How do you differentiate and manage constipation versus bowel obstruction?

Distinguishing Between Constipation and Bowel Obstruction

Constipation and bowel obstruction are distinct clinical entities with different management approaches, with CT scan being the preferred imaging modality for suspected obstruction due to its approximately 90% accuracy in predicting strangulation and need for urgent surgery. 1

Key Differentiating Features

Feature	Constipation	Bowel Obstruction
Pain	Dull, crampy, intermittent	Colicky, more severe, periodic [2]
Stool	Hard, infrequent (<3 BMs/week)	May have obstipation (complete absence) [2]
Vomiting	Uncommon	Common, earlier in small bowel obstruction [2]
Distension	Mild to moderate	More pronounced, sudden onset [2]
Bowel Sounds	Normal or decreased	Initially hyperactive, later hypoactive [1,2]

Diagnostic Approach

History Elements to Focus On

• Prior abdominal surgeries (strongly suggests adhesive obstruction) 1, 3
• Pattern of constipation (chronic vs acute onset)
• Presence of vomiting (suggests obstruction, especially if feculent)
• Recent changes in bowel patterns 2
• Medication use, particularly opioids 1

Physical Examination

• Abdominal distension (more pronounced in obstruction)
• Bowel sounds (hyperactive early in obstruction, absent late)
• Digital rectal examination to assess for fecal impaction 2
• Signs of peritonitis (rebound tenderness, guarding) suggest strangulation or perforation 1

Imaging Studies

1. Plain Abdominal X-ray:

   • Often first-line but has low sensitivity (~70%) for obstruction 1
   • May show dilated bowel loops and air-fluid levels in obstruction
   • Can assess fecal loading in constipation 2

2. CT Scan with Contrast:

   • Gold standard for suspected obstruction 1
   • 90% accuracy in predicting strangulation and need for surgery 1
   • Can identify transition point and exclude other causes 1
   • Shows distinct transition between dilated and normal bowel in obstruction 1

3. Water-Soluble Contrast Studies:

   • Both diagnostic and potentially therapeutic 1
   • If contrast doesn't reach colon within 24 hours, suggests complete obstruction 1, 4
   • Passage of contrast to colon within 5 hours suggests high likelihood of resolution 4

4. Ultrasound:

   • Operator-dependent but useful in specific situations (pregnancy) 1
   • Can detect free fluid suggesting need for urgent surgery 1
   • May have surprisingly good accuracy when performed by experienced operators 5

Management Algorithm

For Constipation:

1. Initial Management:

   • Ensure adequate hydration (8 glasses of water daily)
   • Increase dietary fiber (20-25g daily)
   • Encourage physical activity
   • Optimize toilet positioning 2

2. Pharmacological Management (if lifestyle measures fail):

   • First-line: Polyethylene glycol (PEG) 17g daily mixed in 4-8 oz liquid 2
   • Second-line options:
     • Osmotic laxatives (lactulose, magnesium salts)
     • Stimulant laxatives (senna, bisacodyl)
     • Stool softeners (docusate sodium) 2

3. Treatment Goal: One non-forced bowel movement every 1-2 days 2

For Suspected Bowel Obstruction:

1. Initial Management:

   • Intravenous fluid resuscitation
   • Nothing by mouth
   • Nasogastric tube decompression for significant distension/vomiting 1, 3
   • Correction of electrolyte abnormalities 6
   • Intravenous antibiotics if signs of ischemia 6

2. Decision for Operative vs. Non-operative Management:

   • Immediate Surgery Indications:

     • Peritonitis
     • Signs of strangulation/ischemia
     • Clinical deterioration
     • CT evidence of complete obstruction with concerning features 1

   • Trial of Non-operative Management (for partial obstruction without concerning features):

     • Water-soluble contrast challenge
     • Continued nasogastric decompression
     • Serial abdominal examinations every 4 hours 1, 4
     • Failure to improve within 3-5 days indicates need for surgery 2

Common Pitfalls and Caveats

1. Misdiagnosis Risk: Adhesive small bowel obstruction can mimic simple constipation, especially if intermittent or low-grade 1

2. Radiation-Induced Obstruction: Progressive over many years and can cause both strictures and generalized dysmotility 1

3. Opioid Effects: Can cause both constipation and narcotic bowel syndrome, complicating diagnosis 1

4. Partial vs. Complete Obstruction: Partial obstructions may resolve with conservative management, while complete obstructions typically require surgery 1

5. CT Limitations: Standard CT has lower sensitivity (48-50%) for low-grade or intermittent obstruction; CT enteroclysis may be needed 1

6. Red Flags requiring immediate attention:

   • Severe abdominal pain
   • Significant distension
   • Persistent vomiting
   • Blood in stool
   • Recent onset constipation in older adults
   • Weight loss or anemia 2

By systematically evaluating clinical features and using appropriate imaging, particularly CT scan, clinicians can effectively differentiate between constipation and bowel obstruction, leading to appropriate management decisions and improved patient outcomes.