Opioid toxicity and rotation in advanced pancreatic cancer
A 58-year-old man with locally advanced pancreatic cancer has severe abdominal and back pain. He is taking oral morphine modified-release 100 mg twice daily, with immediate-release morphine 20 mg for breakthrough pain, up to four doses per day. Over the past two weeks he has developed worsening pain (8/10), generalised myoclonus, daytime sedation, and a feeling of "burning" skin. He has also been started on gabapentin 300 mg three times daily. Blood tests: creatinine 180 µmol/L (eGFR 28 mL/min/1.73 m²), albumin 28 g/L, calcium 2.30 mmol/L, bilirubin 12 µmol/L. He is constipated despite regular senna and macrogol. He is anxious and asks whether he should increase his morphine. He lives alone and has a supportive sister nearby.
1. What is the likely diagnosis, and how would you distinguish it from disease progression?
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The likely diagnosis is opioid-induced neurotoxicity, with myoclonus, sedation, and hyperalgesia (the burning skin) in the context of high-dose morphine and renal impairment. Morphine is metabolised to morphine-6-glucuronide and morphine-3-glucuronide; both accumulate in renal failure, and morphine-3-glucuronide is associated with neuroexcitation. Gabapentin can also cause sedation, but it does not explain myoclonus or hyperalgesia.
Disease progression must still be excluded. I would ask about new back pain, leg weakness, bladder or bowel dysfunction, and examine for a sensory level, which could indicate spinal cord compression from metastases. I would check calcium (2.30 mmol/L is not hypercalcaemic), infection markers, and liver function, and arrange imaging if cord compression is suspected. The burning quality may reflect neuropathic cancer pain, but the temporal relationship with morphine escalation and the presence of myoclonus favour opioid toxicity.
The key point is that increasing the morphine dose would likely worsen the toxicity, not improve the pain.
2. Outline an opioid rotation plan, including an equianalgesic calculation and monitoring.
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I would not increase morphine. Because of renal impairment and neurotoxicity, I would rotate to a drug less dependent on renal clearance, such as hydromorphone or fentanyl, and involve palliative care.
Calculation using the total daily morphine dose: 100 mg modified-release twice daily = 200 mg, plus up to 80 mg immediate-release = 280 mg/day. Using an approximate oral morphine-to-hydromorphone ratio of 4–5:1, this is about 56–70 mg hydromorphone/day. Because of incomplete cross-tolerance, reduce by 25–50%: start around 28–35 mg/day, for example hydromorphone 6 mg every 4 hours with 2–3 mg breakthrough, and titrate. An alternative is transdermal fentanyl, but it is less suitable for unstable pain and requires careful conversion; if used, start at the lower end of the calculated range.
Monitoring: hourly pain and sedation scores initially, respiratory rate, myoclonus, and constipation. Give naloxone only for respiratory depression; do not reverse abruptly. Reassess renal function and hydration. Document the conversion and ensure the community team can contact the patient daily.
3. What non-opioid and supportive measures would you add, and what safety-netting is required?
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I would optimise non-opioid analgesia and address the drivers of the pain:
- Gabapentin can be titrated slowly for neuropathic pain, monitoring sedation and renal function.
- Corticosteroids such as dexamethasone 8 mg daily may reduce peritumoural oedema and pancreatic cancer pain.
- Celiac plexus block should be considered for visceral upper abdominal pain.
- Bowel regimen: continue senna and macrogol, add an osmotic laxative if needed, and avoid bulking agents if there is poor mobility.
- Hydration and nutrition: correct dehydration, which worsens metabolite accumulation.
Safety-netting: he lives alone, so I would arrange daily contact, a written escalation plan, and a palliative care nurse review. Advise his sister on recognising sedation and slow breathing. Review all medicines for sedatives. If he develops intractable myoclonus, drowsiness, or respiratory depression, he should be reviewed urgently. The priority is comfort and quality of life, not simply achieving a lower pain score on a higher opioid dose.