Microscopic Colitis (Lymphocytic or Collagenous Colitis)

Microscopic colitis is a syndrome in which patients have (1) chronic diarrhea, (2) normal findings on colonoscopy and a normal-appearing mucosa, (3) histologic evidence of increased cellular infiltrate in the lamina propria, and (4) either (a) full-blown lymphocytic colitis with intraepithelial lymphocytes and increased infiltrate in the lamina propria or (b) collagenous colitis with a collagen band below the epithelium larger than 10 mm ( Fig. 98.1 ).

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Microscopic Colitis: Lymphocytic and Collagenous.

Microscopic colitis has been difficult to define, but numerous reports have clarified the syndrome. Initially described as “collagenous” colitis, with watery diarrhea and deposition of collagen, it was later noted that many patients with chronic watery diarrhea have a lymphocytic infiltrate and a definite intraepithelial lymphocytic infiltrate. During the past decade, many patients have been described with watery diarrhea responsive to therapy and with increased infiltrate only in the lamina propria. It is important to note that watery diarrhea and absence of gross findings on colonoscopy are components of the syndrome. Histologic examination of the biopsy specimen shows no distortion of the crypts, as seen in ulcerative colitis.

One study found microscopic colitis in 9.5% of patients with watery diarrhea. Incidence is reported as 4.2 per 100,000 population, with the lymphocytic type three times more common than the collagenous form. The cause and etiology remain unknown, but it is thought that microscopic colitis is associated with toxins, drugs, or latent autoimmune enteropathy, as well as various diseases (e.g., celiac). Almost 30% of patients with celiac disease have been reported to have some form of microscopic colitis; thus, a similar cause is suspected.