Claire K. Sandstrom
Associate Professor, Emergency Radiology
University of Washington
Advancements in the accessibility, speed, and image quality of MDCT in the last 20 years have guaranteed that MDCT is the preferred imaging modality for evaluation of most conditions presenting in the emergency department, and this is particularly true for imaging after trauma. Vascular injuries, including those involving the thoracic and abdominal aorta, abdominal mesentery, pelvis, cervical vessels, and upper and lower extremities, are an uncommon but potentially lethal outcome of both penetrating and blunt trauma. Historically, diagnosis of vascular injuries relied on open exploration or conventional catheter-based angiography (CTA), both of which are invasive, time-consuming, and not broadly accessible. Today, however, the diagnosis or exclusion of many of these injuries is made on MDCT, oftentimes obviating the need for more invasive techniques. The timing and type of endovascular repair, particularly for aortic injuries, has also evolved.
Most deaths from traumatic aortic injuries still occur at the scene or before the patient reaches the hospital. However, those who reach the emergency department and undergo imaging can now be treated with a high rate of success. The majority of traumatic thoracic aortic injuries (TTAI) are found at the aortic isthmus. Injuries are found less commonly at the aortic root, ascending aorta, distal descending aorta, or at branch vessel origins, with multifocal injuries in up to 18% of patients. Although most aortic injuries are associated with high-energy trauma, there is no consensus as to the precise definition of “high-energy.” Furthermore, direct chest trauma or visible external signs of chest trauma are not necessary for the diagnosis. Therefore, liberal screening with chest CTA is encouraged in any patient with more than minimal deceleration injury.
Initial screening for mediastinal hematoma may be performed with a portable chest radiograph in many institutions. Signs suggesting mediastinal hematoma, and thus raising the possibility of a surgically relevant aortic injury, include right paratracheal stripe thickening, superior mediastinal widening, aortic arch enlargement or irregularity, opacification of the aortopulmonary window, rightward displacement of the trachea or enteric tube, inferior displacement of the left mainstem bronchus, obscuration of the descending aorta, widening of the paraspinal lines, or apical capping. These radiographic features are neither sensitive nor specific, and absence should not preclude chest CTA in high-risk trauma victims. Contrast-enhanced chest MDCT has very high sensitivity and a negative predictive value for acute aortic injuries, and it should be obtained in all at risk patients. Thoracic CTA with cardiac gating or ultrahigh pitch is the diagnostic study of choice, but nongated CTA is sufficient in most patients. Intimal flaps, intraluminal thrombus, intramural hematoma, irregular external aortic contour, focal luminal dilation or saccular outpouching (also known as pseudoaneurysm), or active extravasation are direct signs of aortic injury. Injuries can be graded according to the Society of Vascular Surgery (SVS) system or using a newer system of minimal/moderate/severe injuries that more directly guides management. Minimal aortic injuries have no external aortic contour deformity and intimal tear or intraluminal thrombus less than 10 mm in size (equivalent to SVS grade 1); these injuries do not require operative intervention and instead receive antiplatelet therapy for 4–6 weeks, with optional follow-up imaging. Moderate and severe TTAIs require surgical intervention.
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Between 2002 and 2014, mortality from blunt TTAI decreased from 46.1% to 23.7%, largely as a result of increased use of endovascular rather than open repair. One important recent trend in the management of TTAI involves timing of intervention. Although the risk for rupture of contained TTAI is highest in the first 24 hours, mortality rates and rates of paraplegia and stroke are lower when repair is delayed until after the overall condition of the patient can be stabilized. For all but the most severe aortic injuries—those with active extravasation or a very large contained rupture with large periaortic hematoma—repair is performed in 1–3 days, when the patient’s condition is more stable and concomitant injuries are considered survivable. In the interim, antihypertensives are used to reduce wall stress and risk of rupture.
