Applied Radiology

Dr. Melotti is with the Department of Diagnostic Radiology, University of Maryland Medical Center and Maryland Shock-Trauma Center, Baltimore, MD.

Although not a common sequela of blunt abdominal trauma, bowel and mesenteric injury can be a major cause of early morbidity and mortality if not detected quickly. ^1,2 Bowel and mesenteric injuries occur in approximately 5% of patients following blunt abdominal trauma, the most common mechanism of injury is motor vehicle accidents. ³ Typical locations of injury include the fixed segments of bowel that undergo shearing forces, such as the retroperitoneal duodenal-jejunal junction at the ligament of Treitz and the terminal ileum. ^1,4 The bowel can also sustain injury by compression if caught between the spine and an object such as the steering wheel or seat belt. ^1,4 Due to the nature of such accidents, many patients have more apparent multi-system injuries that can initially mask the diagnosis.

Clinical findings

A reliable history and physical examination are often difficult to obtain in the trauma patient due to distracting injuries, neurologic injury, altered sensorium, or impairment of respiratory function necessitating ventilatory support. The initial physical examination may yield benign results as peritoneal signs may be delayed. The classic clinical triad of rigidity, absent or decreased bowel sounds, and abdominal pain is seen in approximately one-third of patients with bowel and mesenteric injury. ⁵ The presence of an anterior abdominal hematoma or ecchymosis, also known as the "seat-belt sign," may be apparent and suggests a significant impact, although it is not highly correlated with bowel injury ⁶ (figure 1). Hypotension and tachycardia can suggest intra-abdominal bleeding but are not specific for bowel or mesenteric injury, ⁷ and clinical findings can be confounded by injury to other abdominal organs.

Diagnostic evaluation

The timely diagnosis of bowel and/or mesentery injury is vital since a significant delay can lead to peritonitis, sepsis, or continued hemorrhage. Peritoneal lavage can be helpful in diagnosing bowel and mesenteric injury in that it is sensitive for intraperitoneal hemorrhage; however, lavage findings are nonspecific for determining which organ is injured and the extent of injury. ⁸ The presence of bowel contents obtained by lavage is specific for bowel perforation, although this is only detected in 30% of surgically verified bowel perforations. ⁵ An increased white blood cell count in the peritoneal lavage fluid also suggests full-thickness bowel injury, but may not be present until several hours after injury. A drawback of diagnostic peritoneal lavage is that retroperitoneal bowel injuries will not be detected, and the use of lavage limits the ability to diagnose bowel injury with CT since fluid and often air are introduced into the peritoneal cavity.

Although sonography of the abdomen can be used to detect intraperitoneal free fluid, it cannot specify the injured organ. In the majority of patients with isolated bowel and mesenteric injury, intraperitoneal free fluid is often not seen by sonography, thus limiting the diagnostic sensitivity of this modality. ⁹

CT diagnosis

There are many studies in the radiologic and surgical literature that support ^2,10,11 and refute ^12,13 the accuracy of CT in diagnosing bowel and/or mesenteric injury in the blunt trauma patient. Despite continued controversy, CT has increasingly replaced peritoneal lavage as the diagnostic study of choice in many trauma centers. ¹⁴ This is in part due to the widespread use and availability of helical CT in many centers as well as the increased recognition and understanding of CT signs of bowel and mesenteric injury by interpreting radiologists. Scans are typically performed using oral and intravenous iodinated contrast, and preferably prior to the performance of peritoneal lavage.

CT findings specific for full-thickness bowel injury include free intraperitoneal air (without an obvious source such as previous peritoneal lavage or dissection of thoracic air) and extravasated oral contrast. ⁵ In a study by Mirvis et al, ¹⁵ CT findings found to be diagnostic of full-thickness bowel perforation include pneumoperitoneum without an alternate etiology; intramesenteric, intramural, or retroperitoneal free air; direct discontinuity of the bowel wall; and extravasation of oral contrast (figure 2).

Findings suggestive of bowel injury include intraperitoneal or anterior pararenal fluid without CT evidence of a specific abdominal organ injury, and thickened bowel wall or bowel wall hematoma. ¹⁵ Abnormal enhancement of a segment of bowel wall may indicate bowel injury, but is not specific. ^5,8

CT findings indicating mesenteric injury associated with bowel injury include triangular-shaped collections of free fluid between the leaves of the mesentery (mesenteric triangles) or infiltration within the mesenteric fat of fluid or hematoma ⁵ (figure 5). between bowel loops and mesenteric fat infiltration or hematoma ⁵ (figure 3).It is important to note that hemoperitoneum secondary to solid abdominal organ injury does not commonly infiltrate the mesentery ^5,16 or appear as triangular-shaped collections between the mesenteric folds, thus lending greater specificity to this CT finding. Tiny gas bubbles in the leaves of the mesentery can also be seen with bowel injury. ¹⁶ A careful search of the abdominal and pelvic CT images is needed to exclude small amounts of pneumoperitoneum. Common sites of air accumulation include the anterior peritoneal space and porta hepatis region. Review of images in lung or bone windows facilitates detection of minimal amounts of free air (figure 4).

