62-year-old man with worsening pain and muscle jerks
A 62-year-old man with metastatic prostate cancer to bone is seen in the palliative care clinic for worsening pain. His pain had been well-controlled on long-acting morphine 60 mg every 12 hours with short-acting morphine 15 mg every 4 hours as needed. Over the past week, his daughter, who is his caregiver, reports he has required more frequent breakthrough doses, now taking up to 6 per day. Despite this, his pain is now diffuse and he is exquisitely sensitive to touch. She also notes he has become "jumpy" with intermittent muscle jerks. He is oriented but appears distressed. His temperature is 37.1°C (98.8°F), blood pressure is 138/84 mmHg, pulse is 92/min, and respirations are 18/min. Physical examination reveals diffuse tenderness to light palpation over his arms and legs. Brief, random muscle twitches are noted in his extremities. Laboratory studies show: Sodium: 139 mEq/L Potassium: 4.1 mEq/L Calcium: 9.4 mg/dL BUN: 25 mg/dL Creatinine: 1.4 mg/dL (baseline 1.1 mg/dL) Which of the following is the most appropriate next step in management?
See the answer and explanation below:
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B
Explanation
Clinical Pearl:
Opioid-induced neurotoxicity can present with cognitive changes, myoclonus, and paradoxical pain (hyperalgesia/allodynia). It is often precipitated by high opioid doses or renal impairment, and management involves hydration, dose reduction, and opioid rotation.
Explanation:
This patient is presenting with signs and symptoms of opioid-induced neurotoxicity (OIN), specifically hyperalgesia ('exquisitely sensitive to touch') and myoclonus ('intermittent muscle jerks'), in the context of escalating morphine doses and mild renal insufficiency. The accumulation of morphine metabolites, which are cleared by the kidneys, is the likely cause. The most appropriate management for OIN is to rotate to a different opioid, such as hydromorphone or fentanyl, often at a reduced equianalgesic dose. This approach removes the offending agent and its toxic metabolites. Increasing the dose of morphine would be appropriate for progressive disease-related pain but is contraindicated here as it would worsen the neurotoxicity. While adding gabapentin can be useful for neuropathic pain, it does not address the underlying cause of the patient's acute deterioration. Administering naloxone is incorrect as the patient does not have signs of opioid overdose like respiratory depression; this action would precipitate severe withdrawal and a pain crisis. Obtaining a brain MRI to evaluate for central nervous system metastases is not the most appropriate immediate step, as the clinical syndrome is highly suggestive of OIN and this workup would delay necessary treatment.
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