Toxic megacolon. This unfortunate, grave complication of amebiasis can appear during the course of a fulminant colitis. When the disease is fulminating, as it may be in children and younger adults, it resembles acute, rapidly progressive ulcerative colitis. The entire large bowel may be involved and occasionally the terminal ileum as well (even during the acute infection, an incompetent ileocecal valve is common). Only 0.5% of these patients develop this feared transmural disease with extensive muscle lysis damaging the myenteric plexus, resulting in loss of colonic muscle tone with massive dilatation of the large bowel (Fig. 1.13). There is loss of the normal colon landmarks, including haustrations, as well as mucosal destruction and isolated islands of edematous mucosa projecting as pseudopolyps. In some patients, pneumatosis cystoides intestinalis can be appreciated, as well as all the findings of colonic wall destruction similar to ulcerative colitis. A toxic megacolon is a definite contraindication for barium enema examination.

.....

Fig. 1.13 Toxic megacolon in acute amebic colitis, similar radiographically to that seen in ulcerative colitis. Toxic megacolon is an ominous finding in severe amebiasis and is a contraindication for barium enema examination because of the fragile nature of the severely ulcerated and often paper-thin bowel wall. In (A) there is marked distention of the colon with several air-fluid levels in the grossly dilated cecum and distal transverse colon on erect plain film of the abdomen. (B) A barium enema was performed in this case (sound judgment should prevail against such a potentially dangerous study) and shows innumerable small ulcers eroding the mucosa of the greatly distended colon. (Courtesy of Drs. Malcolm Hill and Henry Goldberg, AJR 99:77-83, 1967.)

Unfortunately, multiple careful rectal biopsies and stool examinations may be negative, but the possibility of amebic colitis should be remembered because the results of surgery on the toxic megacolon of amebiasis are almost inevitably disastrous. Antiamebic therapy should always antecede surgery on all patients with colonic amebiasis (see Figs. 1.27 & 1.33). Making the correct diagnosis is a greater problem in climates where both amebic and ulcerative colitis may occur than in the tropics, where ulcerative colitis is rare and toxic megacolon has a much higher probability of being amebic in origin.