Clinical features and diagnosis of blunt thoracic aortic injury

INTRODUCTION — Patients involved in high-energy blunt trauma involving rapid deceleration are at significant risk for blunt aortic injury, which can be life-threatening. It usually occurs at the aortic isthmus just distal to the left subclavian artery, but other sites can be affected. The majority of blunt aortic injuries are due to motor vehicle collision. In the United States, blunt aortic injury is the second leading cause of death behind head injury for individuals aged 4 to 34 [1,2]. It has been suggested that 20 percent of patients with blunt aortic injury survive long enough following the injury to be treated [3].

The presence of risk factors for blunt aortic injury should prompt a diagnostic evaluation, the nature of which depends upon the patient's clinical status. The grade of injury, taken together with the patient's associated injuries and medical comorbidities, determines the timing and type of thoracic aortic repair.

Blunt thoracic aortic injury will be reviewed here. The factors that determine whether an open surgical or endovascular approach should be taken to repair these injuries are discussed separately. (See "Surgical and endovascular repair of blunt thoracic aortic injury".)

MECHANISM OF INJURY — The incidence of blunt thoracic aortic injury is estimated to be between 1.5 and 2 percent of patients who sustain blunt thoracic trauma [4-7]. In a multicenter study involving 274 cases of blunt aortic injury, 81 percent were caused by automobile collisions [8]. Other etiologies of blunt thoracic aortic injury include motorcycle and aircraft crashes, automobile versus pedestrian accidents, falls, and crush injury [9]. Approximately 70 percent of victims are male [10], with approximately 67 percent of patients described as overweight or obese [2].

The main risk factor for blunt thoracic aortic injury is rapid deceleration, either from high-speed motor vehicle collision or falls from a significant height. Injury mechanism and other patient-related factors that increase the risk for blunt thoracic aortic injury are discussed elsewhere. (See "Initial evaluation and management of blunt thoracic trauma in adults", section on 'Epidemiology'.)

Associated injuries — Associated injuries are common and include severe head injury, lung contusion, cardiac contusion, intra-abdominal hemorrhage, long bone fracture, pelvic fracture, spinal fracture, and blunt diaphragmatic rupture [8,11-14]. The incidence of associated injuries in patients with blunt aortic injury was 81 percent in one autopsy series [15]. In a review of the National Trauma Databank, involving 3774 patients over 11 years (2003 to 2013), the median injury severity score (ISS) was 34, indicating multiple associated injuries [11]. The most frequently associated traumatic injuries were those to the head (32.5 percent), abdomen (28 percent), and/or lower extremities (36 percent). In a review of 74 patients, thoracic spine fracture was less common among those with injuries in the proximal aorta (ie, within 5 cm of the subclavian artery) compared with those in the distal zone of the aorta (20 versus 50 percent) [12].

Injuries associated with blunt thoracic aortic injury in unrestrained drivers may differ from those of unrestrained front seat passengers. In one study, injured organs from drivers appeared to be related to thoracic and abdominal compression leading to liver, sternal, and diaphragmatic injury, but for front seat passengers, associated head and neck injuries were more common [16]. The pattern of associated injuries may suggest different pathophysiologic mechanisms for aortic injury. (See 'Pathophysiology of injury' below.)

PATHOPHYSIOLOGY OF INJURY — Most blunt injuries of the thoracic aorta occur at the aortic isthmus just distal to the left subclavian artery [17]. Other locations include the transverse arch, proximal ascending aorta, and descending aorta just proximal to the diaphragm [3,18,19].

A number of theories are used to explain the mechanism of thoracic aortic injury at the isthmus [15,16,20-24]. It is likely that most injuries probably involve a combination of forces.

●The isthmus is thought to be a transition zone between the more mobile ascending aorta and arch and the relatively fixed descending thoracic aorta that allows for stretching with rapid deceleration [15].

●The isthmus may be intrinsically weaker than the remainder of the aorta as evidenced by a series of tensile strength tests conducted on aortic samples [20].

