Inflammation of the colon or colitis can have a number of aetiologies. Causes include inflammatory bowel disease, infections, bowel ischaemia, and iatrogenic causes such as antibiotic-associated colitis (pseudomembranous colitis) and radiation-induced colitis.
The plain abdominal radiograph is an essential examination in patients with acute colitis. It allows an assessment of the extent of the colitis, the state of the mucosa and depth of ulceration and the presence or absence of toxic megacolon and/or perforation.
Involved colon does not usually contain any faeces and in left-sided disease the proximal limit of faecal residue usually indicates the extent of active mucosal lesions. Inflammation of the mucosa leads to the formation of ulcers, superficial or deep. Spasm and colonic narrowing can accompany mucosal inflammation. Fuzzy mucosal edges or absent haustrations indicate active disease.
As inflammation spreads to involve the submucosa, deeper ulceration may be seen. A coarse irregularity of the mucosal edge and absence of the normal haustral pattern are associated with marked inflammation and ulceration. Thickened colonic wall can be identified on the plain radiograph and finger-like indentations of the oedematous bowel margin are known as "thumbprinting". The formation of pseudopolyps (oedematous mucosal islands projecting into the bowel lumen and seen "en face") indicates more extensive inflammation and mucosal destruction. These changes may precede toxic dilatation of the colon.
When there is air in the colonic lumen it is easier to assess the mucosa, but severe mucosal changes can be missed when there is no air within the lumen to outline the mucosa. A "gasless" colon is strongly suggestive of severe disease, especially in a patient with known inflammatory bowel disease.
Toxic megacolon is a fulminating form of colitis with transmural inflammation and extensive ulceration. Patients are systemically toxic and the condition is an indication for urgent surgery. Dilatation of the colon occurs when the ulceration has penetrated the muscle layer. There is breakdown of tissue cohesion and passage of bacteria and toxins across the disrupted mucosal surface. Changes are most obvious in the transverse colon as this is the most anterior part of the colon when the patient is supine and air collects here. Toxic dilatation carries an increased risk of perforation and mortality as high as 30%. Absolute values for the maximum width of colonic lumen above which toxic dilatation is present vary in textbooks between 5cm and 6cm diameter.
This patient failed to respond to medical management, becoming increasingly unwell in hospital and underwent subtotal colectomy and formation of an ileostomy 5 days after admission. Histology of the resected colon showed severe inflammatory changes in keeping with ulcerative colitis.
