A 45-year-old woman on long-term opioid therapy for chronic back pain develops severe constipation unresponsive to standard laxatives. Her physician suspects opioid-induced bowel dysfunction (OIBD). Which investigation is most appropriate to rule out mechanical obstruction before prescribing peripherally acting mu-receptor antagonists?

## Investigation of Choice: Abdominal Imaging (X-ray or CT) ### Why Imaging is Essential **Key Point:** Abdominal imaging is mandatory to exclude mechanical obstruction (bowel impaction, stricture, malignancy) before attributing constipation to opioid-induced bowel dysfunction and initiating treatment. ### Clinical Reasoning **High-Yield:** OIBD is a diagnosis of exclusion. Mechanical obstruction must be ruled out because: 1. **Peripherally acting mu-receptor antagonists (e.g., naloxegol, methylnaltrexone) are contraindicated in mechanical obstruction** — they can precipitate perforation or worsening obstruction. 2. **Imaging findings guide management:** - **Plain abdominal X-ray:** Quick, low-cost screening for impaction, air-fluid levels, megacolon - **CT abdomen:** Higher sensitivity for strictures, masses, volvulus, and other structural causes 3. **Clinical Pearl:** Opioid-induced constipation is functional (no structural lesion); if imaging is abnormal, the diagnosis is NOT OIBD and treatment strategy changes. ### Why Other Tests Are Not Appropriate | Investigation | Why Not First-Line | | --- | --- | | Serum opioid metabolite levels | Does not assess bowel structure or obstruction; not diagnostic for OIBD | | Fecal occult blood test (FOBT) | Screens for GI bleeding, not obstruction; not indicated unless alarm symptoms present | | Serum electrolytes and renal function | Useful for monitoring chronic opioid therapy but does not rule out mechanical obstruction | **Warning:** Starting naloxegol or methylnaltrexone without excluding mechanical obstruction is a common trap — these agents can worsen obstruction and cause perforation. **Mnemonic: OIBD Diagnosis = Exclusion** - **O**pioid-induced constipation - **I**maging first (to exclude obstruction) - **B**owel dysfunction (functional diagnosis) - **D**rug treatment only after imaging clears