●Blunt aortic injury may be the result of a "water hammer" effect in which compression of the abdomen due to a sudden impact occludes the aorta, leading to rupture of the proximal, intrinsically weak isthmus or other susceptible portion of the aorta [20]. This proposed mechanism may explain the association between blunt aortic injury and diaphragmatic rupture [21].

●The "osseous pinch" theory purports that the aorta is trapped between the anterior bony structures (manubrium, first rib, medial clavicles, and sternum) and the vertebral column, leading to focal rupture [23].

Regardless of the force mechanism leading to tissue disruption, it appears that the time course of aortic rupture occurs as two distinct phases separated by an interval of time [16,24]. Rupture of the intimal and medial layers occurs first, followed by an interval of unpredictable duration, and then rupture of the adventitia. For many patients, intimal injury may never progress to a higher-grade injury or rupture. The duration of time between intima/medial injury and adventitial rupture may be from seconds to several years [16]. In an in vitro study of porcine aortic injury, an intimal-medial tear occurred before complete disruption of the entire vessel in 93 percent of specimens [24]. Partial aortic disruption occurred at a mean mechanical stress that was 74 percent of the stress needed for complete rupture. These findings suggest that a sufficient residual strength exists in the aortic wall following the initial injury to allow timely diagnosis and treatment [9].

In an autopsy study, multiple tears occurred in 21 of the 51 subjects in whom aortic tears were found [15]. Tears of the media occurred in more than 60 percent of cases. Similarly, in the porcine model of aortic injury described above, multiple intimal and medial injuries were present in 74 percent of specimens [24].

TRAUMA EVALUATION — We perform initial resuscitation, diagnostic evaluation, and management of the trauma patient with blunt or penetrating trauma based upon protocols from the Advanced Trauma Life Support (ATLS) program, established by the American College of Surgeons Committee on Trauma [25]. The initial resuscitation and evaluation of the patient with blunt trauma is discussed in detail elsewhere. (See "Initial evaluation and management of blunt thoracic trauma in adults" and "Initial evaluation and management of blunt abdominal trauma in adults".)

Early diagnosis of blunt aortic injury is critical. In a multicenter trial performed by the American Association for the Surgery of Trauma prior to the widespread use of endovascular stenting, 20 percent of patients with blunt aortic injury who survived long enough to reach the hospital died within 30 hours of injury from lethal rupture, 8 of them being stable at initial presentation and dying during surgery (15 percent of fatalities) [9]. Clinical findings associated with blunt thoracic aortic injury lack sensitivity and specificity and are discussed below.

CLINICAL FEATURES

History and physical — The trauma patient may complain of chest pain, interscapular pain, or difficulty breathing or swallowing, but these symptoms are not specific for blunt thoracic aortic injury [14]. About 30 percent of patients present with hypotension, and a Glasgow coma scale <8 is present in 41 percent of patients [11].

On physical examination, signs of significant chest wall trauma such as a steering wheel or seatbelt imprint on the surface of the skin may suggest a mechanism of injury consistent with blunt thoracic aortic injury. Other clinical signs that suggest thoracic aortic injury include a cardiac or interscapular murmur (not previously known) or left subclavicular hematoma. Although uncommon, upper extremity hypertension, also known as pseudocoarctation, or bilateral femoral pulse deficit, can also be a sign of aortic injury or rupture.

Plain chest film — A plain anteroposterior chest film is routinely obtained as part of the evaluation of blunt thoracic trauma. Findings on chest radiograph may indicate the need for further testing (algorithm 1), typically using chest computed tomography (CT), which reliably establishes a diagnosis of blunt thoracic aortic injury. (See 'Diagnosis' below.)

Signs consistent with thoracic aortic injury on plain chest radiography are listed below (image 1B); however, it should be noted that blunt thoracic aortic injury can be present in the absence of plain film findings [26,27]. (See "Initial evaluation and management of blunt thoracic trauma in adults", section on 'Abnormal CXR findings'.)

