Toxic Megacolon

From wikiRadiography
Jump to navigation Jump to search

An understanding of toxic megacolon is important for radiographers because the diagnosis can sometimes be expedited by the radiographer and the condition is potentially life-threatening if treatment is delayed. Importantly, the radiographer should be aware of the need to critically examine the supine abdominal film for evidence of abnormal large bowel (dilated, oedematous, effaced), intramural gas, hepatic portal-venous gas and pneumoperitoneum. Consideration should be given to performing supplementary views where there is evidence of critical pathology (e.g. erect CXR for pneumoperitoneum). Inpatients with fulminant colitis may be referred for abdominal plain film every day- this is usually justified. When considering which order should be applied for ward patients waiting for plain film radiography, call for the ? toxic megacolon patient first.


Toxic megacolon is the clinical term for an acute toxic colitis with dilatation of the colon <a class="external" href="" rel="nofollow" target="_blank">(Deepika Devuni,</a>. Radiographers can be confused by this condition- it is not a disease in itself- it is (commonly) a sign of fulminant inflammatory bowel disease.

Importance of Clinical Context

A patient with life-threatening toxic megacolon will usually have obvious clinical signs- they are usually very sick! "A typical clinical presentation is the rapid onset of abdominal pain and diarrhoea, accompanied by a high fever and severe debilitation." Stephen R Baker, The Abdominal Plain Film, Appleton and Lange, 1990, p203

Value of the Plain Film

Stephen R. Baker in his book 'The Abdominal Plain Film' makes the following observation on the value of the plain film in patients with toxic megacolon

There are but very few life-threatening conditions in which the abdominal roetgenogram still remains the sole imaging study required to render an unequivocal diagnosis. Along with pneumoperitoneum, mesenteric ischaemia, small bowel obstruction, colonic volvulus, and emphysematous cholecystitis, toxic megacolon must be included on this short list. In most instances, the plain film appearance is so distinctive that no other diagnosis need be considered. Furthermore, because the barium enema is a dangerous examination in fulminant colitis, exposing the patient to a heightened risk of large bowel perforation, the crucial role of non-contrast abdominal radiographs cannot be overemphasized.

Stephen R Baker
The Abdominal PLain Film
Appleton and Lange, 1990, p203

Serial Plain Films

If toxic megacolon is clinically suspected, patients are usually followed up with plain abdominal radiography every 12-24 hours, depending on the patient’s clinical condition. <a class="external" href="" rel="nofollow" target="_blank">(

The daily abdominal plain film on patients with fulminant colitis can be erroneously interpreted by radiographers as wasteful and an unneccesary radiation hazard for the patient.

Plain Film Appearances
  1. Dilation of the transverse colon/splenic flexure (greater than 6cm diameter).
  2. Alteration of normal wall contour (effacement, thumbprinting, pseudopolyps)
  3. Intraluminal gas
  4. Hepatic portal-venous gas
  5. Pneumoperitoneum


thumbprintingThumbprinting refers to the altered luminal contour of the large bowel wall associated with inflammation. The normal characteristic arcuate plicae and haustral contours are replaced with a coarser pattern (arrowed). The appearance is well named- the suggestion is that the luminal air is indented with protrusions that would approximate pushing your thumb into the bowel.

Prominant air-filled small bowel adjacent to transverse colon may represent sentinal sign.

Bowel Wall Effacement

Toxic Megacolon - wikiRadiographyThe inflammed bowel wall can present on plain film with a variety of appearances. The bowel wall can be thickened (thumbprinting) but can also be thinned as a result of mucosal sluffing. Mucosal sluffing will cause the bowel wall to appear featureless (black arrow). The sluffed-off mucosa can deposit on the bowel wall causing a plain film appearance referred to as pseudopolyps (white arrows).


pseudopolypsPseudopolyps (white arrows) represent mucosal islands in denuded ulcerated colonic wall <a class="external" href="" rel="nofollow" target="_blank">(

Intramural Gas

The Abdominal Plain Film- Mural Gas - wikiRadiographyThis patient has evidence of toxic megacolon involving the transverse colon and descending colon. There is suggestion of intramural gas- care must be taken to differentiate intramural gas from a thin stripe of properitoneal fat (white arrow).
portal venous gasMagnified RUQ

Patient with toxic megacolon.

