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Difficulty: HardEnd-of-Life and Palliative Care

A 68-year-old woman is admitted to the oncology ward for management of intractable bone pain secondary to metastatic breast cancer. Her pain has been reasonably controlled for the past month on high-dose extended-release oral morphine, with immediate-release morphine used as needed for breakthrough pain. Over the past 48 hours, her family reports that she has become increasingly confused, agitated, and restless. On physical examination, the patient is disoriented and exhibits spontaneous, irregular jerking movements of her extremities. When the physician gently palpates her arm to check her pulse, she grimaces and cries out in severe pain. Laboratory studies reveal a blood urea nitrogen of 42 mg/dL and a serum creatinine of 2.4 mg/dL (baseline 0.9 mg/dL one week ago). Which of the following is the most appropriate next step in management?

  1. Discontinue morphine and initiate a continuous intravenous fentanyl infusionAnswer
  2. B
    Administer intravenous naloxone to rapidly reverse the suspected opioid toxicity
  3. C
    Increase the dose of immediate-release morphine to address the worsening pain
  4. D
    Discharge the patient to a home hospice program for comfort care
  5. E
    Request a psychiatric consultation to determine the patient's legal competence

Answer

Discontinue morphine and initiate a continuous intravenous fentanyl infusion
The patient is experiencing opioid-induced neurotoxicity (OIN), a severe complication of opioid therapy characterized by delirium, myoclonus, and hyperalgesia (pain out of proportion to a non-painful stimulus, such as light palpation). This condition is most commonly triggered when a patient on chronic high-dose morphine develops renal impairment. Morphine is conjugated in the liver to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), which rely on the kidneys for clearance. In acute kidney injury, the neuroexcitatory metabolite M3G accumulates, leading to neurotoxicity. The definitive management is opioid rotation. The offending drug must be discontinued and replaced with an opioid that lacks clinically significant active renal metabolites. Fentanyl and methadone are the opioids of choice in patients with severe renal impairment.

Step-by-Step Solution

1
Identify the clinical syndrome based on the patient's symptoms and examination findings.
The combination of confusion, myoclonus (jerking movements), and hyperalgesia (severe pain with light touch) in a patient on high-dose opioids indicates opioid-induced neurotoxicity (OIN).
Recognizing OIN is critical because it requires a completely different management strategy than typical cancer pain progression.
2
Identify the precipitating factor for the new-onset neurotoxicity.
The patient has developed acute kidney injury, evidenced by an elevated BUN and serum creatinine.
Morphine is metabolized in the liver to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), both of which are renally cleared. Renal failure causes these active metabolites to accumulate rapidly.
3
Determine the most appropriate pharmacological intervention.
The current opioid (morphine) must be discontinued. A rotation to an opioid that is safe in renal impairment, such as fentanyl or methadone, should be initiated.
Fentanyl is metabolized primarily into inactive metabolites and does not accumulate significantly in renal failure, making it the preferred choice to achieve analgesia while resolving the neurotoxicity.

Key Concept

Opioid-induced neurotoxicity (OIN) due to accumulation of active morphine metabolites in acute kidney injury requires prompt opioid rotation to a medication safe in renal failure, such as fentanyl or methadone.
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Topics

End-of-Life and Palliative Care

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