●Wide mediastinum (supine chest radiograph [CXR] >8 cm; upright CXR >6 cm)

●Obscured, indistinct, or enlarged aortic knob; abnormal aortic arch contour

●Left "apical cap" (ie, pleural blood above apex of left lung)

●Large left hemothorax

●Displacement of the left main stem bronchus

●Deviation of nasogastric tube rightward

●Deviation of trachea rightward and/or right mainstem bronchus downward

●Wide left paravertebral stripe

Laboratory studies — There are no laboratory studies that are specific for blunt aortic injury. An initially elevated white blood cell count in the trauma patient is common and frequently related only to the physical stress of trauma. Anemia related to acute blood loss is a nonspecific finding.

DIAGNOSIS — Blunt aortic injury may be suspected based upon blunt chest trauma mechanism, associated injuries, physical findings, or findings on plain chest film, but when blunt aortic injury is suspected, we recommend further imaging to definitively identify or exclude blunt aortic injury [28]. The initial evaluation and indication for imaging patients with blunt chest trauma is discussed elsewhere. The specific issues related to blunt aortic injury and findings related to blunt aortic injury are discussed below. (See "Initial evaluation and management of blunt thoracic trauma in adults", section on 'Chest computed tomography for most patients' and 'Trauma evaluation' above.)

Approach to imaging — Contrast-enhanced computed tomographic (CT) angiography of the chest and transesophageal echocardiography (TEE) are the main imaging modalities for diagnosing blunt aortic injury [29]. More than one imaging modality and/or repeated imaging may be required to establish the diagnosis. Given the high sensitivity and specificity of CT angiography for thoracic aortic injury and the wide availability of CT in the emergency setting, CT angiography is recommended for the diagnosis of blunt traumatic aortic injury in hemodynamically stable patients [30]. TEE is a valuable alternative in the assessment of hemodynamically unstable ventilated patients who require prompt assessment. The main advantage of TEE is its portability and repeatability, particularly in multiply injured patients [31]. TEE can be performed at the bedside, or in the operating room prior to exploration. In contrast, transthoracic echocardiography is not adequate for the accurate diagnosis of blunt aortic injury since it provides limited window to the aortic isthmus and therefore should not be used for this indication [32].

TEE and CT have similar diagnostic accuracies for the identification of subadventitial injuries of the aortic isthmus [33]. Multidetector CT with multiplanar reconstructions and 3D reformations has replaced helical CT in later series [34,35]. Although TEE appeared more sensitive than earlier CT angiography for the diagnosis of superficial traumatic thoracic aortic injuries and for the detection of associated cardiac trauma [33], this is not applicable to multidetector CT angiography. The potential additional diagnostic value of 3D TEE remains unknown in this setting.

Thoracic aortography is not routinely used to identify blunt aortic injury because it is invasive and associated with delays related to the need to set up the interventional suite and call in appropriate personnel (interventionalist, nursing [36]). Aortography may also fail to detect a tear until the development of pseudoaneurysm [29]. However, aortography may be needed where newer-generation CT scanning is not available [37], and when TEE cannot be alternatively be used in ventilated patients. The normal aortic contour and typical findings of blunt aortic injury on aortogram are given in the images (image 1A-E). Compared with multidetector chest CT, the sensitivity of aortography may be slightly inferior [38,39]. In a study of 494 patients, of whom 71 had blunt thoracic aortic injury, the sensitivity of CT approached 100 percent compared with 92 percent for aortography [39]. In another review, 272 patients were followed a mean of 615 days from the time of blunt chest trauma after blunt thoracic aortic injury was excluded by chest CT [40]. No procedures for injuries to the aorta or great vessels were needed, and there were no deaths related to blunt aortic injury.

Magnetic resonance (MR) imaging avoids ionizing radiation and iodinated contrast, and although good-quality images of the aorta can be obtained, this study is uncommonly used in the diagnosis of blunt thoracic aortic injury due to the long-time need for image acquisition, the need for patient immobility, and MR-compatible support system. MR cannot be used if any metallic objects are present, including those that may have been acquired at the time of the injury (eg, bullets). MR may be used in the follow-up of minimal or equivocal intimal injuries, especially in young patients, as a strategy to reduce radiation dose because of repeat CT studies.

