Bowel and mesenteric injuries in blunt abdominal injury

INTRODUCTION Injuries to the bowel and mesentery are a subset of blunt abdominal injuries that occur infrequently, affecting approximately 5% of patients who suffer severe abdominal trauma.[1] They are the most frequently missed injuries in abdominal trauma, which can be attributed to the varied anatomical configuration and length of the bowel. The long, convoluted configuration of suboptimally distended bowel and the variable, irregular shape of mesentery make it challenging to evaluate them wholly and in a structured manner. The failure to detect subtle signs suggestive of bowel and mesenteric injuries may lead to delayed diagnosis, which results in increased mortality. In our institution, high-velocity road traffic accidents are the predominant (53%) cause of significant bowel and mesenteric injuries. Management of these injuries in our institution is often determined by computed tomography (CT) imaging findings and the patient's clinical status, with surgical exploration reserved for significant mesenteric bleeds and perforated bowel injuries. Conservative management is adopted in the absence of severe traumatic injuries on CT in a haemodynamically stable patient, with interval CT imaging performed in intervals of 8–12 h in selected patients to assess for interval changes. For isolated mesenteric arterial injuries without significant bowel injury, minimally invasive transcatheter angioembolisation is the treatment of choice due to its high success rate and quick recovery. The CT findings in bowel and mesenteric injuries can be subtle and are thus missed by radiologists with less experience. In this article, we present and discuss a spectrum of CT features of blunt bowel and mesenteric injuries, ranging from conspicuous signs to subtle and indirect signs. We also share some imaging pearls in the evaluation of bowel and mesenteries injuries. MULTIPHASIC COMPUTED TOMOGRAPHY Multidetector CT is the imaging modality of choice for evaluation of blunt abdominal trauma in haemodynamically stable patients. The CT scanner is a fundamental and indispensable imaging modality in modern tertiary institutions and can rapidly acquire cross-sectional images of high spatial resolution for evaluation of a polytrauma patient. Computed tomography has a sensitivity of 69%–95% and a specificity of 94%–100% for bowel injury.[2,3,4] Our institution's trauma imaging protocol has a two-phase, dual-pass CT protocol, which comprises an 'arterial' phase (25–35 s) scan followed by a 'porto-venous' phase (60–70 s) scan. Preliminary images are reviewed by the radiology resident, and delayed phase (3–5 min postcontrast injection) is performed if there is suggestion of active bleed. Oral contrast is not routinely used, as it delays the scan, has questionable utility and runs the risk of aspiration in a severely injured patient. Studies have shown that omission of oral contrast does not affect the accuracy of detection of bowel injury on CT.[5] An alternative single-pass, biphasic injection protocol (also known as the 'Camp Bastion protocol') can be used for trauma imaging. This method was devised and proven to facilitate imaging in the military setting and subsequently adopted for civilian use.[6,7] It comprises an initial bolus of contrast for the porto-venous phase and a second bolus of contrast for the arterial phase, with a single acquisition at approximately 60 s. The advantages include lower radiation dose, shorter acquisition time and fewer images to review. The drawback is that detection of active bleeding and small pseudoaneurysms is more difficult than in conventional dual-phase imaging.[7] COMMON SITES OF INJURY Understanding the mechanism of injury can be useful in predicting the site of injury. Mechanisms include direct crush injuries (anterior–posterior compression), acceleration–deceleration shear injuries and burst injuries from rapidly increased intraluminal pressure[8] [Figure 1]. Direct crush injuries result in bowel contusions and perforation at the site of trauma. Acceleration–deceleration injuries cause bowel/mesentery tears and avulsions, specifically at the sites of fixation, where shearing forces occur during rapid deceleration [Figure 2] in high-velocity road traffic accidents.[9]Figure 1: Mechanism of blunt bowel injuries.Figure 2: Coronal CT image (porto-venous phase) shows mesenteric haematoma (arrow) and free gas (arrowhead) located in the sigmoid mesentery, just adjacent to the mid-sigmoid colon. Intraoperative finding shows sigmoid colon perforation.COMPUTED TOMOGRAPHY FINDINGS OF BLUNT BOWEL AND MESENTERIC INJURIES Specific and direct signs of bowel injury Bowel wall defect A defect in the bowel wall is the most specific sign of bowel injury.