What is the differential diagnosis and management for bowel symptoms, including possible impaction?

Differential Diagnosis of Bowel Symptoms

For bowel symptoms, prioritize distinguishing between functional constipation with possible impaction, inflammatory bowel disease, infectious colitis, and bowel obstruction through targeted history, digital rectal examination, and selective laboratory and imaging studies.

Key Clinical Features to Assess

History Elements

• Duration and pattern: Acute symptoms (<2 weeks) suggest infection or ischemia, while chronic symptoms (>4 weeks) point toward IBD, functional disorders, or structural abnormalities 1
• Stool characteristics: Bloody diarrhea suggests IBD or infectious colitis; hard infrequent stools with overflow soiling indicates impaction 1, 2
• Associated symptoms: Weight loss and rectal bleeding raise concern for colorectal cancer or IBD; abdominal distension with colicky pain suggests obstruction 1
• Medication history: Opioids cause constipation in ~50% of users; NSAIDs can mimic IBD 3, 4
• Previous surgeries: Prior abdominal surgery increases adhesive bowel obstruction risk (85% sensitivity) 1

Physical Examination Priorities

• Digital rectal examination: Essential to identify fecal impaction—a full rectum or hard stool mass confirms the diagnosis 3, 2, 5
• Abdominal examination: Distension has a positive likelihood ratio of 16.8 for bowel obstruction; peritoneal signs indicate perforation or ischemia 1
• Perianal inspection: Look for fecal soiling (suggests overflow from impaction), fissures, or fistulas (suggests Crohn's disease) 2, 1

Differential Diagnosis

Primary Considerations

Fecal Impaction

• Chronic constipation leads to rectal distension with hard stool accumulation 2
• Overflow incontinence (paradoxical diarrhea) occurs when liquid stool leaks around the impaction 2
• Risk factors include elderly age, immobility, opioid use, and neurological conditions 3, 6
• Yes, impaction is possible and common, particularly in high-risk populations 5, 6

Inflammatory Bowel Disease (IBD)

• Ulcerative colitis: Continuous inflammation from rectum proximally with bloody diarrhea 1
• Crohn's disease: Discontinuous lesions, strictures, perianal disease 1
• Consider when symptoms persist >14 days despite treatment 1

Infectious Colitis

• Bacterial (Salmonella, Shigella, Campylobacter, C. difficile): Acute bloody diarrhea 1
• Parasitic (Giardia, Entamoeba): Chronic diarrhea with travel history 1
• CMV colitis: In immunocompromised patients 1, 7

Bowel Obstruction

• Small bowel: Adhesions (55-75%), hernias (15-25%), malignancy (5-10%) 1
• Large bowel: Cancer (60%), volvulus (15-20%), diverticular disease (10%) 1
• Presents with colicky pain, distension, nausea/vomiting 1

Other Considerations

• Irritable bowel syndrome: Normal inflammatory markers, faecal calprotectin <100 μg/g 1
• Ischemic colitis: Elderly with cardiovascular disease, acute onset 1, 7
• Medication-induced: NSAIDs, mycophenolate, immune checkpoint inhibitors 7, 8

Laboratory Investigations

First-Line Tests

• Complete blood count: Leukocytosis suggests bacterial infection or ischemia; anemia (Hb <13 g/dL men, <12 g/dL women) suggests IBD or malignancy 1
• C-reactive protein: Elevated in active inflammation 1
• Electrolytes and renal function: Assess for dehydration and pre-renal failure from obstruction 1
• Faecal calprotectin: <100 μg/g makes IBD unlikely; >250 μg/g warrants urgent gastroenterology referral 1

Additional Tests When Indicated

• Stool culture and C. difficile toxin: For acute diarrhea, especially if bloody 1
• Iron studies: Ferritin <30 μg/g (or <100 μg/g with inflammation) indicates iron deficiency 1
• Lactate and arterial blood gas: If ischemia suspected (elevated lactate, metabolic acidosis) 1

Imaging Studies

Plain Radiography

• Acute abdominal series (KUB): First-line for suspected obstruction or impaction 6
• Identifies dilated bowel loops, air-fluid levels, and large fecal masses 6

Cross-Sectional Imaging

• CT abdomen/pelvis with IV contrast:

  • Gold standard for bowel obstruction—identifies transition point and cause 1
  • Detects complications: perforation, abscess, ischemia 1
  • Wall thickening >10 mm suggests severe inflammation; strictures with upstream dilation (>4 cm) indicate obstruction 1

• MR enterography:

  • Preferred for young patients with suspected Crohn's disease (avoids radiation) 1
  • T2 hyperintensity indicates mural edema/inflammation 1

Endoscopy

• Colonoscopy with biopsies:

  • Required for definitive IBD diagnosis—obtain ≥2 biopsies from inflamed and normal-appearing mucosa 1
  • Differentiates IBD from infectious colitis (preserved crypt architecture in infection) 1
  • Avoid in acute severe colitis or suspected perforation 1

• Flexible sigmoidoscopy: Safer alternative in acute colitis; can differentiate IBD from CMV or C. difficile 1

Management of Fecal Impaction

Immediate Treatment

Manual disimpaction is first-line when impaction confirmed on digital rectal exam 3, 2, 5

• Digital fragmentation and extraction of stool 3, 2
• Follow with glycerin suppositories or enemas 3

Pharmacological Disimpaction

• Polyethylene glycol (PEG) 3350: Oral or via nasogastric tube for proximal impaction 5, 9
• Enemas: Phosphate, mineral oil, or warm water enemas for distal impaction 3
• Contraindications for enemas: Neutropenia, thrombocytopenia, intestinal obstruction, recent colorectal surgery, severe colitis, recent pelvic radiotherapy 3

Maintenance Prevention

• PEG 17 g daily: Most effective osmotic laxative with best safety profile 3
• Stimulant laxatives (senna, bisacodyl): Prophylactic use for opioid-induced constipation 3
• Fiber supplementation: Psyllium 20-30 g daily with adequate hydration 3

Critical Pitfalls to Avoid

• Do not assume overflow diarrhea is infectious: Always perform digital rectal exam to exclude impaction 2
• Stop PEG if rectal bleeding, worsening pain, or diarrhea develops: These indicate serious complications 4
• Do not delay imaging in suspected obstruction: Moderate-severe upstream dilation (>4 cm) requires urgent evaluation for transition point 1
• Recognize that faecal calprotectin 100-250 μg/g is indeterminate: Repeat testing or refer if clinical suspicion remains high 1
• Post-disimpaction colonoscopy is mandatory: Evaluate for underlying structural causes (cancer, stricture) once impaction resolves 5