Large bowel obstruction (LBO) is often impressive on imaging, on account of the ability of the large bowel to massively distend. This condition requires prompt diagnosis and treatment.
On this page:
Terminology
Bowel obstruction may be complete or incomplete 6:
complete or high grade obstruction means that no fluid or gas can proceed beyond the obstruction
incomplete, low grade or partial obstruction means that fluid or gas are still able to proceed
Epidemiology
Large bowel obstructions are far less common than small bowel obstructions, accounting for only 20% of all bowel obstructions 4.
Clinical presentation
The classic presentation is with abdominal pain, distension, and failure of passage of flatus and stool. As dilatation of the colon increases, the risk of perforation also increases. Perforation may occur at the site of obstruction, or more proximally secondary to ischaemic change, which may be implied by the presence of intramural gas or decreased mural enhancement. Signs of peritonism, sepsis, and shock may develop when perforation occurs.
Pathology
Aetiology
The underlying aetiology of large bowel obstructions is age-dependent, but in adulthood, the most common cause is colonic cancer (50-60%), typically in the sigmoid 1-4.
The second most common cause in adults is acute diverticulitis (involving the sigmoid colon). Together, obstructing tumours and acute diverticulitis account for 90% of all causes of large bowel obstruction. While adhesions are the leading cause of small bowel obstruction, for practical purposes, they do not tend to cause large bowel obstruction. Overall causes of large bowel obstruction include 4:
-
malignancy
colorectal carcinoma (most common, 50-60%)
pelvic tumours: direct spread or metastatic disease
-
caecal volvulus (1-3%)
sigmoid volvulus (3-8%)
ischaemic stricture (see ischaemic colitis)
faecal impaction/faecaloma (most common cause in debilitated elderly)
hernias (uncommon) 5
Radiographic features
Large bowel obstructions are characterised by colonic distension proximal to the obstruction, with collapse distally. In some cases, the point of obstruction and site of obstruction are not the same, with the point of obstruction located distal to the apparent cut-off point, e.g. an obstructing sigmoid tumour may present with an apparent cut-off at the splenic flexure.
In general the colon is considered dilated if it is over 6 cm in diameter, with the caecum having an upper limit of 9 cm 1. See the separate article on the 3-6-9 rule. A caecal diameter of 12 to 15 cm increases the risk for caecal rupture 7.
Plain radiograph
colonic distension: gaseous secondary to gas-producing organisms in faeces
collapsed distal colon: very few or no air-fluid levels are found in the large bowel because water is reabsorbed 7
-
small bowel dilatation, which depends on
duration of obstruction
incompetence of the ileocaecal valve
rectum has little or no air 7
In advanced cases one may see the stigmata of an ischaemic colon, namely:
intramural gas (pneumatosis coli)
free intra-abdominal gas (pneumoperitoneum)
CT
CT is the most widely used modality for assessment of large bowel obstructions and is not only able to confirm the diagnosis and localise the location of obstruction but in most instances also is able to identify the cause.
The large bowel will be distended with a thinned stretched wall but should enhance (unless ischaemic). If the ileocaecal valve is competent then the small bowel may be mostly collapsed.
It should be traced distally until a transition is found. The cause is often present at this point, although sometimes the obstructing lesion is distal to the apparent transition point.
Complications, such as those of ischaemia or perforation, should be assessed for 5.
Imaging features will depend on the underlying cause and are thus discussed separately (see above).
Treatment and prognosis
Treatment depends on the underlying cause and presentation, but in all cases, resuscitation and correction of electrolyte imbalances should be carried out.
In patients who have signs of peritonitis clinically or features suggestive of perforation or necrosis then emergency laparotomy is required to avoid overwhelming sepsis 2. This is usually resection of the affected bowel and causative lesion, bring out the remaining proximal large bowel or small bowel (if caecum resected) as a colostomy/enterostomy, and oversewing the rectal stump (Hartmann procedure) with second operation weeks or months later to re-anastomose the bowel 2.
In patients without overt peritonitis, but evidence of an obstructing lesion (e.g. colorectal carcinoma) surgery is also carried out relatively expediently. Traditionally a three-stage operation,
decompressive colostomy proximal to the obstruction
resection of the lesion
re-anastomosis
was the primary mode of surgical treatment, although with advances in anastomosis technique single-stage procedures with primary anastomosis are more commonly performed 2.
In some patients, especially those not considered fit to have a general anaesthetic, or who have extensive systemic disease, a self-expanding colonic stent may be used to decompress the colon 2.
Differential diagnosis
When a clear transition point, especially when a causative lesion is identified, then there is little doubt in the diagnosis. The main problem occurs in patients where an abrupt transition point is not present, in which case a number of other possibilities should be entertained:
-
no transition point
often history contains cause for ileus (e.g. surgery)
-
a long duration of obstruction and/or incompetent ileocaecal valve can lead to prominent small bowel dilatation, however, the absence of distension of the colon is usually a giveaway
-
although no abrupt transition point is present, a less well-defined zone of calibre change is usually present, most frequently in the region of the splenic flexure 3
-
toxic megacolon secondary to Clostridioides difficile colitis
C. difficile infection is usually preceded by antibiotic use or chemotherapy and is therefore usually encountered in unwell, hospitalised patients with significant co-morbidity
bowel wall thickening usually a prominent feature
-
usually, bowel wall is thickened but can be thinned and dilated
absent or poor wall enhancement
usually respects vascular territories
Unable to process the form. Check for errors and try again.