Intravascular ultrasound (IVUS) or TEE can help clarify the diagnosis when chest CT is abnormal or equivocal [41-43]. IVUS has emerged as a valuable diagnostic tool for the precise characterization of blunt aortic injury, particularly when CT is equivocal [41]. The use of IVUS at the time of aortography increases the diagnostic accuracy and facilitates accurate endograft sizing and graft deployment, if needed [44]. IVUS is more often used in stable patients with grade III injuries [45].

CT findings — CT angiography of the chest is a highly sensitive and specific test for thoracic aortic injury and is the diagnostic test of choice (table 1) [4-6,14,29,35,37,39,40,46-48]. With multidetector CT, multiplanar reformations, and ultrafast acquisitions, the sensitivity and specificity are 98 to 100 percent, including minimal aortic injury [49].

Findings on chest CT indicative of blunt thoracic aortic injury can be divided into direct signs (evidence of injury to the aortic wall) and indirect signs. The direct signs include:

●Intimal flap

●Luminal and mural thrombus

●Focal aortic wall outpouching

●Sudden change in aortic caliber

●Aortic contour abnormality

●Evidence of vascular wall disruption such as periaortic contrast extravasation

The indirect signs include periaortic or mediastinal hematoma. Findings on chest CT angiography indicative of blunt thoracic aortic injury are listed below and illustrated in the images (image 2A-C) [50]. The grade of injury identified on CT angiography determines the approach to treatment [4,6,46]. (See 'Aortic injury grading' below and "Management of blunt thoracic aortic injury", section on 'Approach to management'.)

TEE findings — TEE provides another means to evaluate the thoracic aorta and may be most useful for hemodynamically unstable patients, especially when they are mechanically ventilated and cannot be transported safely to undergo emergency CT angiography of the chest [51]. TEE can be performed to rule out aortic injury in the emergency department, or in the operating room for those patients with indications for immediate surgical intervention.

TEE is a powerful imaging modality for the diagnosis of blunt aortic injury involving the region of aortic isthmus. In experienced hands, the sensitivity and specificity of TEE ranges between 91 and 100 percent and between 98 and 100 percent, respectively [32,52-56]. The reliability of TEE in the diagnosis of blunt rupture of the aortic isthmus requires meticulous examination by an experienced echocardiographer. The diagnostic accuracy of TEE for the identification of injury involving other aortic segments is less known. Nevertheless, TEE may allow diagnosis of blunt injuries to the ascending aorta [57], whereas traumatic injuries to aortic branches are usually outside the field of view of TEE probes.

The TEE diagnosis of subadventitial thoracic aortic injury requires the identification of a disruption of the aortic wall with flow on both sides of the lesion that can be identified by color Doppler imaging [53]. TEE findings associated with subadventitial injury are most frequently observed in the region corresponding to the aortic isthmus (typically approximately 25 to 35 cm from the incisors, immediately distal to the origin of the left subclavian artery).

●Typically, a thick and irregular intraluminal flap is seen traversing the lumen of the aortic isthmus in the transverse view (image 3). Since this lesion corresponds to disruption of both intimal and medial aortic layers, it should be considered a "medial flap." In the longitudinal view, the medial flap is nearly perpendicular to the aortic wall since traumatic lesions are usually confined to a few centimeters (image 4).

●An abnormal aortic contour is also commonly observed due to the acute formation of a localized false aneurysm (pseudoaneurysm) [53,58]; the wall of the false aneurysm consists of the adventitial layer under pressure. Depending on the size of this false aneurysm, the thoracic aorta may or may not be enlarged.

●Color Doppler echocardiography typically depicts similar blood flow velocities on both sides of the medial flap (image 5). In addition, a mosaic of colors surrounding the disrupted aortic wall may be observed, reflecting local blood flow turbulence (image 5) [53].

●Intraluminal obstruction caused by the disrupted aortic wall may result in a pseudo-coarctation syndrome (image 6) and associated ischemia. In this case, TEE depicts usually an intermittent aortic obstruction with a maximal pressure gradient greater than 20 mmHg using continuous wave Doppler [32]. Similar injury of the ascending aorta is rarely identified using TEE [43] since it usually results in death on scene.

