Ultrasound of the Week: Bowel Perforation
Author: Mark Perkowski, MS4
Edited by: Chris Parker, DO and Molly Hartrich, MD
A 58-year-old woman with no significant past medical history presented to the emergency department (ED) with left lower quadrant abdominal pain. She reported intermittent abdominal pain for the past three weeks, which suddenly worsened four hours prior to arrival. Her last bowel movement was earlier that morning. She denied a history of cancer or abdominal surgeries. She denied fever, chills, nausea, vomiting, hematochezia or melena.
The patient was initially saturating 93% on room air, which increased to 100% on 3 liters nasal cannula. Except for mild tachycardia, other vital signs were within normal limits. On exam, the abdomen was severely distended with left lower quadrant tenderness with palpation and percussion. Labs showed a lactate of 5.1 but no leukocytosis and was otherwise unremarkable. A point-of-care ultrasound (POCUS) was performed, confirming the diagnosis.
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Bedside ultrasound of abdomen showed dilated bowel loop with a diameter measurement greater than the 2.5 cm cutoff, concerning for small bowel obstruction (SBO; Figure 1). The presence of echogenic stripes between the peritoneum, abdominal wall, and intra-abdominal organs with intraluminal reverberation artifact suggests pneumoperitoneum (Figure 2; Figure 3). There was no free intra-abdominal fluid. Abdominal x-ray revealed free air under the diaphragm (Figure 4), supporting the diagnosis of perforated viscus.
Figure 1. Bedside ultrasound of the left lower quadrant showing a dilated bowel loop, with a diameter measurement greater than the 2.5 cm, raising concern for a small bowel obstruction. Pockets of echogenicity surrounding the bowel with associated shadowing suggests pneumoperitoneum, limited abdominal view.
Figure 2. Bedside ultrasound demonstrating enhanced peritoneal stripe sign (EPSS) of the right upper quadrant (red arrow). The presence of echogenic stripes between the peritoneum and liver suggests pneumoperitoneum, limited abdominal view.
Figure 3. Bedside ultrasound demonstrating reverberation artifact, or “A-lines in the abdomen” (red arrow), suggests pneumoperitoneum, limited abdominal view. This clip was taken during a different case but provides an excellent example of the reverberation artifact.
Figure 4. Abdominal x-ray showing free air underneath the diaphragm (white arrow), anterior-posterior view.
PEARLS & CONCLUSION
A perforated viscus, also known as bowel perforation, is a full-thickness disruption of the intestinal wall, which results in peritoneal structures being exposed to gastrointestinal contents and can be a life-threatening cause of sepsis. There are four basic mechanisms that lead to bowel perforation, including erosion (e.g., malignancy, ulcerative disease), infection (e.g., diverticulitis, appendicitis), ischemia (e.g., bowel obstruction), and physical disruption (e.g., trauma, iatrogenic injuries).  The prevalence of bowel perforation varies with etiology, but patient outcomes can be devastating with significant morbidity and mortality. Patients with diffuse peritonitis due to bowel perforation have a mortality rate between 8.8% to 15%, depending on the location and etiology of the perforation.  It is crucial to rapidly diagnose and treat these patients to improve outcomes.
The gold standard for diagnosis is computer tomography (CT). Chest x-ray is used to rapidly diagnose free air underneath the diaphragm. However, POCUS is more sensitive than radiography for identifying pneumoperitoneum, at 92% versus 78% respectively, with both demonstrating a similar specificity of 53%.  POCUS is useful in the assessment of pneumoperitoneum and is valuable in the ED setting in developing a differential diagnosis and guiding clinical decisions, such as expediting advanced imaging, treatment, and consultation.
A perforated viscus can cause pneumoperitoneum, which can have the following findings on ultrasound:
Enhanced peritoneal stripe sign (EPSS): Typically, the transition between abdominal wall and peritoneal fluid or tissue causes a “peritoneal stripe,” which is more echogenic than its surroundings (Figure 2). In areas where air accumulates with pneumoperitoneum, ultrasound waves are interrupted and scattered before reaching deeper structures. This causes high-amplitude linear echoes, which further enhance the peritoneal stripe. [1,2] EPSS is most clearly visualized adjacent to intra-abdominal solid organs (especially the liver).
Reverberation artifact (“A-lines in the abdomen”): The same phenomenon which causes A-lines in healthy lung can cause reverberatory artifact to appear due to the interruption of the normal peritoneal fluid-tissue or tissue-tissue interface (Figure 6).  The presence of gas within healthy bowel can also cause this finding, so caution must be exercised. These artifacts are especially visible on the attempted imaging of the bladder in the present case (Figure 5).
Figure 5. Bedside ultrasound demonstrating reverberation artifact, or “A-lines in the abdomen” (red arrow). There is an impedance mismatch between air and surrounding tissue that leads to decreased visualization of surrounding structures.
Figure 6. Bedside ultrasound demonstrating enhanced peritoneal stripe sign (EPSS) with associated reverberation artifacts (white arrows), or “comet-tail” artifact. The curved arrow indicates a calculus in the gallbladder. 
“Comet-tail” reverberatory artifacts: A consequence of the highly reflective interruption of air between the peritoneal fluid-tissue or tissue-tissue interfaces. The finding is similar to a reverberation artifact, but the sound waves become trapped in this area and are delayed in returning to the probe, leading to attenuation and a rapidly fading “comet-tail” (Figure 6). 
Studies have shown various dynamic techniques for the visualization of pneumoperitoneum with POCUS. The “scissor maneuver” is a dynamic ultrasound maneuver that involves increasing pressure on one end of the probe and then releasing it while keeping contact with the skin. This results in the reverberation artifact decreasing and increasing, respectively, as intraperitoneal air escapes from under the probe and then returns (Video 1). This technique is most effective when scanning anterior to the liver, where air is directly trapped between the liver and anterior abdominal wall.  The “shifting gas artifact” involves shifting the patient into a 45-degree left lateral decubitus position and observing the appearance of reverberation artifact in the right paramedian view (liver and gallbladder), which is typically not present when the patient is supine (Figure 7). This also can help differentiate air in the bowel (a possible normal finding) versus intraperitoneal air. 
Video 1. Bedside ultrasound demonstrating “scissor maneuver” in a patient with pneumoperitoneum. Pressure is initially applied, and free air is displaced, allowing for visualization of the bowels. Notable is the appearance of reverberation and “comet-tail” artifact, limited abdominal view.
General surgery and colorectal surgery were immediately consulted. CT abdomen/pelvis was performed, and patient was then taken directly from the CT scanner to the operating room for exploratory laparotomy and flexible sigmoidoscopy. The patient was found to have a perforation of the transverse colon as well as a cecal ulcer and a sigmoid stricture. The patient had feculent peritonitis and required a subtotal colectomy.
Indiran, V., Kumar, R. V., & Jefferson, B. (2018). Enhanced peritoneal stripe sign. Abdominal Radiology, 43, 3518-3519.