Large bowel obstruction
Large bowel obstruction (LBO) are often impressive on imaging, on account of the ability of the large bowel to massively distend. This condition requires prompt diagnosis and treatment.
Presentation is typically with abdominal pain, distension and failure of passage of flatus and stool. Eventually signs of peritonism, sepsis and shock develop, when perforation occurs.
As dilatation of the colon increases, the risk of perforation is also increased. Perforation may occur at the site of obstruction, or more proximally secondary to ischaemic change. Perforation may be implied by the presence of intramural gas.
The underlying aetiology of large bowel obstructions is age-dependant, 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:
- colorectal carcinoma (most common, 50-60%)
- pelvic tumours; direct spread or metastatic disease
- colonic diverticulitis
- ischaemic stricture (see ischaemic colitis)
- faecal impaction/faecoloma (most common cause in debilitated elderly)
- hernias (uncommon) 5
Large bowel obstructions are characterised by colonic distension proximal to the obstruction, with collapse distally. It should be noted that in some cases the point of obstruction and site of obstruction are not the same, with the point of obstruction located distal to to apparent cut-off point, e.g. an obstructing sigmoid tumour may present with aparent cut-off at the splenic flexure.
- colonic distension: gaseous secondary to gas-producing organisms in faeces
- collapsed distal colon
- small bowel dilatation, depends on
- duration of obstruction
- incompetence of the ileocaecal valve
In advanced cases one may see the stigmata of an ischaemic colon, namely:
CT is the currently 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 ileocecal 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 a little way distal from the apparent transition point.
Complications should be assessed for 5:
- strangulation refers to ischaemia loops of bowl within the hernial orifice
- incarceration refers to a irreducible hernia and cannot be assessed on imaging alone but a narrow neck with a large hernial sac is suggestive
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's procedure) with a second operation weeks or months later to reanastomose 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
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.
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)
small bowel obstruction
- long duration of obstruction and / or incompetent ileocaecal valve can lead to prominent small bowel dilatation, however absence of distension of the colon is usually a give away
- 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 Clostridium 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
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