[10] However, this imaging finding is rarely seen on CT as the bowel needs to be distended to appreciate a mural defect. A perforated bowel would typically collapse because the surrounding bowel wall closes in around a defect in the wall. This explains the low sensitivity of this imaging finding of only up to 22%.[11] A defect in the bowel wall more commonly manifests as bowel wall irregularity, thickening or nonenhancement, which will be covered subsequently. Extraluminal gas The presence of extraluminal gas is a specific sign of bowel injury and has a reported specificity of 99%–100% and sensitivity of 32%–44%.[12,13] Intraluminal gas enters the peritoneal space via a traumatic bowel defect and remains in the peritoneal space, which can readily be detected on CT scan lung window. The ease of detecting extraluminal gas makes this imaging finding more common than visualising the bowel wall defect, with 47% of patients with bowel injury presenting with extraluminal gas [Table 1]. In some cases, the presence of a small pocket of extraluminal gas may be the only sign of bowel injury and serves as an important clue in locating the site of perforation [Figure 3].Table 1: Proportion of CT findings of mesenteric and bowel injuries (N=30).Figure 3: Coronal CT image (porto-venous phase) shows focal jejunal mural thickening (arrowhead) with adjacent pockets of extraluminal gas (arrow). Intraoperative finding shows jejunal haematoma with focal full-thickness tear and perforation.Specific and direct signs of mesenteric injury Significant mesenteric vascular injuries Mesenteric lacerations manifest as contrast extravasation and/or pseudoaneurysm formation on CT, where the involved vessels have a beaded or truncated appearance[14] [Figure 4]. A pseudoaneurysm is a full-thickness defect of the vascular wall, with the escaping blood encased by surrounding soft tissues, giving it a saccular appearance. Pseudoaneurysms are isodense to the blood pool and retain their shape in all phases. Contrast extravasation is synonymous with active bleeding (100% specificity)[14] and is depicted by contrast pooling, which becomes less dense in the venous and/or delayed phases. Thirty percent of our patients with mesenteric injuries presented with pseudoaneurysm and/or contrast extravasation [Figures 5 and 6, Table 1], which require immediate surgical or interventional radiological management.Figure 4: (a) Axial and (b) coronal CT images (arterial phase) show beaded appearance of mesenteric vessel (arrows) and haemoperitoneum (arrowhead). Intraoperative finding shows mesenteric laceration and haematoma.Figure 5: (a) Axial CT image (arterial phase) shows mesenteric haematoma surrounding a focal arterial blush, suspicious for a pseudoaneurysm (arrow). (b) Angiogram image of the selective catheter angiography of the superior mesenteric artery confirms the finding of a pseudoaneurysm (arrows). This was treated with coil embolisation.Figure 6: Axial CT images show mesenteric laceration with active bleed and progressive pooling of extravascular contrast on the (a) arterial and (b) porto-venous phases (arrows).Subtle and indirect signs of bowel and mesenteric injuries Abdominal wall bruising The presence of abdominal wall bruising is associated with more than 65% of intra-abdominal injuries, as compared to 8% when no bruise is present.[15] This happens as external traumatic energy is more likely to cause injury to the abdominal wall before reaching the intra-abdominal organs. Hence, the radiologist needs to look out for bowel and mesenteric injuries (and other solid organ injuries) in the presence of this sign. Focal bowel wall thickening Detection of focal bowel wall thickening can be challenging due to the variable extent of bowel loop distension. Focal bowel wall thickening in bowel injury may mimic collapsed or constricted bowel secondary to peristalsis. An adjunct finding in detecting bowel wall injury is the presence of fat stranding adjacent to the thickened bowel wall. It has been suggested that focal bowel wall thickness of at least 3–4 mm is suspicious for bowel injury.[11] In our study, 27 percent of patients who had experienced bowel injury had focal bowel wall thickening [Figures 7 and 8, Table 1].Figure 7: Axial CT image (porto-venous phase) shows a localised segment of mid-jejunal mural oedema and thickening (circle). Intraoperative finding showed partial-thickness jejunal laceration.Figure 8: Axial CT image (delayed phase) shows a segment of slightly hyperdense jejunal mural thickening (circle), suspicious for intramural haematoma. This finding may be subtle and difficult to appreciate when the bowel is collapsed. The small amount of adjacent mesenteric fat stranding in this case is a very helpful adjunct finding.Abnormal bowel enhancement In the context of blunt abdominal injury, CT finding of irregular enhancement of bowel wall is suggestive of full-thickness tear.