Opioid-induced hyperalgesia

Abstract

Opioid-induced hyperalgesia (OIH) is defined as a state of nociceptive sensitisation caused by exposure to opioids. The condition is characterised by a paradoxical response whereby a patient receiving opioids for the treatment of pain could actually become more sensitive to certain painful stimuli. The type of pain experienced might be the same as the underlying pain or might be different from the original underlying pain. Observational, cross-sectional, and prospective controlled trials have examined the expression and potential clinical significance of OIH in humans. Most studies have been conducted using several distinct cohorts and methodologies utilising former opioid addicts on methadone maintenance therapy, perioperative exposure to opioids in patients undergoing surgery, cancer patients on opioids, and healthy human volunteers after acute opioid exposure using human experimental pain testing. There are several proposed mechanisms for OIH; among these mechanisms involving the central glutaminergic system, spinal dynorphins, descending facilitation, genetic mechanisms, and decreased reuptake and enhanced nociceptive response have been described as the important mechanisms. Clinicians should suspect OIH when opioid treatment’s effect seems to wane in the absence of disease progression, particularly if found in the context of unexplained pain reports or diffuse allodynia unassociated with the original pain, and increased levels of pain with increasing dosages. The treatment involves reducing the opioid dosage, tapering them off, or supplementation with NMDA receptor inhibitors.