Aetiology

Approximately 60% of large bowel obstructions are caused by an underlying colorectal malignancy.[1][3]​​ Diverticular strictures are a common benign cause, accounting for about 10% to 20% of obstructions.[4][13]​​​ Colonic volvulus (sigmoid or caecal) accounts for about 10% to 15% of patients with large bowel obstruction.[3][4]​ Sigmoid volvulus is more usually seen in frail or older patients.[2]​ Caecal volvulus is more rare and more commonly seen in younger patients.[1][2]​[14]​ 

In some regions, such as Africa, sigmoid volvulus accounts for up to 50% of patients with intestinal obstruction.[9]​[10]​[11]​[12]​​ Ileosigmoid knotting is a rarer variant of sigmoid volvulus. Other, rarer causes include hernias, other abdominal or pelvic malignancies, or endometriosis.

Pathophysiology

The colon proximal to the cause of mechanical obstruction (e.g., malignancy, colonic volvulus, benign stricture) dilates and, with increased colonic pressure, mesenteric blood flow is reduced producing mucosal oedema with transudation of fluid and electrolytes into the colonic lumen. This can produce dehydration and electrolyte imbalances. With progression, the arterial blood supply becomes jeopardised with mucosal ulceration, full thickness wall necrosis, and eventual perforation.[15] This provides conditions for bacterial translocation, which can produce septic complications.[16] The caecum is the usual site of rupture, as it has the largest diameter and is where the bowel wall is thinnest, resulting in faecal soilage of the peritoneal cavity, and sepsis.[1][17]​ An incompetent ileocaecal valve may allow for some decompression of the colon into the small intestine and thus delay the progression to ischaemia.

Colonic volvulus arises following axial rotation of the colon on its mesenteric attachments: the sigmoid colon is the most frequently affected segment (76%), then the caecum (22%).[9]​[18] Rotation can be clockwise or anti-clockwise. Once the volvulus has a 360° twist, then a closed loop obstruction is produced. Fluid and electrolyte shifts result from fluid secretion into the closed loop producing an increase in pressure and tension on the colonic wall that will eventually impair colonic blood supply. This results in ischaemia, necrosis, and perforation.

Classification

Clinical classification

No formal classification exists, but large bowel obstruction may be divided according to whether the obstruction has a true mechanical source or has an underlying functional basis (e.g., pseudo-obstruction). Most causes have a relatively rapid onset, although rarer causes may produce a more chronic picture, particularly with an underlying functional cause. Mechanical obstruction may be complete or partial.

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