Spinal opioid analgesia. A critical update

Small spinal (intrathecal or extrathecal) doses of opioids induce a long-lasting and regional analgesic effect in various experimental animal models. Nowadays extrathecal morphine administration is considered an established method of controlling postoperative and cancer-induced pain conditions. The potency of morphine applied by the spinal route is higher than when the drug is applied by the intravenous (IV) route. Opioids which are more lipophilic than morphine will provide a marginally better analgesic effect when administered by the spinal route as compared with the IV route. Several controlled clinical trials in postoperative patients have demonstrated that a single dose of morphine administered by the spinal route gives a more long-lasting action than a similar IV dose. It is not known whether frequent patient-controlled administration of morphine may provide equally good analgesia without additional side effects. The use of spinal morphine in the treatment of cancer-related pain is based on clinical experience only. There are risks in replacing opioid administration by the oral or IV route with spinal opioids. Morphine should only be used in selected cases until the advantage of spinal opioid analgesia to control postoperative and cancer pain has been clearly defined in well-designed clinical studies. Spinal morphine dosages must be individualised according to the intensity of the nociceptive stimuli and should take into account intra-individual variability in drug responses due to pharmacokinetic and pharmacodynamic factors.