Microscopic colitis is a subtype of inflammatory bowel disease that is characterized clinically by a chronic, watery diarrhea with normal endoscopic appearance but histologic evidence of chronic inflammation. The etiology is unknown. Microscopic colitis is subdivided into 2 primary types: collagenous colitis and lymphocytic colitis, determined by histologic appearance.

Microscopic colitis is more common in adults older than 60 years and has an incidence of approximately 8.6 cases per 100 000 person-years. Female patients are more likely to have collagenous colitis, but there is no clear sex predilection in lymphocytic colitis.

The most common symptom is a chronic, watery diarrhea. Hematochezia, melena, and bright red blood per rectum (BRBPR) are uncommon. Some patients will experience weight loss and malaise. Abdominal pain, cramps, bloating, nausea, and fecal incontinence have also been reported. Undiagnosed and untreated, symptoms from microscopic colitis are often continual, although they may not progressively worsen.

Patients with autoimmune diseases such as type 1 diabetes mellitus, celiac disease, and autoimmune thyroiditis are at increased risk of developing microscopic colitis. Additionally, smoking and many medications are associated with microscopic colitis.