Pseudomembranous colitis

The CT showed a remarkable wall thickening of the colon, pericolonic inflammation and ascites. These findings suggested a diagnosis of pseudomembranous colitis. Bacteriological examination of the stool demonstrated the presence of Clostridium difficile and confirmed the diagnosis. Anticlostridial treatment was instituted with prompt resolution of the clinical signs.

Clostridium difficle colitis (CDC) is a condition in which an insult to normal gut flora, most commonly caused by antibiotics or chemotherapy agents, allows colonisation of the colon by the organism and production of its characteristic toxins. These toxins produce a spectrum of clinical findings including diarrhoea, abdominal pain, fever, leukocytosis, sepsis and perforation. Characteristic pseudomembranes composed of fibrin, inflammatory cells and cellular debris are seen in pathological gross examination and have earned this disease the name "pseudomembranous colitis" (PMC).

The gold standard for diagnosis is a microbiological stool assay for CD. Endoscopic examination of the colon with biopsy could also be used to make the diagnosis, but this method is less sensitive and mechanical manipulation of the diseased colon can by risky.

Plain films may show small or large bowel dilation with haustral thickening and "thumb printing" indicating bowel wall oedema. Barium enema is contraindicated in the acute illness or when the colon is dilated. Non-specific findings include ulceration, mucosal destruction and inflammatory pseudopolyps.

CT is very useful in the diagnosis of CDC especially when the disease is not suspected clinically. Common CT findings include wall thickening; low attenuation mural thickening, corresponding to mucosal and submucosal oedema; the "accordion sign"; the "target sign" (also called the "double halo sign"); pericolonic stranding and ascites.

The most common CT finding in patients with PMC is colonic wall thickening, which usually ranges from 3mm to 32mm in diameter. Mural thickening may be circumferential, eccentric, smooth or polypoid. Inflammation of the mucosa and colon wall may enhance markedly following IV contrast administration. The target sign, which consists of two or three concentric rings of different attenuation, was originally described in ulcerative colitis and Crohn's disease, but is also reported in PMC. This sign indicates mucosal and submucosal oedema and inflammation.

The accordion sign is seen when orally administered contrast material becomes trapped between markedly thickened haustral folds, giving the appearance of alternating bands of high attenuation (contrast material) and low attenuation (oedematous haustra). The accordion sign is highly suggestive of PMC, although it is usually seen only in advanced cases. Its appearance may vary depending on the degree of oedema of the haustral folds and the amount of contrast material trapped between the folds. Although the accordion sign is highly suggestive of PMC, other findings such as wall thickening, pericolonic stranding and ascites are not very specific and can be seen in a variety of inflammatory or infectious diseases of the colon.

Pericolonic stranding may be identified, but is usually mild, reflecting the primary mucosal and submucosal nature of PMC. The relative paucity of pericolonic stranding and inflammation in PMC, in combination with marked colonic wall thickening, helps to differentiate this disorder from other types of colitis. Ascites tends to occur in severe cases of PMC. Because ascites is uncommon in other inflammatory bowel diseases, it may be a helpful clinical finding. However, ascites may also be described in other types of colitis. The imaging findings of PMC are not specific and may be simulated by other disorders that also cause focal or diffuse bowel wall thickening. PMC can be confused with the acute stage of ulcerative and granulomatosis colitis, inflammatory colitis and ischaemic colitis. Entities that cause thickening of the colon unrelated to colitis, such as leukaemic infiltration, colonic lymphangiectasia, and haemorrhage, should also be considered as part of the differential diagnosis.

Because the disease may progress rapidly and become fatal, the radiologist should raise the possibility of CDC on the basis of these CT findings. Appropriate anticlostridial treatment should be instituted before obtaining bacteriological confirmation.