A 42-year-old woman on chronic morphine therapy for cancer pain develops severe constipation unresponsive to standard laxatives. Her physician suspects opioid-induced bowel dysfunction. Which investigation is most appropriate to rule out mechanical obstruction before attributing symptoms solely to opioid side effects?

## Investigation of Choice for Opioid-Induced Constipation with Obstruction Concern ### Why Abdominal CT is the Best Choice **High-Yield:** Abdominal CT with oral and IV contrast is the gold standard imaging investigation to exclude mechanical bowel obstruction in patients with opioid-induced constipation. It provides high sensitivity (>95%) and specificity for detecting obstruction, volvulus, perforation, and other surgical pathology that may mimic opioid-related constipation [cite:Harrison 21e Ch 297]. **Key Point:** Abdominal CT is essential because: - **Sensitivity & Specificity:** Detects mechanical obstruction, adhesions, tumors, and fecal impaction with >95% accuracy - **Non-invasive:** Avoids endoscopy-related risks in constipated patients - **Comprehensive:** Evaluates entire abdomen and pelvis in one study - **Guides management:** Determines if surgical intervention or conservative management is needed ### Why Other Investigations Are Inadequate | Investigation | Limitation | Clinical Role | |---|---|---| | **Serum morphine concentration** | Measures drug level, not bowel pathology; does not differentiate functional vs. mechanical obstruction | Therapeutic drug monitoring; irrelevant to obstruction diagnosis | | **Fecal chymotrypsin assay** | Assesses pancreatic exocrine function, not bowel obstruction | Used for pancreatic insufficiency diagnosis, not opioid-related constipation | | **Serum electrolytes & renal function** | Detects metabolic derangements from prolonged obstruction but does not identify the cause | Supportive investigation; does not confirm or exclude mechanical obstruction | ### Clinical Pearl **Warning:** Do NOT assume all constipation in opioid-treated patients is functional. Opioids increase risk of mechanical obstruction through adhesions, tumor progression (in cancer patients), and impaction. CT imaging is mandatory before attributing symptoms solely to opioid side effects. **Mnemonic: OBSTRUCT (Imaging Choice for Opioid Constipation)** - **O**bstruction ruled out → CT imaging essential - **B**owel pathology assessment → CT shows mechanical causes - **S**urgical vs. medical → CT guides management strategy - **T**umor or adhesions → CT detects these complications - **R**apid, non-invasive → CT is first-line imaging - **U**rgent exclusion needed → Do not delay imaging - **C**ontrast-enhanced study → Oral + IV contrast optimal - **T**herapeutic decision-making → CT result determines next step