Only about 5% of blunt aortic injuries involve the abdominal aorta. Sufficiently rare that many radiologists may never encounter one during their careers, this injury should be specifically sought when patients present with blunt abdominal trauma, such as a seatbelt sign or abdominal impact on the steering wheel, and have spinal fractures (particularly flexion-distraction injuries), duodenal or small bowel injuries, or pancreatic injuries. Isolated blunt abdominal aortic injuries (BAAI) are also rare. Two-thirds of BAAIs occur between the renal arteries and the aortic bifurcation, and up to one-quarter also have injuries involving the thoracic aorta. Abdominal CTA is the diagnostic study of choice, though venous-phase abdominal MDCT is sufficient for diagnosis and preintervention planning in most cases. In patients stable enough to be evaluated on MDCT, the most common appearance of BAAI is intimal flaps or intimal thrombi without external aortic contour deformity. Pseudoaneurysms of the abdominal aorta are only seen in 16% but require repair, either open or endovascular, depending on location. In the absence of external contour abnormality, BAAI can be managed nonoperatively with antiplatelet therapy and beta blockers. It is important to note that neither TTAI or BAAI can be excluded on a unenhanced CT because injuries, particularly intraluminal thrombi and intimal flaps, may not be accompanied by periaortic hematoma or stranding.
Injuries of the mesenteric vasculature are also uncommon in blunt trauma patients, more often resulting from penetrating trauma. Unfortunately, these are frequently lethal due to exsanguination, reflecting the difficulty in obtaining control of the proximal superior mesenteric artery (SMA), as well as back-bleeding from the valveless portomesenteric venous system. Though uncommon at initial laparotomy, bowel infarction and subsequent sepsis and multiple organ system failure are responsible for the bulk of delayed deaths from mesenteric vascular injury. Classification systems by the American Association for the Surgery of Trauma-Organ Injury Scale (AAST-OIS) and by Fullen et al. are both anatomy-based, reflecting the greater surgical difficulty and poorer outcomes associated with more proximal mesenteric arterial or venous injuries. Although immediate operative evaluation is appropriate in any patient with penetrating trauma to the peritoneum or with blunt trauma in extremis, patients with hemodynamic stability following blunt abdominal trauma can be imaged with contrast-enhanced MDCT. On MDCT, direct signs of surgically important mesenteric vascular injuries include mesenteric vascular beading, abrupt termination, or active extravasation. Intraperitoneal low- or intermediate-density free fluid is highly sensitive for either bowel or mesenteric injury, as is abnormal bowel wall thickening or enhancement, and surgical exploration is appropriate when any of these are found on MDCT. The absence of intraperitoneal free fluid has a high negative predictive value for surgically important mesenteric or bowel injury. Isolated mesenteric stranding or hematoma without active extravasation does not necessarily need surgical exploration, but these patients should be monitored carefully for delayed presentation of CT-occult bowel injury or mesenteric injury resulting in bowel ischemia.
Hemorrhage from pelvic ring injuries can be significant and life-threatening. Arterial hemorrhage accounts for 15–20% of pelvic bleeding, and low-pressure bleeding from venous structures or fractured edges of cancellous bone account for the remainder. These low-pressure bleeding sites are usually controlled by pelvic sheeting, external fixation, or internal pelvic packing, whereas arterial hemorrhage is amenable to endovascular control. Early triage to angiography may be considered for those patients with obturator ring fractures displaced at least 1 cm or pubic symphyseal diastasis of at least 1 cm, as these are independent predictors of major hemorrhage. If a patient with pelvic ring injuries is hemodynamically stable, multiphase MDCT can improve the sensitivity and specificity of detection of pelvic bleeding. Ideally, an arterial phase is obtained to identify arterial injury, as opposed to venous injury, and an additional phase differentiates active bleeding from pseudoaneurysm. Unenhanced MDCT or dual-energy CT with virtual unenhanced images may be necessary to identify bone fragments that mimic pseudoaneurysm or active extravasation. Absence of contrast extravasation on MDCT has a high negative predictive value for clinically significant pelvic bleeding. When conventional catheter-based pelvic angiography is performed, whether before or after MDCT, injection of the bilateral internal iliac veins and the bilateral external iliac veins should be performed.
