What are the main causes of rectosigmoid colitis?

Main Causes of Rectosigmoid Colitis

Rectosigmoid colitis is primarily caused by ulcerative colitis (proctosigmoiditis), which represents the most common etiology, accounting for up to 60-80% of newly presenting ulcerative colitis cases. 1

Primary Etiologies

Ulcerative Colitis (Proctosigmoiditis)

• Idiopathic inflammatory bowel disease is the predominant cause, characterized by continuous mucosal inflammation extending from the rectum into the sigmoid colon 2
• The pathogenesis involves a response to environmental triggers (infections, drugs, or other agents) in genetically susceptible individuals 2
• Smoking paradoxically decreases the risk of ulcerative colitis through unknown mechanisms 2
• Peak incidence occurs between ages 10-40 years, though 15% of patients are diagnosed after age 60 2

Crohn's Disease

• Can affect the rectosigmoid region with patchy, discontinuous inflammation rather than the continuous pattern seen in ulcerative colitis 2
• Characterized by transmural inflammation (affecting all bowel wall layers) versus the mucosal-limited inflammation of ulcerative colitis 3
• Rectal sparing is common in Crohn's disease but rare in ulcerative colitis (occurring in only up to 3% of cases) 3
• Smoking increases the risk of Crohn's disease 2

Microscopic Colitis

• Increasingly recognized cause of chronic watery diarrhea with rectosigmoid involvement 2
• Comprises two subtypes: lymphocytic colitis and collagenous colitis 2
• Average age of onset is 62-65 years, with nine-fold female predominance in collagenous colitis 2, 4
• Key causative factors include:
  • Medications: NSAIDs, proton pump inhibitors, and selective serotonin reuptake inhibitors 2, 4
  • Smoking and alcohol consumption 4
  • Autoimmune diseases and bile acid malabsorption 4

Infectious Causes

• Must be excluded before finalizing inflammatory bowel disease diagnosis 2
• Include Clostridium difficile, Yersinia, Salmonella, Shigella, and Campylobacter species 5
• Cytomegalovirus colitis can mimic ulcerative colitis 2
• Intestinal tuberculosis (particularly relevant in endemic regions) typically affects the ileocecal region but can involve the colon 5

Important Diagnostic Considerations

Distinguishing Features

• Ulcerative colitis: Continuous inflammation starting from the rectum, extending proximally; rectal involvement present in >97% of untreated cases 3
• Crohn's disease: Skip lesions, rectal sparing common, transmural inflammation with potential granulomas on biopsy 3
• Microscopic colitis: Normal endoscopic appearance despite chronic watery diarrhea; diagnosis requires colonic biopsy 2

Critical Pitfall

• In children with newly diagnosed ulcerative colitis, atypical rectosigmoid histology with patchy or absent inflammation can occur in up to 42% of cases at disease onset, potentially leading to misdiagnosis as Crohn's disease 6
• Biopsy specimens from the rectosigmoid alone may not reveal microscopic colitis in some cases; biopsies from the descending colon are also necessary 2

Medication-Induced Considerations

• Patients with refractory symptoms should avoid potential medication triggers including NSAIDs, proton pump inhibitors, and selective serotonin reuptake inhibitors 2

Unclassifiable Cases

• In 5-15% of inflammatory bowel disease cases, definitive distinction between ulcerative colitis and Crohn's disease cannot be made despite complete evaluation, termed indeterminate colitis or IBD-unclassified 2, 3