaneurysms and intimal flaps, moreover, can be challenging on MR because of the generally inferior spatial resolution compared with MDCT (Fig. 94-7).

FIGURE 94-6 Traumatic aortic injury with intimal flap and pseudoaneurysm as depicted on axial CT (A) and oblique sagittal CT (B, arrow) images. Oblique sagittal postcontrast T1-weighted MR image (C) obtained 1 day after CT demonstrates the same findings (arrow).

FIGURE 94-7 Axial CT image (A) and oblique sagittal reconstruction (B) demonstrate a faint intimal flap (arrowhead) with a very small pseudoaneurysm (arrow). Oblique sagittal postcontrast T1-weighted MR image (C) acquired 1 day after CT demonstrates less clearly than on CT the small pseudoaneurysm (arrow) but highlights the limits of spatial resolution of MR imaging in this clinical setting.

In rare patients, severe associated traumatic injuries preclude acute treatment of an aortic injury, and a chronic pseudoaneurysm develops that may be managed conservatively. MR is an appropriate modality for close imaging follow-up of these patients (Fig. 94-8). Key sequences include ECG-gated cine steady-state free precession, three-dimensional steady-state free precession MR angiography, and gadolinium-enhanced three-dimensional MR angiography of the chest. ECG gating is particularly helpful for evaluation of the aortic root.

FIGURE 94-8 Axial CT image (A) demonstrates a large acute pseudoaneurysm (asterisk) with surrounding mediastinal hematoma. Axial postcontrast T1-weighted MR image (B) acquired 3 years later shows no interval increase in size of the pseudoaneurysm (asterisk).

Classic Signs

Ductus Diverticulum

The ligamentum arteriosum, the remnant of the ductus arteriosus, inserts along the anterior undersurface of the aortic isthmus. At this location, a common variant is a small anterior bulge of the aortic lumen, known as a ductus bump or ductus diverticulum. A ductus diverticulum (Fig. 94-9) demonstrates a smooth contour and obtuse angles with the aortic wall and is distinct from the appearance of an aortic intimal tear with pseudoaneurysm (Fig. 94-10), in which there are acute angles and often an irregular contour.

FIGURE 94-9 The ductus diverticulum (arrow) is a shallow depression with smooth contours and obtuse margins with the aorta. This is distinct from the appearance of a traumatic aortic pseudoaneurysm (compare with Figs. 94-1 and 94-10), which has acute margins with the aorta.

FIGURE 94-10 Traumatic aortic pseudoaneuryms (arrow) with acute margins of a traumatic pseudoaneurysm (similar to that shown in Fig. 94-1).

Medical

Identification of aortic injury is critical; 40% of patients who survive to reach the emergency department but are not diagnosed will die within 24 hours.⁶ There is no role for medical management of traumatic aortic injury. However, a patient will occasionally present with an enlarging aortic pseudoaneurysm years after a traumatic event (Fig. 94-11). This is due to either a rare case of survival of a missed minor traumatic aortic injury or a known injury that was not treated surgically because of severe concomitant injury. Over time, there is gradual progression from intimal tear to pseudoaneurysm. Intervention will usually be required. In other cases, the presenting feature of a missed aortic injury may be due to thrombus formation on the damaged intima, which introduces the potential complication of distal embolization.

FIGURE 94-11 Volume rendered (A) and maximum intensity projection (B) images demonstrate a chronic calcified pseudoaneurysm (A, arrow) from a remote traumatic event.

Surgical/Interventional

The standard therapy for traumatic aortic injury is surgical repair. The surgical approach is commonly through a left lateral thoracotomy, although unusual sites of aortic injury may require an alternative approach. For example, a mid-arch dissection may be more easily approached through a median sternotomy.¹⁰ An open repair is associated with high morbidity and mortality, not least due to the usually extensive accompanying injuries. Emergent surgical repair carries a 15% to 50% mortality rate.^11–13 The recent introduction of endovascular techniques to treat these patients has led to a reduced mortality rate and has also enabled the treatment of patients who are not candidates for open thoracic surgery. Recent data demonstrate that use of endovascular techniques in place of open repair in aortic trauma has increased from 0% to 64% of cases from 1997 to 2007, with concomitant reduction by more than 50% in mortality and paraplegia rates.^14,15

Endovascular stent graft placement requires a proximal aortic neck of at least 15 mm for safe graft attachment (Fig. 94-12). In the case of aortic isthmus injury, the lesion is frequently too close to the left subclavian artery to provide a suitable attachment site (“landing zone”) for the stent graft. In this case, the left subclavian artery may be intentionally covered, provided patency of both vertebral arteries can be demonstrated before graft placement as the vertebral arteries enable collateral filling of the subclavian artery (i.e., left subclavian “steal” filling through left vertebral artery). In most patients, this will suffice. If vertebrobasilar or arm ischemia develops, a carotid-subclavian transposition or shunt can be performed.¹⁶

FIGURE 94-12 Traumatic aortic injury (arrowheads) in the mid descending thoracic aorta, distant from the arch vessels on oblique sagittal CT reconstruction (A). The injury was successfully treated with endovascular stent graft placement (before stenting, B; after stenting, C).

(Courtesy of Dr. K. Kolbeck.)