How do you differentiate and manage diverticulitis versus ulcerative colitis (UC) in patients presenting with similar symptoms?

Differentiating Diverticulitis from Ulcerative Colitis

CT imaging is the gold standard for diagnosing acute diverticulitis, while colonoscopy with biopsy is essential for confirming ulcerative colitis—but these conditions can coexist and require careful clinical, radiological, and endoscopic correlation to distinguish.

Clinical Presentation Patterns

Diverticulitis

• Left lower quadrant pain that is severe and persistent, typically in patients over 40 years of age 1
• Fever, leukocytosis, and elevated CRP are common with acute inflammation 1
• Constipation or diarrhea may occur, but rectal bleeding is uncommon in uncomplicated diverticulitis 2
• Symptoms develop acutely over days, not chronically relapsing as in UC 1

Ulcerative Colitis

• Bloody diarrhea with mucus is the hallmark presentation, often with urgency and tenesmus 1, 3
• Rectal bleeding occurs in up to 90% of UC flares 1
• Symptoms follow a relapsing-remitting pattern over months to years 1
• Fecal calprotectin >150 mg/g strongly suggests active UC inflammation 1, 4
• Left-sided or continuous colonic involvement starting from the rectum 1

Diagnostic Workup Algorithm

Step 1: Initial Assessment

• Obtain complete blood count, CRP, and stool studies (including C. difficile toxin) to exclude infection in both conditions 1, 5
• Check fecal calprotectin if UC is suspected—levels >150 mg/g indicate active inflammation with 95.4% accuracy in symptomatic patients 1, 4

Step 2: Imaging

• CT abdomen/pelvis with IV contrast is the diagnostic test of choice for suspected diverticulitis 1
  • Uncomplicated: bowel wall thickening, pericolic fat stranding, diverticula 1
  • Complicated: abscess (>4 cm), extraluminal gas, distant free fluid 1
• CT findings help classify severity using the WSES classification system 1

Step 3: Endoscopic Evaluation

• Avoid colonoscopy during acute diverticulitis due to perforation risk—defer until inflammation resolves (4-6 weeks) 1, 6
• For suspected UC, perform flexible sigmoidoscopy or full colonoscopy with biopsies from at least 5 sites including rectum and ileum 1
• Obtain minimum of 2 biopsies per site from both inflamed and uninflamed mucosa 1

Key Endoscopic and Histologic Distinctions

Ulcerative Colitis Features

• Continuous mucosal inflammation starting distally in the rectum with proximal extension 1
• Diffuse erythema, friability, ulceration, and loss of vascular pattern 1
• Histology shows basal plasmacytosis, crypt distortion, and diffuse chronic inflammation 1
• Inflammation involves diverticular orifices when diverticula are present 7

Diverticulitis Features

• Inflammation is localized to diverticular orifices and peri-diverticular mucosa, sparing inter-diverticular areas 7
• Segmental involvement corresponding to areas with diverticula 7
• Histology shows acute inflammation without chronic architectural changes 8, 6

Critical Diagnostic Pitfalls

Overlapping Conditions

• Segmental colitis associated with diverticulosis (SCAD) affects inter-diverticular mucosa without involving diverticular orifices—distinct from both UC and acute diverticulitis 6, 7
• Diverticular colitis can mimic UC or Crohn's disease with chronic mucosal inflammation in areas with diverticula 8, 6
• UC patients have lower prevalence of diverticulosis (10.8% vs 27.8% in general population), suggesting UC may be protective 9

When Conditions Coexist

• In UC with diverticulosis (UCD), inflammation affects the entire colonic mucosa including diverticular orifices, not just peri-diverticular areas 7
• Incidence of mucosal inflammation with diverticular disease is only 2-2.3% in clinical practice 7
• If UC is established and patient develops acute left lower quadrant pain with fever, consider acute diverticulitis superimposed on UC 9, 7

Management Implications

Diverticulitis Management

• Uncomplicated diverticulitis: outpatient management with oral antibiotics or observation (antibiotics not always required per recent evidence) 1
• Complicated diverticulitis with abscess <4 cm: antibiotics ± percutaneous drainage 1
• Abscess >4 cm or peritonitis: surgical consultation for possible resection 1

UC Management

• Mild-moderate UC: combination topical mesalamine 1g daily plus oral mesalamine 2.4-4.8g daily 3, 10
• Moderate-severe symptoms with fecal calprotectin >150 mg/g: proceed directly to treatment adjustment without requiring endoscopy first 1, 5
• Acute severe UC: IV methylprednisolone 40-60 mg/day with assessment for rescue therapy (infliximab or cyclosporine) if no response by day 3-5 3, 5

Diverticular Colitis Management

• Treat with high-fiber diet, antibiotics, and/or aminosalicylates (mesalamine) 8, 6
• Consider sigmoid resection for persistent or recurrent symptoms despite medical therapy 8, 6

When to Suspect Alternative Diagnosis

• Rectal sparing in suspected UC: consider Crohn's disease or SCAD, though can occur in 3% of UC patients on topical therapy 1
• Right-sided inflammation with diverticula: more common in UC with diverticulosis (22.2%) than in diverticulosis alone (0.8%) 9
• Granulomas on biopsy: suggests Crohn's disease rather than UC, though can occur in diverticular colitis 8, 2