What are the possible causes, diagnostic work‑up, and initial management for an adult with computed tomography evidence of colitis confined to the ascending colon?

Colitis in the Ascending Colon: Diagnostic Approach and Management

When CT demonstrates colitis isolated to the ascending colon in an adult, you must immediately pursue stool studies for infectious etiologies—particularly C. difficile, bacterial pathogens, and cytomegalovirus—while simultaneously assessing for ischemic colitis, inflammatory bowel disease (especially Crohn's disease with right-sided predominance), and medication-related causes.

Key Diagnostic Considerations

Distribution Pattern Significance

• Right-sided or ascending colon involvement is atypical for most Western colitis patterns and narrows your differential significantly 1.
• Isolated ascending colon colitis suggests specific etiologies: ischemic colitis (particularly in elderly or those with cardiovascular risk factors), C. difficile infection (which can involve any colonic segment), Crohn's disease (which has predilection for terminal ileum and right colon), infectious colitis from specific organisms, or medication-induced colitis 1, 2, 3.
• Right-sided ischemic colitis carries higher mortality risk than left-sided disease and often presents atypically without the classic bloody diarrhea 4.

CT Imaging Findings to Document

• Measure bowel wall thickness: >4 mm indicates active inflammation; >10 mm suggests severe disease with increased mortality risk 5, 3.
• Look for specific patterns: circumferential symmetric thickening with fold enlargement suggests ischemia; asymmetric involvement with mesenteric fat stranding and engorged vasa recti suggests Crohn's disease 1, 3.
• Document pericolonic stranding, mesenteric vessel engorgement, fluid-filled distention, pneumatosis, or free air—these findings indicate severity and potential complications 1, 5.
• The "accordion sign" (contrast trapped between thickened haustral folds) is relatively specific for C. difficile colitis but can occur in other severe inflammatory conditions 5, 3.

Immediate Diagnostic Workup

Stool Studies (Priority #1)

• Obtain C. difficile PCR (nucleic acid amplification test) immediately—sensitivity 80-100%, specificity 87-99%—as CDI can present with isolated right-sided involvement 5, 6.
• Send comprehensive bacterial stool culture for Salmonella, Shigella, Campylobacter, STEC (including E. coli O157:H7), and Yersinia 5.
• Order stool ova and parasite examination if the patient has recent travel, immunocompromise, or risk factors for parasitic infection 5.
• Measure fecal calprotectin or lactoferrin to confirm intestinal inflammation and gauge severity 1, 5.

Laboratory Assessment

• Obtain CBC with differential: leukocytosis >15 × 10⁹/L indicates severe disease; paradoxical leukopenia with neutrophilia suggests neutropenic enterocolitis or overwhelming sepsis 5, 6.
• Comprehensive metabolic panel to assess renal function (creatinine >1.5× baseline = severe disease), electrolytes, albumin, and liver enzymes 5, 6.
• Serum lactate >4 mmol/L is an urgent red flag for ischemia, perforation, or fulminant infection requiring immediate surgical consultation 5, 6.

Endoscopic Evaluation Timing

• Defer colonoscopy initially in favor of flexible sigmoidoscopy with biopsies only if stool studies are negative and diagnosis remains uncertain after 48-72 hours 1, 6.
• Full colonoscopy is contraindicated in severe or fulminant colitis due to perforation risk 6.
• Endoscopic biopsies can distinguish Crohn's disease (transmural inflammation, granulomas) from ulcerative colitis (mucosal inflammation only), ischemia (ghost cells, hyalinized lamina propria), and infection 1.

Differential Diagnosis by Etiology

Infectious Causes

• C. difficile infection: Can involve any colonic segment; recent antibiotic exposure is the primary risk factor; CT may show pancolitis or segmental involvement with marked wall thickening (often >15 mm) 1, 5, 6, 3.
• Cytomegalovirus colitis: Consider in immunocompromised patients (HIV, transplant, immunosuppressive therapy); requires tissue biopsy showing viral inclusions 1.
• Bacterial enterocolitis: Campylobacter, Salmonella, Shigella, STEC can cause right-sided colitis; presence of fecal leukocytes or pus confirms invasive infection 5.

Ischemic Colitis

• Right-sided ischemic colitis is less common but more dangerous than left-sided disease, often occurring in elderly patients with cardiovascular disease, recent hypotension, or vasculopathy 4, 3.
• CT typically shows circumferential symmetric wall thickening with fold enlargement; "thumbprinting" on plain films is a classic but late finding 3.
• Absence of bloody diarrhea does not exclude ischemia—right-sided ischemia often presents with pain and ileus rather than bleeding 4.

