Collagenous colitis (CC) is a distinctive form of colitis associated with chronic watery diarrhea. Along with lymphocytic colitis, it is a member of a group of conditions called microscopic colitis because the colon is endoscopically and radiologically normal, but histologically abnormal. Both collagenous colitis and lymphocytic colitis (LC) occur in middle-aged to older patients; CC is more common in females while LC tends to affect the sexes equally. The cause is unknown but thought to be autoimmune in origin; there is often an association with other autoimmune diseases and abnormal HLA haplotypes.
Histologically, CC and LC are associated with increased mucosal chronic inflammation and an increase in intraepithelial lymphocytes. CC is distinguished from LC by the presence of a thickened band of collagen that lies under the surface epithelium and is thought to impair absorption of water.
The microscopic abnormalities are similar to those seen in Celiac disease, and there is a positive association between celiac disease and these colitides. CC and LC may be self-limiting, or progressive and severe; treatment included anti-diarrheal agents, sulphasalazine and in refractory cases, steroids.