There is hepatic portal-venous gas (white arrows)

There is intramural gas (black arrows)


pneumoperitoneumPatients who demonstrate plain film evidence of toxic megacolon are more likely to have pneumoperitoneum. This patient has evidence of toxic megacolon and scattered lucent densities overlying the liver. The liver is a homogenous organ and will usually present as a homogenous density on abdominal plain film. If the liver is not a homogenous density, pneumoperitoneum should be considered and an erect CXR or decubitus AXR performed.

Hepatic portal-venous gas

<img align="bottom" alt="portal venous gas" src="" title="portal venous gas"/>This is a magnified view of the erect abdominal plain film RUQ. The portal venous gas is clearly seen as well as intramural gas (black arrow) and surgical sutures (white arrow) in a patient with fulminant inflammatory bowel disease.


1. Patients with toxic megacolon may demonstrate a remarkable similarity in the appearance of the large bowel on abdominal plain film. In particular, the mid-to-distal transverse colon only may appear to be involved. You should be mindful that this may be due to the highlighting of some of the diseased colon by the presence of intra-luminal gas. When the patient is supine, the transverse colon/splenic flexure are the least dependent segments of the large bowel- i.e. gas will tend to collect in the uppermost part of the bowel. If you can't see the remainder of the large bowel, how do you know whether it is similarly diseased? A prone abdominal film may reveal the diseased segments of large bowel to extend beyond that demonstrated in the supine image. Decubitus projections (left and right) will usually demonstrate involvement of both ascending and descending colon. The presence of toxic megacolon is more important than the extent- supplementary abdominal views should only be performed where their are sufficient indications.

2. Toxic megacolon is not well named. It is undeniably potentially toxic to the patient but it is not that 'mega'. When you compare the typical large bowel dilation that can occur with sigmoid volvulus (and some pseudo-obstructions), it might be more apt to have named it mild rather than mega.

Case 1
Day 1
This 63 year old lady presented to the Emergency Department with the following history

- diarrhoea for 5 weeks
- 15 kg weight loss in 5 weeks
- decreased appetite
- deranged electrolytes
- tachycardia
- low albumin

She was referred for chest and abdominal radiography.

The abdominal plain film demonstrates
  • thickened mucosa in the transverse and descending colon
  • mildly dilated transverse colon
  • pseudopolyps (arrowed)
No pneumoperitoneum was demonstrated.

The patient was admitted to hospital.
Day 2
Toxic Megacolon - wikiRadiography
The patient was referred for abdominal plain film radiography on day 2. The appearances are similar to day 1
Day 3
Toxic Megacolon - wikiRadiography
The patient was referred for abdominal plain film radiography on day 3. The appearances are similar to day 1and 2
Day 4
Toxic Megacolon - wikiRadiography

The patient was referred for abdominal plain film radiography on day 4. The appearances are largely unchanged
Day 5
Toxic Megacolon - wikiRadiography
The patient was referred for abdominal plain film radiography on day 5. The patient's condition had deteriorated. The transverse colon is slightly less dilated. There are scattered gas densities evident, particularly overlying the liver- these gas densities were not evident on previous imaging. There is a subtle suggestion of demonstration of the falciform ligament.

The senior radiographer on duty reviewed the imaging and noted the changes suggestive of pneumoperitoneum. The patient was recalled to the department for a decubitus abdominal plain film. The ward nurses advised that the patient could not come to the department- she was already on the way to the operating theatre.

At operation, most of the colon was found to be diseased. There were two sites of perforation of the large bowel. There was extensive faeces throughout the abdominal cavity. A partial colectomy was performed.