Large bowel obstruction is a common diagnosis and accounts for approximately 25% of all bowel obstructions. Regardless of the cause, obstruction is mechanical in nature. They can range from partial obstructions to complete obstruction. Depending on the cause of the obstruction, surgical management may vary from immediate surgery to 2-3 days of bowel rest and watchful waiting. The most common cause of bowel obstruction in the US is adhesions, predominantly from previous surgery. Other important causes include volvulus, which in adults is concerning for underlying malignancy. In the elderly population, fecal impaction is potentially a source.

The typical symptoms of patients are lower abdominal pain, nausea, vomiting, and constipation/obstipation. Obstipation is unreliable as it may not be complete even in the setting of full obstruction as colonic flora distal to the obstruction may continue to make some amount of gas.

Imaging findings on radiograph typically include dilated loops of bowel filled with fluid, gas (may have multiple air-fluid levels in loops of bowel) and/or stool. The 3, 6, 9 rule is easy to remember for criteria for bowel obstruction. On radiograph:
Small bowel is considered dilated when it measures over 3 cm in diameter.
Colon is considered dilated when the diameter exceeds 6 cm.
The cecum is not considered dilated until 9 cm.

Cecum over 10 cm in diameter is at increased risk for rupture/perforation. It is important in the setting of dilated colon to assess the cecum as (according to Laplace's Law) it should be the most dilated in the setting of obstruction. If it is not dilated along with the distal portions of the colon, it is possible that the patient has pseudo-obstruction, which would be managed clinically in a completely different way. Additional findings include a transition point in dilation of the colon with stool and gas proximal to the obstruction and normal colon distally. Remember that haustra folds
are incomplete rings seen around the large bowel wall and these are what differentiates large bowel from small bowel (small bowel lines are complete rings around the bowel).

Another sign of obstruction is lack of visualized bowel in the abdomen. This is termed paucity of gas and it results from lack of interface between different radiographic densities such as air and fluid. This will, however, be picked up on CT.

Once a large bowel obstruction is suggested on radiograph, CT may be indicated to find the source of the obstruction and guide clinical management. The differential diagnosis here is extensive and management will vary depending on what is the cause.