Lack of analgesic effect of opioids on neuropathic and idiopathic forms of pain.

The aim of the present study has been to assess the responsiveness of various types of chronic pain to opioids given i.v. and tested against placebo in a double-blind, randomized fashion. Pain classified as primary nociceptive was effectively alleviated (P greater than 0.001) while neuropathic deafferentation pain was not significantly influenced by morphine or equivalent doses of other opioids. Also 'idiopathic' pain, defined as chronic pain with no or little demonstrable pathology, failed to respond. The results were not related to whether the patients were regular users of narcotic analgesics or not. The outcome of our double-blind opioid test has proved useful to justify a continued, or discontinued, use of narcotic medication in individual patients. It may also support the indication and choice of invasive stimulation procedures (spinal cord or brain). The results of the study illustrate the misconception of chronic pain as an entity and highlight the importance of recognizing different neurobiological mechanisms and differences in responsiveness to analgesic drugs as well as to non-pharmacological modes of treatment. The opioid test has thus become a valuable tool in pain analysis and helpful as a guide for further treatment.