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Microscopic colitis is a chronic inflammatory disease of the colon characterized by chronic watery non-bloody diarrhea, with normal or near-normal colonoscopic appearance, but distinct histopathologic abnormalities on biopsy.

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Microscopic Colitis (Lymphocytic & Collagenous Colitis) - An Underdiagnosed Form of IBD

It encompasses two main histologic subtypes:

1. Collagenous colitis – characterized by a thickened subepithelial collagen band.
2. Lymphocytic colitis – characterized by increased intraepithelial lymphocytes without a collagen band.

Both forms share similar clinical and radiologic features and are often grouped under the term microscopic colitis.

Etiopathogenesis

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Pathophysiology involves abnormal mucosal immune response, leading to inflammation and disruption of epithelial barrier integrity, with resultant secretory diarrhea.

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Exact cause is unknown, but it is believed to be multifactorial, involving:

• Autoimmune mechanisms – associated with celiac disease, rheumatoid arthritis, thyroiditis, type 1 diabetes.
• Drug-induced:
  • NSAIDs
  • Proton pump inhibitors (especially omeprazole, lansoprazole)
  • SSRIs (sertraline)
  • Statins
• Bile acid malabsorption
• Infectious triggers – post-infectious immune activation
• Smoking – strongly linked to collagenous colitis.

Clinical Features

• Chronic watery diarrhea (non-bloody) – intermittent or persistent.
• Abdominal cramps and urgency.
• Nocturnal diarrhea possible.