Pain Medicine
The pain medicine domain tests mechanism-based assessment, analgesic safety, opioid rotation, and interventional indications in acute, chronic, and cancer pain. Questions frequently ask for the first-line drug or the next safe step.
Core rules
- Mechanism: classify nociceptive, neuropathic, and nociplastic pain using IASP terminology. Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system.
- Neuropathic pain: first-line options are gabapentinoids, tricyclic antidepressants, or duloxetine; topical lidocaine 5% is useful for postherpetic neuralgia. Opioids are not first-line.
- Cancer pain: follow the WHO ladder, prescribe regular plus breakthrough analgesia, add laxatives with opioids, and use co-analgesics. Celiac plexus block is indicated for upper abdominal cancer pain.
- Opioid safety: distinguish tolerance, dependence, and addiction. Screen for risk, monitor sedation and respiratory rate, and prescribe naloxone where overdose risk is high.
- Interventions: medial branch radiofrequency ablation after diagnostic facet blocks; epidural steroid for selected radicular pain; sympathetic block for CRPS; trigger-point injection for myofascial pain.
- NSAIDs and paracetamol: avoid NSAIDs in renal impairment, GI bleeding risk, and decompensated heart failure; paracetamol maximum 4 g/day, reduced to 3 g/day in frailty or liver disease.
Worked example
A patient on oral morphine 120 mg/day develops myoclonus and is rotated to oral oxycodone. Using a common 1.5:1 oral morphine:oxycodone ratio, the equianalgesic dose is 120 ÷ 1.5 = 80 mg/day. Because cross-tolerance is incomplete, reduce by 25–50%: start 40–60 mg/day divided, with breakthrough oxycodone at 10–15% of the total daily dose. Reassess sedation and pain within 24 hours; local conversion tables vary.
Traps that cost marks
- Prescribing opioids without a bowel regimen.
- Using pethidine in renal failure, where normeperidine accumulates.
- Missing serotonin syndrome risk with tramadol.
- Confusing addiction with pseudoaddiction or physical dependence.
- Performing epidural steroid injection without checking anticoagulation.
- Forgetting breakthrough doses when converting opioids.