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Difficulty: MediumEnd-of-Life and Palliative Care

A 68-year-old man is evaluated in the hospital on the second day of admission for poor oral intake and lethargy. He has a history of non-small cell lung cancer with diffuse osseous metastases. For the past two months, his bone pain has been well-controlled on a regimen of extended-release morphine 90 mg twice daily with immediate-release morphine as needed. Over the past 24 hours, the patient has become increasingly confused and reports seeing unfamiliar people in his room. The nurse notes that he has developed sudden, brief, involuntary muscle jerks in his upper extremities. When the nurse gently touches his arm to check his blood pressure, the patient screams in agony, stating that the light touch is excruciating. Vital signs are temperature 37.1°C (98.8°F), blood pressure 108/68 mmHg, pulse 94/min, and respiratory rate 16/min. Laboratory studies show a serum creatinine of 3.4 mg/dL, which is elevated from a baseline of 1.0 mg/dL one month ago. Which of the following is the most appropriate next step in management?

  1. Rotate the current analgesic regimen to intravenous fentanylAnswer
  2. B
    Increase the dosage of the extended-release morphine
  3. C
    Administer intravenous naloxone
  4. D
    Obtain a non-contrast CT scan of the head
  5. E
    Transfer the patient to the intensive care unit for continuous electroencephalogram monitoring

Answer

The most appropriate next step is to rotate the current analgesic regimen to intravenous fentanyl.
The correct answer involves rotating the analgesic regimen to intravenous fentanyl. This patient is experiencing opioid-induced neurotoxicity (OIN), a syndrome characterized by delirium, myoclonus, and hyperalgesia (or allodynia, which is pain from a non-painful stimulus like light touch). Morphine is primarily metabolized in the liver to morphine-6-glucuronide (M6G, which provides analgesia) and morphine-3-glucuronide (M3G, which has no analgesic properties but causes neurotoxicity). Both metabolites are cleared by the kidneys. In the setting of acute kidney injury (evidenced by his elevated creatinine of 3.4 mg/dL), M3G accumulates and crosses the blood-brain barrier, causing central nervous system hyperexcitability. The definitive management of OIN is to decrease the dose of the offending opioid or, more commonly, rotate to a different opioid that does not have renally cleared toxic metabolites. Fentanyl and methadone are hepatically cleared and lack neurotoxic active metabolites, making them the preferred choices in patients with severe renal impairment.

Step-by-Step Solution

1
Identify the patient's new-onset clinical signs.
The patient has developed delirium (hallucinations, confusion), myoclonus (involuntary muscle jerks), and allodynia/hyperalgesia (excruciating pain from a light touch).
Recognizing this specific triad of symptoms is essential for differentiating a new pain crisis or brain metastasis from medication toxicity.
2
Correlate the symptoms with the patient's current medications and laboratory findings.
The patient is on high-dose morphine and has newly developed acute kidney injury (creatinine 3.4 mg/dL).
Morphine is metabolized into morphine-3-glucuronide (M3G), which is cleared by the kidneys. Renal failure causes M3G to accumulate.
3
Formulate a diagnosis based on the clinical picture.
The diagnosis is opioid-induced neurotoxicity (OIN) caused by the accumulation of the neurotoxic metabolite M3G.
M3G accumulation directly causes central nervous system hyperexcitability, leading to myoclonus, hyperalgesia, and delirium.
4
Select the most appropriate management strategy.
Rotate to an opioid that is safe in renal impairment, such as fentanyl or methadone.
Fentanyl is metabolized in the liver to inactive metabolites and is safe to use in patients with severe renal impairment. In addition to opioid rotation, hydration may be considered to help clear the accumulated toxic metabolites.

Key Concept

Opioid-induced neurotoxicity (OIN) presents with a classic triad of delirium, myoclonus, and hyperalgesia/allodynia. It typically occurs when opioids with renally cleared active metabolites (like morphine) accumulate due to acute kidney injury. The primary treatment is opioid rotation to an agent without active neurotoxic metabolites, such as fentanyl or methadone.
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Topics

End-of-Life and Palliative Care

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