CT findings can be categorized as those associated with a need for immediate surgery and those that may be observed. CT findings sensitive for surgical bowel injury include extraluminal gas and free fluid. Specific findings included extraluminal oral contrast and extraluminal gas.

In one of the largest studies to date, Killeen ¹⁷ evaluated not only the accuracy of helical CT for detecting bowel and mesenteric injury, but also its accuracy in determining which patients required surgical management. CT was found to be 88% accurate in detecting bowel injury, and 88% accurate in determining surgical versus nonsurgical cases.

In diagnosing mesenteric injury, specific findings include active mesenteric bleeding, bowel wall thickening associated with mesenteric hematoma, and mesenteric hematoma alone ¹⁷ (figure 5). Mesenteric infiltration and free fluid suggest injury to the mesentery, although these are not diagnostic findings for injuries requiring surgical repair.

In the study by Killeen, ¹⁷ CT was found to be 96% accurate in diagnosing mesenteric injury and 75% accurate in determining surgical versus nonsurgical cases. CT findings found to be sensitive for surgical mesenteric injury are free intraperitoneal fluid and mesenteric hematoma; specific findings include active bleeding and bowel wall thickening associated with a mesenteric hematoma (figure 6). These findings are similar to those of a study of mesenteric injury by Dowe et al. ¹⁸ Patients with CT findings of mesenteric bleeding or bowel wall thickening associated with mesenteric hematoma or infiltration were found to have a high incidence of mesenteric or bowel injury requiring surgery. Focal mesenteric hematoma or infiltration alone was found to be nonspecific, occurring in both bowel and mesenteric injuries requiring surgery and those that did not.

Mimics and masks of bowel and mesenteric injury

Certain pathologic processes can either mask or mimic the CT findings of bowel and mesenteric injury. Prolonged shock can produce relatively diffuse bowel wall thickening ¹⁹ and obscure bowel wall contusion (figure 7). Shock bowel tends to produce diffuse small bowel wall thickening associated with mesenteric edema, whereas blunt trauma to the bowel is more likely to manifest as focal or localized wall thickening (figure 8). Increased central venous pressure (CVP) can cause periportal low density on CT, bowel wall and mesenteric edema, free peritoneal fluid, and retroperitoneal edema, ²⁰ thus mimicking the intraperitoneal fluid, mesenteric edema, and bowel wall thickening seen with primary bowel injury. Again, the changes occurring with elevation of CVP are diffuse in extent rather than focal as usually occurs with bowel injury (figure 9). Signs of shock bowel, increased CVP, and bowel injury can coexist.

Conclusion

Despite its relative rarity, bowel and mesenteric injury is a major cause of morbidity and mortality if not diagnosed in a timely manner. ⁴ Controversy remains in the radiologic and surgical literature concerning the diagnostic accuracy of CT in diagnosing these injuries. Widespread availability of helical CT at emergency centers and increasing familiarity by radiologists with diagnostic and suspicious CT findings should lead to increased confidence and reliance on this diagnostic modality. The increasing adoption of nonoperative management of liver and spleen injuries requires high confidence in the exclusion of surgical bowel and mesenteric injury by CT. Further data from large, prospective studies should increase the confident reliance on CT as an expeditious and accurate diagnostic study for bowel and mesenteric injuries in the trauma patient. AR

References

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2. Janzen DL, Zwirewich DJ, Breen DJ, Nagy A: Diagnostic accuracy of helical CT for detection of blunt bowel and mesenteric injuries. Clin Radiol 53:193-197, 1998.

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4. Mirvis SE, Young JWR: Imaging in Trauma and Critical Care, pp. 191-197. Baltimore. Williams & Wilkins, 1992.

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6. Porter RS, Zhao N: Patterns of injury in belted and unbelted individuals presenting to a trauma center after motor vehicle crash: Seat belt syndrome revisted. Ann Emerg Med 32:418-424, 1998.

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14. Allen GS, Moore FA, Cox CS Jr., et al: Hollow visceral injury and blunt trauma. J Trauma 45:69-78, 1998.

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17. Killeen KL: Accuracy of helical computed tomography in the detection of bowel and mesenteric injuries following blunt abdominal trauma. Presented at the 85th Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, IL, November 28- December 3, 1999.

18. Dowe MF, Shanmuganathan K, Mirvis SE, et al: CT findings of mesenteric injury after blunt trauma: implications for surgical intervention. AJR 168:425-428, 1997.

19. Mirvis SE, Shanmuganathan K, Erb R: Diffuse small-bowel ischemia in hypotensive adults after blunt trauma (shock bowel): CT findings and clinical significance. AJR 163:1375-1379, 1994.

20. Shanmuganathan K, Mirvis SE, Amoroso M: Periportal low density on CT in patients with blunt abdominal trauma: Association with elevated venous pressure. AJR 160:279-283, 1993.