Inflammatory Bowel Disease

• Crohn's disease has predilection for terminal ileum and right colon; look for skip lesions, asymmetric involvement, mesenteric fat stranding, and fistulas/abscesses on CT 1, 3.
• Isolated right-sided ulcerative colitis is rare in Western populations but has been reported; requires endoscopic and histologic confirmation 7.

Medication-Induced Colitis

• NSAIDs are associated with increased risk of colitis, particularly in patients on immune checkpoint inhibitors 1.
• Immune checkpoint inhibitor-induced colitis typically involves the descending colon more than ascending, but any segment can be affected; histology shows marked mixed inflammatory infiltrate 1.

Initial Management Strategy

Empiric Antibiotic Therapy (When Indicated)

• If patient has severe systemic illness (fever, leukocytosis >15 × 10⁹/L, AKI, lactate elevation), start empiric broad-spectrum antibiotics immediately: piperacillin-tazobactam or meropenem to cover gram-negatives, anaerobes, and enterococci 5.
• Add oral vancomycin 125 mg q6h immediately if C. difficile is clinically suspected (recent antibiotics, severe leukocytosis, typical CT findings) without awaiting PCR results 5, 6.
• For severe C. difficile with ileus or toxic appearance, use triple therapy: oral/NG vancomycin 125 mg q6h + IV metronidazole 500 mg q8h + rectal vancomycin 500 mg in 100-500 mL saline retention enema q6h 6.

Supportive Care

• Aggressive IV crystalloid resuscitation targeting urine output >0.5 mL/kg/h to reverse AKI and maintain perfusion 5.
• Correct electrolyte abnormalities, particularly potassium and magnesium 6.
• Avoid antiperistaltic agents (loperamide) and minimize opioids—both increase risk of toxic megacolon 5, 6.
• Provide pharmacologic VTE prophylaxis with subcutaneous heparin unless contraindicated 5.

When to Withhold Antibiotics

• For mild, uncomplicated presentations without systemic toxicity, fever, or significant leukocytosis, consider observation with supportive care alone while awaiting stool studies 5.
• If inflammatory bowel disease flare is suspected and infection is ruled out, antibiotics may worsen outcomes and should be avoided 1.

Surgical Consultation Criteria

Immediate Surgical Input Required

• Worsening abdominal pain, rising leukocytosis despite antibiotics, lactate >4 mmol/L, or CT evidence of perforation, pneumatosis, or toxic megacolon (transverse colon >5.5 cm) 5, 6.
• Fulminant C. difficile (WBC >35 × 10⁹/L, lactate >5 mmol/L, end-organ dysfunction) may warrant subtotal colectomy even before completing 48-hour antibiotic trial 6.
• Right-sided ischemic colitis with peritoneal signs or CT evidence of transmural infarction requires urgent surgical exploration 4.

Surgical Procedure of Choice

• For fulminant colitis requiring surgery, subtotal colectomy with end ileostomy is the preferred operation 6.

Monitoring and Response Assessment

• Perform daily assessments of stool frequency, abdominal examination, vital signs, leukocyte count, CRP, and creatinine to gauge treatment response 5.
• Expect clinical improvement within 3-5 days; lack of progress mandates repeat imaging and possibly flexible sigmoidoscopy with biopsies 5, 6.
• Do not repeat C. difficile PCR after completing therapy unless symptoms recur—PCR may remain positive for weeks despite cure 5, 6.

Antibiotic De-escalation and Duration

• When stool cultures identify a specific pathogen, narrow to the most targeted antibiotic based on susceptibility testing 5.
• If all cultures remain negative after 48-72 hours and patient shows clinical improvement, consider stopping IV antibiotics or switching to oral agents 5.
• Limit total antibiotic course to 7 days for non-C. difficile infections with adequate response, and to 10-14 days for confirmed C. difficile infection 5, 6.

Common Pitfalls to Avoid

• Do not assume left-sided distribution for all colitis—right-sided involvement changes your differential and management 4, 7.
• Do not rely on absence of bloody diarrhea to exclude ischemia—right-sided ischemia often presents with pain and ileus alone 4.
• Do not delay surgical consultation in patients with severe systemic toxicity, rising lactate, or worsening clinical status despite antibiotics 5, 6.
• Do not perform full colonoscopy in severe or fulminant colitis—perforation risk is prohibitive 6.
• CT has only 52% sensitivity for C. difficile colitis—negative CT does not exclude CDI if clinical suspicion is high 5.