Opioids are the most potent centrally acting analgesic drugs for the treatment of pain. For the past years, since the discovery of spinal opioid receptors, the use of spinal opioids has been adopted in clinical practice in the hope of producing intense segmental analgesia that was devoid of the dose-limiting side effects associated with systemic opioid administration. Experimental studies have demonstrated that after their perispinal administration, liposolubility is inversely proportional to their spinal selectivity, which is higher for the most water-soluble drug, morphine, than for other more lipophilic drugs, such as fentanyl and sufentanil. Clinical trials have shown that epidural morphine in the form of extended-release liposome injections gives good analgesia for a period of 48 hours, with no need for epidural catheterization. Conversely, fentanyl is the most appropriate opioid in ambulatory surgery and seems to have the strongest effect at the spinal cord administered epidurally as a bolus and supraspinally using continuous epidural infusion. Epidural methadone and hydromorphone are suitable alternatives for analgesia in the postoperative period, given that they have intermediate pharmacokinetic characteristics with respect to the two aforementioned groups of opioids. All opioids administered intrathecally will produce some degree of spinally mediated analgesia. The main differences are related to their duration of action, rate of clearance, and the pathways by which the drugs reach their receptors in the brain. In general, lipophilic opioids produce short-term analgesia (1-4 hours), which is very useful for immediate postoperative pain. However, morphine produces intense analgesia for up to 24 hours with doses as low as 100 µg.

Opioids are considered a “gold standard” in clinical practice for the treatment of postoperative pain. The spinal administration of an opioid drug does not guarantee selective action and segmental analgesia in the spine. Evidence from experimental studies in animals indicates that bioavailability in the spinal cord biophase is negatively correlated with liposolubility, and is higher for hydrophilic opioids, such as morphine, than lipophilic opioids, such as fentanyl, sufentanil and alfentanil. Epidural morphine sulphate has proven analgesic efficacy and superiority over systemically administered morphine for improving postoperative pain. However, pain relief after a single epidural injection of morphine could last less than 24 hours. Techniques used to administered and prolong opioid epidural analgesia, can be costly and inconvenient. Moreover, complications can arise from indwelling epidural catheterization, particularly in patients receiving anticoagulants. Clinical trials have shown that epidural morphine in the form of extended-release liposome injections (EREM) gives good analgesia for a period of 48 hours, with no need for epidural catheterisation. Intrathecal morphine produces intense analgesia for up to 24 hours with a single shot, and clinical recommendation is to choose the minimum effective dose and do not exceed 300 μg to prevent the delay respiratory depression.

Bujedo BM. Korean J Pain. 2014 Jul;27(3):200-209. http://dx.doi.org/10.3344/kjp.2014.27.3.200

The itching or pruritus is a very annoying secondary effect that appears after the neuraxial administration (epidural or intrathecal) of opioid drugs. Sometimes, it can even be more unpleasant that the own pain in or by itself. Both, either prevention or treatment remains a challenge in the clinical practice of care for these patients. A wide variety of medications with different mechanisms of action has been used and focused on its prevention and treatment, with widely varying results. The objective of this article is to review the literature and summarize the current evidence of the mechanisms and pharmacological treatments available to handle pruritus induced by spinal opioids. The source of articles of this review was obtained through PubMed, Medline and Scopus until December 2016. The most useful drugs are opioid mu antagonists, as naloxone, and mixed opioids kappa agonists /mu antagonists, as nalbuphine and butorphanol, the latter being able in addition to maintaining the analgesia. They have also shown some effectiveness, but to a lesser degree, from serotonin 5 HT3 receptor antagonists, as ondansetron, administered prophylactically, and the antagonists of dopaminergic receptors D2, as dehidrobenzoperidol. Finally, sub-anesthesic low dose of propofol and oral mirtazapine have been used with poor results.