|LETTER TO THE EDITOR
|Year : 2018 | Volume
| Issue : 1 | Page : 54-55
Intrathecal morphine: Present status
Abhijit Sukumaran Nair, Basanth Kumar Rayani
Department of Anaesthesiology, Basavatarakam Indo-American Cancer Hospital and Research Institute, Hyderabad, Telangana, India
|Date of Web Publication||30-Apr-2018|
Dr. Abhijit Sukumaran Nair
Department of Anaesthesiology, Basavatarakam Indo-American Cancer Hospital and Research Institute, Hyderabad, Telangana 500034
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Nair AS, Rayani BK. Intrathecal morphine: Present status. Indian J Pain 2018;32:54-5
Intrathecal (IT) morphine has been used successfully to manage acute postoperative pain in patients undergoing obstetric, urological, general, orthopedic, and spine surgeries. It provides long-lasting analgesia and better pain scores when used in appropriate dose, and it is cost-effective. IT morphine is considered gold standard for managing obstetric postoperative pain. Enhanced Recovery After Surgery (ERAS) Society consensus statement recommends a dose of 200–250 μg in less than 75-year-old patients and less than 200 μg in more than 75-year-old patients undergoing major abdominal surgeries. The randomized-controlled study by Koning et al. showed that IT morphine is a more effective method of postoperative analgesia in laparoscopic colonic surgery than intravenous opioids within the ERAS program. IT opioids bind to G protein–linked opioid receptors at pre- and postsynaptic levels in laminae I and II of dorsal horn. On activation of μ- and κ-receptors (via potassium-channel opening) and δ-receptors (via closure of calcium channels), the release of excitatory transmitters, glutamate and substance P, is reduced from presynaptic C fibers, which reduces transmission of nociceptive stimulus by decreasing intracellular calcium. IT morphine increases lumbosacral adenosine levels that open potassium channels causing hyperpolarization of nerve fibers, which reduces overall neuronal activity. IT morphine reduces release of gamma-aminobutyric acid and glycine from dorsal-horn neurons. IT morphine is 129–1737 times more hydrophilic than fentanyl with low octanol/water coefficient, which leads to slow diffusion into epidural space. Fentanyl has high octanol/water partition coefficient (860), which leads to high volume of distribution in nonreceptor sites in spinal cord epidural fat, myelin, and white matter. IT morphine binds to high-affinity receptors in the dorsal-horn receptor sites but has less binding to nonreceptor sites in myelin and white matter compared with fentanyl. This leads to small “volume of distribution” within spinal cord and high concentration within cerebrospinal fluid for a longer duration, which provides a wide band of analgesia. Therefore, morphine has the best bioavailability when injected intrathecally. However, there is a possibility of dose-dependent late-onset respiratory depression (between 3.5 and 12 h). The risk factors for respiratory depression are advanced age, use of long-acting sedatives, positive pressure ventilation, and presence of respiratory disease.
As IT morphine provides segmental analgesia, the side effects observed are less frequent when compared with systemic opioids. The usually observed side effects are sedation, sweating, delayed gastric emptying, urinary retention, pruritus, nausea and vomiting, and respiratory depression. Pruritus is dose dependent with an incidence of 0%–100%. Pruritus is mostly observed in obstetric patients, which could be due to altered hormonal milieu during gestational period. Pruritus can be treated with 5HT3 antagonists or propofol. Antihistaminics have no role in managing opioid-induced pruritus as histamine release is not mediated via opioid receptors. Partial opioid agonist nalbuphine has been found superior in reducing opioid-induced pruritus with a dose of 2.5–5 mg without causing drowsiness. Constipation and ileus are unlikely to happen as the peripherally located μ-receptors are spared. Till date, there is no evidence of neurotoxicity observed after use of IT morphine.
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