Mediastinal hematomas can be accurately diagnosed using TEE and are observed in many patients with blunt aortic injury [59,60]. Diagnosis is based on the presence of an increased distance between TEE probe and the anteromedial aortic wall and/or the presence of an ultrasound signal consistent with blood between the posterolateral aortic wall and the left visceral pleura (image 4). Mediastinal hematomas are usually large when associated with aortic injury compared with those secondary to other traumatic injuries (eg, laceration of small mediastinal vessels, vertebral fractures) [60].

Aortic dissections and intramural hematomas have been described using TEE in patients sustaining severe blunt chest trauma [58,61]. Several TEE findings help distinguish acute aortic dissection from blunt aortic injury (table 2 and image 3 and image 4) [53]. Importantly, aortic dissection results in two distinct channels, and the false lumen in aortic dissection usually has a slower velocity than the true lumen. By comparison, with blunt aortic injury, blood flow velocities as evidenced by color Doppler imaging appear similar on both sides of the medial flap (image 5). A second potential diagnostic problem is that protuberant atherosclerotic changes of the aorta may be challenging to differentiate from partial injury and may mask underlying localized subadventitial thoracic aortic tears [54,62]. Finally, although more frequently depicted at the level of the ascending aorta than in the region of the aortic isthmus, linear artifacts within the aortic lumen may be misdiagnosed as medial flaps. TEE diagnostic criteria of these intra-aortic linear artifacts have been validated, thus limiting the risk of false positive results [63].

TEE also allows accurate diagnosis of less severe blunt aortic injuries. Superficial traumatic aortic injuries are too small and superficial to exert an excessive pressure on the adventitial layer. Therefore, both the aortic diameter and contour usually remain unchanged, and no or little blood flow turbulence is evidenced using color Doppler mapping. These injuries may be isolated or associated with a subadventitial aortic injury. Intimal tears appear as thin, mobile, and short (usually <10 mm) intraluminal appendages of the aortic wall, commonly located at or in the immediate vicinity of the aortic isthmus [53].

Wall thrombi within the aortic lumen may also be observed as a result of violent chest trauma. These thrombi may be voluminous and highly mobile, resulting in arterial emboli. They are typically located in the proximal descending thoracic aorta, which is otherwise free of atherosclerotic changes.

Aortic injury grading — Classification schemes for blunt aortic injury vary in their complexity [13,56,64-70]. We prefer to use a relatively simple system (Society of Vascular Surgery Classification) for grading the severity of aortic injury, which may help provide more consistency in reporting and formulating recommendations (figure 1) [13,64]:

●Type I: Intimal tear or flap (image 1C and image 1D)

●Type II: Intramural hematoma (image 2A)

●Type III: Pseudoaneurysm (image 1B and image 1D)

●Type IV: Rupture (eg, periaortic hematoma, free rupture) (image 2C and image 2B)

The Vancouver and Harborview classification systems includes size, contrast extravasation, and external contour abnormality [70].

The meaning of "minimal aortic injury" remains unclear, and the term has been used to represent relatively small lesions, including isolated intimal defects without extravasation of blood, and for describing anything less than a small pseudoaneurysm. One grading system defines minimal aortic injury as an injury with an intimal flap of less than 1 cm, without external contour deformity, and no (or minimal) periaortic mediastinal hematoma [65]. As high-resolution imaging has become more available, these subtle arterial lesions, which may have been previously undetected, are being identified more commonly. In addition, increasing experience with conservative treatment and follow-up of aortic injuries is expanding the types of injuries included in minimal aortic injury [11,71]. Approximately 10 to 30 percent of patients with blunt aortic injury have changes identified by multidetector CT that are characterized as "minimal" [71-73].

DIFFERENTIAL DIAGNOSIS — Chest pain associated with blunt chest trauma is nonspecific, and pain can be a manifestation of injury to nearly any structure within the chest. Other non-aortic injuries related to blunt chest trauma are discussed elsewhere. (See "Initial evaluation and management of blunt thoracic trauma in adults" and "Initial evaluation and management of chest wall trauma in adults".)

