When a patient presents with acute onset of pain, diffuse tenderness to palpation and free air on imaging the diagnosis of a perforated small bowel is easily entertained; however, the underlying cause is often more elusive. Broad etiologies should be considered first. Bowel obstruction and inflammatory processes are the most common cause of small bowel perforation in developed countries, while infectious etiologies are the most common worldwide.

It is important to emphasize that oftentimes the precise cause may not be known, but this should not delay definitive therapy which usually includes prompt surgical intervention. Axiomatically, time should not be wasted obtaining other imaging and laboratory studies that will not change management.

Because the natural history of intestinal perforation leads to peritonitis, it is not common to require extensive imaging before laparotomy. However, given the acute nature of most presentations of bowel perforation, there are instances where preoperative imaging may be helpful. The erect chest X-ray radiograph is best to demonstrate free air under the diaphragm, which, in the absence of a secondary contributing cause, is an indication for surgical exploration (Fig. 22.1). Abdominal radiographs may show free air as well. A left lateral decubitus radiograph can be helpful to distinguish from any air that may be seen in the stomach and could be helpful if other films are equivocal. In obtunded or immobilized patients, a chest X-ray obtained in the reverse Trendelenburg position may closely resemble an upright projection. Many patients with non-specific abdominal pain may have cross-sectional imaging of the abdomen and pelvis in the ED prior to surgical consultation, which may clearly yield a diagnosis of pneumoperitoneum. While the advantage of a preoperative CT scan is that it may also indicate the location or underlying pathology of the perforation, it is often unnecessary as it may waste valuable time. If the patient has received oral contrast, extraluminal contrast can be seen in addition to air. Depending on the acuity of the perforation, the patient may or may not have peritonitis. If peritoneal signs are present, they can aid in the confirmation of perforated viscus [1, 2].