Non-analgesic effects of opioids: the cognitive effects of opioids in chronic pain of malignant and non-malignant origin. An update.

Opioids constitute the basis for pharmacological treatment of moderate to severe pain in cancer pain and non-cancer pain patients. Their action is mediated by the activation of opioid receptors, which integrates the pain modulation system with other effects in the central nervous system including cognition resulting in complex interactions between pain, opioids and cognition. The literature on this complexity is sparse and information regarding the cognitive effects of opioids in chronic pain patients is substantially lacking. Two previous systematic reviews on cancer pain and non-cancer pain patients only using controlled studies were updated. Fourteen controlled studies on the cognitive effects of opioids in chronic non-cancer pain patients and eleven controlled studies in cancer pain patients were included and analyzed. Opioid treatment involved slightly opposite outcomes in the two patient groups: no effects or worsening of cognitive function in cancer pain patients and no effect or improvements in the chronic non-cancer pain patients, however, due to methodological limitations and a huge variety of designs definite conclusions are difficult to draw from the studies. In studies of higher quality of evidence opioid induced deficits in cognitive functioning were associated with dose increase and the use of supplemental doses of opioids in cancer patients. Future perspectives should comprise the conduction of high quality randomized controlled trials (RCTs) involving relevant control groups and validated neuropsychological assessments tools before and after opioid treatment in order to further explore the complex interaction between pain, opioids and cognition.