Most peripheral vascular traumatic injuries result from penetrating trauma in civilian or military settings and involve the femoral or popliteal arteries of the lower extremity. Following blunt trauma, popliteal arterial injuries are found in 30% of knee dislocations, as well as accompanying some displaced femoral or tibial plateau fractures. Open mid-shaft tibial and fibular fractures commonly have injuries to the anterior and posterior tibial arteries. Although traumatic injuries of the torso usually take precedence over extremity injuries, active extremity bleeding may require direct pressure, tourniquet, or direct clamping to prevent life-threatening hemorrhage. Potential complications of peripheral vascular trauma include exsanguination, acute ischemia, tissue necrosis, reperfusion injury, and need for amputation. Thus, prompt diagnosis and treatment of peripheral vascular injuries aims to prevent life-threatening blood loss and restore perfusion to the extremity, with increased likelihood of limb salvage, if definitive treatment is performed within 6 hours.
Lower extremity CTA is the diagnostic study of choice for noninvasive evaluation of lower extremity vascular trauma. Any patient with hard signs of vascular trauma, including active hemorrhage, an expanding or pulsatile hematoma, a wound with bruit or thrill, a distal pulse deficit, or distal ischemic changes, should undergo CTA, unless emergency surgical intervention is necessary. Even those with lower extremity injuries, without hard signs of vascular injury, may still benefit from lower extremity CTA if the ankle-brachial index (ABI) is reduced below 0.9 (sensitivity 87–100%, specificity 80–100%). For those with ABI above 0.9, the likelihood of vascular injury requiring surgery is low, though these patients may still be observed with serial exams for 24–48 hours. One important protocol issue with lower extremity CTA on newer ultrafast scanners is that the scan may “outrun” the contrast bolus in the distal lower extremity, particularly in patients with lower cardiac output. For this reason, at my institution, our lower extremity CTA protocol includes the arterial phase from abdomen or pelvis through the toes, followed 7 seconds later by an immediate delayed (late arterial) phase from knees to toes. Inclusion of both lower extremities in the reconstructed FOV is helpful, even if the injury is unilateral, to provide internal comparison.
Upper extremity CTA is less commonly performed and is more variable in technique. If the upper extremity CTA is performed in isolation, the arm may be positioned above the patient’s head to improve image quality and reduce radiation dose, as long as the patient’s injuries permit such positioning, whereas if the CTA is performed concurrent with a chest CTA, the arm can be positioned at the patient’s side. Always consider contrast injection contralateral to the injured arm to avoid a nondiagnostic scan because of venous extravasation or extensive streak artifact. If both upper extremities require evaluation, a central line should be used for contrast injection. Furthermore, the suspected location of vascular injury may affect the scan range. Proximal injuries, such as those from scapulothoracic dissociation, may only require evaluation of the upper arm to the level of the elbow. If more distal evaluation of the forearm, hand, or fingers is required, some advocate a two-part CTA protocol with different energies and fields of view (100–120kV for aortic arch to elbow, small field of view and 80–100kV for elbow to fingertips, when elbow is positioned above the head).
CTA findings of vascular injury in the upper and lower extremities requiring intervention include vascular occlusion, dissection, extravasation, transection, pseudoaneurysm, and arteriovenous fistula. Differential considerations include preexisting peripheral arterial atherosclerotic disease, nonocclusive vascular spasm, extrinsic compression from adjacent bone fragments or compartment syndrome, or acute embolic occlusion, such as from proximal aortic injury.
Vascular trauma requires prompt recognition and appropriate treatment to prevent significant mortality or morbidity. Today, MDCT is by far the most common technique by which these injuries are diagnosed following trauma. Since vascular injuries are uncommon, many radiologists might not feel adept at imaging them, recognizing them, and characterizing them. It is imperative that an arterial phase MDCT protocol be developed for use in high-risk patients, that intravenous contrast be used in all cases, and that suspicious imaging findings are conveyed to the trauma team appropriately and urgently. These patients may benefit from referral to a level 1 trauma center for definitive treatment.
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