Bowel obstruction

Bowel obstructions are common and account for 20% of admissions with "surgical abdomens". Radiology is important in confirming the diagnosis and identifying the underlying cause.

Bowel obstructions are usually divided according to where the obstruction occurs, and since imaging appearances, underlying pathology, and treatment differ, these are discussed separately:

• small bowel obstruction
• large bowel obstruction

Clinical presentation

The classic clinical features of bowel obstruction are:

• colicky abdominal pain
• vomiting
• abdominal distension
• absolute constipation

The order and timescale in which these appear to vary depending on whether there is large or small bowel obstruction. Absolute constipation and pain are more prominent early on in large bowel obstruction while vomiting is the predominant early feature for small bowel obstruction.

Additional features are dependant on the exact underlying pathology.

Radiographic features

Imaging plays an important role in both diagnosing bowel obstruction as well as helping determine the choice and timing of appropriate management. The main aims of imaging in cases of suspected bowel obstruction are:

• differentiate true mechanical obstruction from ileus or constipation
• localise the site of obstruction
• identify an underlying cause
• assess for complications (e.g. ischaemia or perforation)
• assess the viability of bowel segments involved

Plain radiograph

Plain abdominal radiographs are used as a screening tool for bowel obstruction but do not exclude this if they are normal. The main findings include dilated bowel loops (see 3-6-9 rule) with or without the presence of fluid levels. Erect chest radiographs can also be used to screen for complicating perforation.

CT

Plain radiographs are increasingly being replaced by CT to assess for bowel obstruction. Although the precise findings vary with the underlying pathology and localisation of the obstruction, common findings include:

• a distinct transition point where bowel calibre changes from normal to abnormal
• dilated bowel loops proximal to the transition point
  • small bowel >3.0 cm
  • large bowel >5 cm
• a small bowel size threshold of 2.5 cm increases sensitivity for partial small bowel obstruction
• collapsed or normal calibre bowel distal to the transitional point
• bowel wall thickening
• surrounding mesenteric fat stranding indicating inflammation
• twisting of the mesentery in cases of volvulus 

If bowel obstruction is identified it is important to assess for complications and assess the viability of the involved bowel:

• pneumoperitoneum indicating perforation
• bowel ischaemia
• strangulation
  • as for ischaemia but for bowel within a hernial sac

Treatment and prognosis

Once intestinal obstruction is confirmed imaging findings can guide the timing of any planned surgical intervention. In cases where there are signs of ischaemia (i.e. strangulated bowel obstruction), emergency surgery may be needed to salvage bowel. The presence of frank perforation and peritonitis will need surgical washout.

The extent of involved bowel may determine whether or not a laparoscopic approach can be employed as well as help guide the size of the opening surgical incision.