[16] This occurs as transection of arterial vessels interrupts the flow of blood to the bowel tissue, which results in hypoenhancement. Although this imaging feature of bowel ischaemia has a high specificity of 96%, it has a moderate sensitivity of 62%.[17] On the other end of this imaging spectrum, bowel hyperenhancement can happen in areas of reperfusion injury due to reactive hyperaemia or haemorrhage.[18] In mesenteric congestion, downstream occlusion of venous outflow results in severe oedema of the bowel wall, which can show a targetoid pattern of enhancement.[19] Twenty percent of patients had abnormal bowel enhancement, and two of these patients had hypoenhancing bowel [Figure 9, Table 1].Figure 9: Coronal CT image (porto-venous phase) show (a) hyperenhancing bowel loop proximal to the site of bowel transection (arrow) and (b) hypoenhancing bowel loop distally (arrowhead).Mesenteric fluid and stranding The presence of high-density fluid or ill-defined stranding in the mesentery is likely secondary to mesenteric laceration or bowel injury. It is known to be a sensitive but nonspecific sign.[13,14] The presence of high-density fluid in the mesentery without associated solid organ injury is suggestive of underlying bowel or mesenteric injury.[16] The adjacent bowel will have to be scrutinised carefully for any abnormal bowel wall thickening/enhancement [Figure 9]. Mesenteric haematoma Mesenteric haematoma, defined as "a well-defined high-density collection within the mesentery", indicates mesenteric laceration. It often has a triangular-shaped appearance within the interloop of bowel.[20] In the absence of active contrast extravasation or pseudoaneurysm, patients may not need to undergo urgent surgical exploration. In our study, eight (27%) patients had mesenteric haematoma [Figures 10 and 11, Table 1].Figure 10: Coronal CT image (porto-venous phase) shows haemoperitoneum with a triangular area of increased density (around 60 HU), that is, the 'sentinel clot' (arrow), localised to the right iliac fossa. The bowel loop adjacent to the clot is hypoenhancing (arrowhead). Intraoperative finding shows distal ileal mesenteric laceration with associated serosal tear of the distal ileum.Figure 11: Axial CT image (porto-venous phase) shows small mesenteric haematoma (around 40 HU) in the ileal mesentery (arrow). This is very subtle and missed on initial review. Postmortem finding shows mesenteric laceration.Caveat for bowel and mesenteric injuries Extraluminal free gas Pneumothorax, pneumomediastinum, diaphragmatic rupture and mechanical ventilation are some of the causes of extraluminal free gas, mimicking bowel perforation[14,20] [Figure 12]. However, in the context of trauma, free intraperitoneal gas should prompt the radiologist to search carefully for other associated signs of bowel injury.Figure 12: (a) Axial CT image (arterial phase) shows left retroperitoneal gas (arrow) likely secondary to the left-sided pneumothorax due to left-sided rib fractures (not shown). Left flank subcutaneous emphysema (arrowhead) and mesenteric stranding (circle). Intraoperative finding showed jejunal contusion. (b) Coronal CT image (porto-venous phase) shows stranding in the mesentery (circle).IMAGING PEARLS Some of the imaging findings of blunt abdominal trauma can be subtle and may be overlooked by the radiology trainees, as they are under time pressure to review the scans in an emergency setting. We list five imaging pearls to serve as a guide for junior radiologists. Screen 'high-risk' areas such as the ligament of Trietz, ileocaecal junction and sigmoid colon. Utilise lung or bone window to look for small gas pockets, as this can be subtle in soft tissue window. In the presence of extraluminal air or free fluid, scrutinise adjacent bowel loops carefully for features of injury. Location of free fluid or extraluminal gas is suggestive of the site of injury.[14] Utilise a narrow window setting and measure the Hounsfield units to detect a small amount of haemoperitoneum or sentinel clot, which can be subtle on soft tissue window.[15] Pay particular attention to the delayed phase when there is a focus of hyperattenuating fluid in the mesentery in earlier phases. If the hyperattenuating focus enlarges or pools in the delayed phases, it suggests active bleeding in the mesentery. CONCLUSION The CT findings of significant blunt bowel and mesenteric injuries range from obvious specific signs to those that are subtle and easily missed. Delayed diagnosis of bowel and mesenteric injuries may lead to significant bowel-related morbidity or even mortality. It is, therefore, important for the radiologist to recognise the spectrum of imaging findings, so as to ensure timely and appropriate interventions. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest. SMC CATEGORY 3B CME PROGRAMME Online Quiz: https://www.sma.org.sg/cme-programme Deadline for submission: 6 pm, 03 July 2024