What is the differential diagnosis (DDX) of erosive rectosigmoiditis?

Differential Diagnosis of Erosive Rectosigmoiditis

The differential diagnosis of erosive rectosigmoiditis includes ulcerative colitis, Crohn's disease, infectious colitis (bacterial, viral, parasitic), ischemic colitis, drug-induced colitis, radiation colitis, and less commonly Behçet's disease, cytomegalovirus infection, and eosinophilic colitis. 1

Primary Inflammatory Bowel Disease Considerations

Ulcerative Colitis

• Most likely diagnosis when inflammation is continuous and confluent, extending proximally from the rectum with clear demarcation 1
• Rectal involvement is present in >97% of untreated cases 2
• Mucosal-only inflammation (not transmural) with diffuse basal plasmacytosis on histology 1
• Crypt architectural distortion and mucosal atrophy develop after 4 weeks of symptoms 1
• Proctosigmoiditis represents 60-80% of newly presenting ulcerative colitis cases and generally has a milder course 3
• Disease remains localized in 90% of patients, with only 10% showing proximal extension over time 4

Crohn's Disease

• Consider when lesions are discontinuous (skip lesions), with rectal sparing, or when strictures/fistulae are present 1, 2
• Transmural inflammation affecting all bowel wall layers 2
• Non-caseating granulomas on histology (pathognomonic when present) 2
• Perianal disease strongly suggests Crohn's rather than ulcerative colitis 1, 5
• Patchy chronic inflammation with focal crypt architectural abnormalities 1

Infectious Etiologies

Bacterial Infections

• Salmonella, Shigella, and Campylobacter produce endoscopic features similar to ulcerative colitis 1
• Yersinia and cytomegalovirus enterocolitis resemble Crohn's disease 1
• Clostridium difficile can superimpose on IBD, complicating diagnosis 1
• Obtain stool cultures and C. difficile toxin assay before diagnosing IBD 1
• Infectious colitis typically shows preserved crypt architecture with acute inflammation only 1

Viral and Parasitic Infections

• Cytomegalovirus colitis shows variable colonoscopic findings: patchy erythema, micro-erosions, deep ulcers, or pseudotumors 1
• More common in immunocompromised patients (HIV, post-transplant) 1
• Test for ova, cysts, and parasites based on travel history 1

Non-Infectious, Non-IBD Causes

Ischemic Colitis

• Common differential diagnosis, particularly in older patients with vascular risk factors 1
• Typically affects watershed areas (splenic flexure, rectosigmoid junction) 1
• Acute presentation with abdominal pain and bloody diarrhea 6

Drug-Induced Colitis

• NSAIDs, immunosuppressants, and chemotherapy agents can cause erosive colitis 1
• History of medication use is critical 1

Radiation Colitis

• Occurs in patients with prior pelvic radiation therapy 1
• Temporal relationship to radiation exposure is diagnostic 1

Other Rare Causes

• Behçet's disease, eosinophilic colitis, and segmental colitis associated with diverticulitis (SCAD) 1, 7
• Most erosive lesions without surrounding inflammation are nonspecific (84% in one series) 7

Diagnostic Approach

Endoscopic Evaluation

• Perform complete ileocolonoscopy with biopsies from at least five sites (including ileum and rectum), taking at least two biopsies per site, even from normal-appearing mucosa 1, 2
• Document lesion distribution (continuous vs. discontinuous), rectal involvement, and presence of skip lesions 1, 2
• Flexible sigmoidoscopy alone is insufficient for definitive diagnosis 2

Histopathological Features

• Biopsies from both inflamed and uninflamed segments are essential 1
• Look for granulomas (Crohn's), basal plasmacytosis (UC), preserved crypt architecture (infection), or eosinophilic infiltration 1, 2
• In 5-15% of cases, differentiation between Crohn's and UC is impossible (IBD-unclassified) 1, 2

Laboratory and Imaging

• Obtain complete blood count, CRP, albumin, liver function, iron studies, and renal function 1
• Fecal calprotectin >100 μg/g supports IBD diagnosis (sensitivity 93%, specificity 96%) 2
• Perform cross-sectional imaging (MRI or CT enterography) to evaluate small bowel involvement and complications 2

Critical Pitfalls

• Acute symptoms (<2 weeks) favor infection or ischemia; chronic symptoms (>4 weeks) suggest IBD or structural abnormalities 6
• Histologic findings may be nonspecific early in disease course, requiring careful follow-up 7
• Two patients with initially nonspecific acute inflammation were later diagnosed with Crohn's disease on follow-up 7
• When symptoms or lesions fail to improve, repeat endoscopy and biopsies are mandatory 7
• Superimposed infections (C. difficile, CMV) can mimic IBD flares in known IBD patients 1