Neuraxial Opioid-Induced Pruritus

Neuraxial opioid induced pruritus is itch, often facial, after intrathecal or epidural opioids. It is the commonest side effect of neuraxial opioids.

Epidemiology. It affects 30% to 100% of patients after intrathecal morphine (up to 60% to 100% in obstetric patients) and 20% to 70% after epidural opioids. It starts 1 to 4 hours after the dose with lipophilic agents earlier, and lasts up to 12 to 24 hours.

Risk factors. Obstetric patients, higher dose (intrathecal morphine over 100 microgram), younger age and prior history.

Pathophysiology.

Mu opioid receptor activation in the spinal trigeminal nucleus and dorsal horn leads to central itch. It is not histamine mediated. Serotonin and prostaglandin pathways contribute.

Itching of the face, neck and upper chest (or generalised) without rash, starting 1 to 4 hours after injection. Sleep disturbance and scratching.

Red flags.

• Urticaria, wheeze or hypotension (allergy).
• Sedation or slow respiratory rate.

Clinical. Check for rash, drug allergy and respiratory depression.

Differential diagnosis.

• Drug allergy.
• Other drug induced itch.
• Cholestasis of pregnancy.
• Skin disease.

Reassure the patient. Treat if distressing.

• Ondansetron: 4 mg IV once (4 to 8 mg), repeated once after 8 hours; maximum 16 mg daily in this use; monitor QTc; avoid in long QT; reduce in severe hepatic impairment.
• Nalbuphine: 2.5 to 5 mg IV every 4 to 6 hours as needed; maximum 20 mg daily for this indication; monitor sedation and analgesia reversal; use lower doses in older adults.
• Naloxone: 40 to 80 microgram IV bolus, or infusion 0.25 to 1 microgram/kg/h; maximum 400 microgram; monitor pain, respiratory rate; may reverse analgesia.
• Naltrexone: 6 mg orally once; maximum 9 mg for this indication; avoid in opioid dependence and hepatic failure.
• Propofol: 10 mg IV (low dose) single dose; maximum 20 mg; monitor sedation; reduce in older adults.

Antihistamines (diphenhydramine 25 mg IV or orally) work through sedation; use with caution in sedated patients and older adults.

Prevention. Ondansetron 4 mg IV at the time of spinal reduces incidence, and a lower intrathecal morphine dose (50 to 100 microgram) reduces itch.

Monitoring. Itch score, sedation and respiratory rate for 24 hours.

Referral and escalation. Acute pain service for refractory itch.