Pain Medicine: Mechanisms, Assessment and Safe Prescribing
Mechanisms matter
Distinguish nociceptive, neuropathic and nociplastic pain. Neuropathic pain typically has burning, shooting or electric qualities with allodynia or hyperalgesia; nociplastic pain is central sensitisation without clear tissue or nerve injury. The examiner will test whether you can name the mechanism and then match the drug class: gabapentinoids and tricyclics for neuropathic pain, opioids for severe nociceptive pain, and multimodal adjuvants (ketamine, lidocaine infusion, regional blocks) for complex perioperative pain.
Assessment and red flags
Always screen for red flags before treating: progressive neurological deficit, cauda equina features (saddle anaesthesia, bladder/bowel dysfunction), fever with spinal pain, unexplained weight loss, and pain worse at night or at rest. Use a structured history (site, radiation, quality, timing, aggravating/relieving factors) and validated tools (NRS, DN4 for neuropathic features).
Opioid safety
The exam loves opioid stewardship. Know equianalgesic conversion principles, tolerance versus opioid-induced hyperalgesia, and the signs of opioid toxicity (pinpoint pupils, respiratory depression, sedation). Naloxone is titrated (small aliquots, repeated) because a full reversal can precipitate severe pain and withdrawal. Beware of the trap of confusing delirium with uncontrolled pain in the elderly.
Worked example
A patient on 60 mg oral morphine daily needs a switch to IV. Using an oral-to-IV ratio of about 3:1, that is roughly 20 mg IV morphine per 24 hours, then reduce by 25-50% for incomplete cross-tolerance and titrate to effect. Always state the reasoning, not just the number.
Chronic pain principles
Emphasise function over pain score, self-management, graded activity, psychological therapies, and deprescribing where benefit is absent.