Pain Medicine: Acute and Chronic Pain Management
Pain items demand mechanism-based classification, safe multimodal dosing and competence in opioid conversion. Acute and chronic pain are examined separately but share pharmacology.
Mechanisms and assessment
- Nociceptive (somatic or visceral) versus neuropathic versus nociplastic; assess function as well as the numeric rating score.
- Peripheral and central sensitisation explain hyperalgesia and allodynia; pre-emptive analgesia targets central sensitisation.
Acute pain rules
- Multimodal: paracetamol 1 g every 6 hours (maximum 4 g/day), NSAIDs unless contraindicated (AKI, ulcer, coagulopathy, fracture healing), titrated opioids, ketamine 0.1–0.3 mg/kg, regional blocks.
- PCA: morphine 1 mg bolus with 5-minute lockout, no background infusion in opioid-naive patients.
- Neuraxial analgesia requires clotting and infection checks; monitor for local anaesthetic systemic toxicity and epidural haematoma.
Chronic pain and opioids
- Neuropathic pain: gabapentinoid, tricyclic or SNRI (duloxetine) first-line; opioids are not first-line.
- Oral morphine 30 mg is roughly equivalent to parenteral morphine 10 mg (3:1); oral morphine 10 mg approximates oral oxycodone 5 mg.
- Screen for misuse; watch for opioid-induced hyperalgesia and androgen suppression.
Worked example
A patient on oral morphine 60 mg/day needs conversion to PCA. 60 mg oral equals 20 mg parenteral morphine per 24 hours. Reduce by 50% for incomplete cross-tolerance to 10 mg/24 h; prescribe 1 mg bolus with a 6-minute lockout, no basal rate, and reassess within 24 hours.
Traps
Pethidine causing neurotoxicity in renal impairment; codeine in children who are CYP2D6 ultrarapid metabolisers; continuing NSAIDs in hypovolaemia or after fracture.