The specific etiology of MCO is usually more definitive (Table 8.3). The most common cause is colorectal cancer, accounting for 33–60 % of mechanical obstructions [1, 12–14], with three-quarters of these cancers being adenocarcinomas [4]. Overall, 10–30 % of colorectal cancer patients will develop obstruction [1, 4, 15]. Volvulus causes about 10–15 % of obstructions and chronic diverticular disease (abscess and stricture) accounts for 10 % [15]. In addition to primary colorectal cancers, metastatic tumors to the abdomen, including ovarian and uterine cancers, can lead to extrinsic compression of the colonic lumen [14]. Benign strictures due to ischemia, diverticular disease, diverticulitis and inflammatory bowel disease (secondary to acute inflammation or chronic strictures), nonsteroidal anti-inflammatory agents (NSAIDs), and high-dose pancreatic enzymes can cause MCO [12, 15–19]. Intussusception, adhesions, hernia, fecal impaction, and endometriosis are less frequent causes [12, 15, 20]. Very rarely, infectious sources, including Actinomyces, Taenia saginata, botulism, and Salmonella, have been reported to cause mechanical colonic obstruction [15].

Radiographic imaging is . critical in the diagnosis and management of either type of colonic obstruction. In ACPO, plain abdominal X-ray typically reveals massive gas-filled dilation of colon without air-fluid levels and little or no small bowel dilation (Figs. 8.1 and 8.2). Stool and gas can be seen distal to the dilated segment since a mechanical obstruction is not present [22]. Careful attention should be paid to the amount of stool in the rectal vault to exclude distal stool impaction, which would be managed differently than ACPO. The cecum and right colon are usually the sites showing dilation of the largest diameter, averaging 10–16 cm on radiographs and conferring the highest risk of perforation due to Laplace’s law [5, 23]. In MCO, plain abdominal films reveal dilation proximal to the obstruction with air-fluid levels in the colon and small bowel. Distal to the obstruction, the colon is decompressed and devoid of stool and air [12]. Cecal volvulus typically displays a markedly distended loop of large bowel extending from the right lower quadrant to the epigastrium or left upper abdomen (Fig. 8.3). Sigmoid volvulus can present with an inverted-U or a coffee bean shape on X-ray due to massive dilation (Fig. 8.4) [24]. Upright abdominal and chest films are more useful than supine films in determining if free air due to perforation is present. If bowel ischemia is present, plain films may reveal thumbprinting due to mucosal edema and submucosal hemorrhage [5].

Unfortunately, plain radiographs have poor sensitivity in diagnosing colonic obstruction. In a study of 120 patients, the sensitivity was only 33 % while the specificity was 100 %; subsequent CT imaging increased the sensitivity to 67 % [25]. In another series of 140 cases, plain abdominal X-ray alone had an 84 % sensitivity and 72 % specificity for colonic obstruction [15]. Plain abdominal imaging may also not be reliable in differentiating between ACPO and MCO. In another series, 30 % of patients diagnosed with MCO on plain X-ray actually had ACPO, whereas 20 % of those diagnosed with ACPO had mechanical obstruction [15].

CT with oral or rectal contrast is advised in all suspected cases to differentiate ACPO from mechanical obstruction and to assess for evidence of complications (Fig. 8.5). Contrast CT studies have the added ability to characterize bowel mucosa for signs of ischemia or perforation [3]. Water-soluble contrast enema is preferred over barium enema due to the risk of barium impaction at the site of obstruction and barium peritonitis if perforation is present [4]. CT findings that are characteristic of ACPO include preserved haustral markings and luminal dilation in the absence of an obstructive lesion [23]. If mechanical colonic obstruction is present, the source is very likely to be seen on these studies (Fig. 8.6). Volvulus can be diagnosed by the presence of a bird’s beak pattern on contrast studies. On CT imaging, sigmoid volvulus is characterized by limbs of the twisted loop converging toward a fulcrum point, which appears as a “whirl sign” when the view plane is orthogonal to the rotation axis of the loop. In most cases, the whirl sign is found in the left lower abdomen with a craniocaudal axis (Fig. 8.7). The rectum and the upstream colon are usually flat, whereas the twisted loop is highly distended and located in the anterior part of the abdomen. Cecal volvulus is the torsion of a mobile cecum around its own mesentery, which often results in a closed-loop obstruction; twisted terminal ileum, distended cecum, and twisted ascending colon are seen. Cecal volvulus may occur by three mechanisms: type 1 develops from clockwise axial torsion or twisting of the cecum around its mesentery; type 2 loop volvulus develops from counterclockwise axial torsion of the cecum around its mesentery; and type 3 or cecal bascule involves upward folding of the cecum as opposed to axial twisting [24]. In most cases of cecal volvulus, the whirl sign is found in the right part of the abdomen with a lateral or an anteroposterior axis (Fig. 8.8). Pneumatosis or gas in mesenteric veins in concert with bowel wall thickening strongly suggests that bowel infarction has occurred [5, 24].

Initial management of . ACPO and MCO is conservative unless there is significant concern for present or impending complications. Endoscopic interventions are central to the management of both MCO and ACPO and are detailed in a separate chapter. ACPO can often be managed conservatively, with reported success rates ranging widely from 20 to 92 % [2–4, 8]. Cases unresponsive to conservative measures after 24–48 h, symptom duration more than 3–4 days, and colonic diameter more than 10–12 cm warrant further treatment [8]. MCO can also be managed conservatively for a short time interval while preparing for more definitive endoscopic or surgical therapy. Close monitoring with serial abdominal examination and plain abdominal radiographs obtained every 12–24 h should be performed to monitor for peritoneal signs suggestive of ischemia or impending perforation while conservative measures are being instituted [2].