Intrathecal Opioids (10 Marks)

Definition

Mechanism of Action

• Opioid receptors (mainly mu, also kappa and delta) are densely expressed in the substantia gelatinosa (lamina II) of the spinal dorsal horn.
• Receptors are located:
  • Presynaptically on the terminals of small primary afferent C-fibers - activation inhibits release of excitatory neurotransmitters (substance P, glutamate).
  • Postsynaptically on second-order dorsal horn neurons - activation causes hyperpolarization and reduced neuronal firing.
• Net effect: interruption of nociceptive transmission at the first synapse in the pain pathway, producing analgesia without blocking touch, proprioception, or motor fibers (unlike local anesthetics).
• Lipid solubility determines onset, spread (cephalad migration in CSF), and duration - lipophilic drugs (fentanyl, sufentanil) act fast and stay localized; hydrophilic drugs (morphine) have delayed onset but longer duration and greater cephalad spread (Goodman & Gilman's Pharmacological Basis of Therapeutics; Morgan and Mikhail's Clinical Anesthesiology, 7e).

Commonly Used Drugs and Doses

Synergy with Local Anesthetics

Advantages

• Excellent, prolonged analgesia with a single small dose
• No or minimal motor blockade or sympathectomy - preserves ability to push in labor, useful in patients who cannot tolerate sympathetic block (aortic stenosis, tetralogy of Fallot, Eisenmenger syndrome, pulmonary hypertension)
• Reduces systemic opioid requirement and associated systemic side effects
• Long-lasting postoperative analgesia (morphine) reducing need for repeated dosing

Disadvantages / Side Effects

1. Respiratory depression - the most serious complication; centrally mediated.
   • Early (within 2 hours): due to vascular/systemic absorption, more common with lipophilic opioids (fentanyl, sufentanil).
   • Late/delayed (up to 6-24 hours): due to cephalad spread of hydrophilic morphine in CSF to the brainstem respiratory centers - never described beyond 2 hours with fentanyl/sufentanil, but a recognized risk with morphine.
2. Pruritus - most common side effect, more frequent with intrathecal/epidural than IV opioids, centrally mediated (naloxone-reversible), independent of dose or opioid type; face/trunk more than extremities.
3. Nausea and vomiting - centrally mediated via chemoreceptor trigger zone.
4. Urinary retention - dose-dependent suppression of detrusor contraction, common especially with intrathecal morphine/sufentanil.
5. Sedation
6. Rarely: fetal bradycardia reported with intrathecal sufentanil in labor (not consistently confirmed in later studies).

Contraindications

• Patient refusal, coagulopathy, local/systemic infection, raised intracranial pressure, true opioid allergy, uncorrected hypovolemia (relative, mainly for accompanying local anesthetic component).

Monitoring / Management

• Continuous respiratory rate and sedation score monitoring for at least 12-24 hours after intrathecal morphine (per most institutional protocols), given delayed respiratory depression risk.
• Pulse oximetry, naloxone infusion (0.1-0.2 mg/h IV) available to treat side effects (pruritus, respiratory depression) without fully reversing analgesia.
• Antiemetics (ondansetron) for nausea/pruritus.

Clinical Uses

• Labor analgesia (spinal opioid alone or CSE)
• Cesarean section postoperative analgesia (intrathecal morphine, single-shot spinal)
• Postoperative analgesia after major surgery (orthopedic, cardiac, abdominal)
• Chronic cancer and non-cancer pain via implanted intrathecal drug delivery systems (pumps)

Sources

• Morgan and Mikhail's Clinical Anesthesiology, 7e, "Intrathecal Opioids" and "Regional Anesthetic Techniques," Ch. 41
• Miller's Anesthesia, 10e, "Opioids," "Pruritus," "Respiratory" sections, Ch. 41/spinal additives
• Goodman & Gilman's Pharmacological Basis of Therapeutics, Table 23-2, "Sensory Versus Affective Dimensions of Pain"
• Roberts and Hedges' Clinical Procedures in Emergency Medicine, "Device Complications"