Other aortic pathologies can produce symptoms, particularly chest or back pain, similar to the pain associated with blunt aortic injury. The pain from an acute aortic dissection is described as searing and often begins in the chest, migrating into the abdomen over time. Aortic dissection may affect other arch vessels, leading to other symptoms (cerebral embolism, upper extremity ischemia) that are not as likely in patients with only a blunt thoracic aortic injury.

Thoracic aortic imaging will differentiate aortic dissection and other thoracic pathologies from the typical appearance of blunt thoracic aortic injury. However, in patients with blunt chest injury, blunt aortic injury and other aortic pathologies can coexist. Blunt chest trauma may have precipitated rupture in patients with preexisting thoracic aortic pathology, such as chronic aortic dissection or aortic pseudoaneurysm due to erosion of ulcerated plaque [74]. Alternatively, thoracic dissection (much like myocardial infarction or acute cerebrovascular ischemia) may have been a precipitating event for the traumatic injury [75-79]. For those with risk factors for spontaneous aortic dissection, it is important to make this distinction because treatment differs. (See "Epidemiology, risk factors, pathogenesis, and natural history of thoracic aortic aneurysm and dissection", section on 'Etiology and risk factors'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Thoracic trauma".)

SUMMARY AND RECOMMENDATIONS

●Blunt thoracic aortic injury – The incidence of blunt thoracic aortic injury is between 1 and 2 percent of those patients who sustain blunt thoracic trauma. Blunt thoracic aortic injury is a major cause of rapid death following motor vehicle collision. Prompt diagnosis is critical since up to 20 percent of patients who initially reach hospital alive may die subsequently from aortic rupture before undergoing a corrective procedure. (See 'Introduction' above and 'Mechanism of injury' above and "Management of blunt thoracic aortic injury", section on 'Introduction'.)

●Risk factors – The clinical evaluation of the trauma patient should assess the risk for blunt thoracic aortic injury based upon injury mechanism. The main risk factor for blunt thoracic aortic injury is rapid deceleration, either from high-speed motor vehicle collision or falls from a significant height. The majority of blunt injuries to the thoracic aorta occur at the aortic isthmus just distal to the left subclavian artery, and a number of theories are used to explain the predilection for this site. (See 'Mechanism of injury' above and 'Pathophysiology of injury' above.)

●Imaging – The routine evaluation of the patient with blunt thoracic injury includes a plain chest radiography. For patients in whom there is clinical suspicion for blunt aortic injury, given the high sensitivity and specificity of CT angiography for thoracic aortic injury and the wide availability of CT in the emergency setting, CT angiography is recommended as a first-line imaging modality for the diagnosis of blunt aortic injury in hemodynamically stable patients. TEE is an alternative that is particularly useful for the assessment of hemodynamically unstable mechanically ventilated patients but requires experienced operators. (See 'Approach to imaging' above.)

•Plain radiography findings suggestive of thoracic aortic injury on clinical evaluation or the chest radiograph may indicate the need for further imaging using CT angiography of the chest or transesophageal echocardiography (TEE), but a normal mediastinal contour fails to exclude the diagnosis in a patient at risk (algorithm 1).

•Findings on chest CT indicative of blunt thoracic aortic injury can be divided into direct signs (injury to the aortic wall) and indirect signs. The direct signs include findings of an intimal flap, luminal and mural thrombus, focal aortic wall outpouching, sudden change in aortic caliber, aortic contour abnormality, or any evidence of vascular wall disruption, such as periaortic contrast extravasation. The indirect signs include periaortic or mediastinal hematoma.

•The TEE diagnosis of subadventitial thoracic aortic injury involves identification of disruption of the aortic wall (medial flap) with flow on both sides of the flap identified by color Doppler imaging. Other associated findings include abnormal aortic contour due to pseudoaneurysm and evidence of mediastinal hematoma. TEE also enables diagnosis of superficial traumatic aortic injuries as well as traumatic aortic dissection or intramural hematoma, and potentially associated cardiac injuries.

●Aortic injury grading – Aortic injuries can be graded based upon CT findings as follows (see 'Aortic injury grading' above):

•Type I: Intimal tear

•Type II: Intramural hematoma

•Type III: Pseudoaneurysm

•Type IV: Rupture