Opioid responsiveness.
Cancer pain generally responds in a predictable way to analgesic drugs and drug therapy is the mainstay of treatment. A small proportion of patients, of the order of 20%, have pain that does not respond well to conventional analgesic management. Because opioid analgesics are the most important part of this pharmacological approach, a terminology has developed which centres around whether or not pain will respond to opioid analgesics. The terms opioid-responsive-pain and opioid-non-responsive pain, or opioid-resistant-pain, have been used to differentiate between patients whose pain falls into these two broad groups. This terminology is not satisfactory because it implies an all or none phenomenon, that is that pain either does or does not respond to opioid analgesics. Rarely is there such a clear distinction in practice. This is because the end point when titrating dose against pain with strong opioid analgesics is not simply pain relief or lack of relief: adverse effects may limit dose titration. It is preferable to describe patients with pain which is relatively less sensitive to opioids and/or patients where there is an inbalance between analgesia and unwanted effects as having "opioid-poorly-responsive pain". A pragmatic definition of opioid-poorly-responsive pain is pain that is inadequately relieved by opioid analgesics given in a dose that causes intolerable side effects despite routine measures to control them. Included in this definition is so called paradoxical pain which is not a distinct entity. Neuropathic pain is the most common form of opioid-poorly-responsive pain. The underlying pathophysiology remains unclear but abnormal metabolism of morphine is not the cause of a poor response to this drug. Patients with opioid-poorly-responsive-pain should be considered for treatment with the same opioid by an alternative (spinal) route or with an alternative opioid agonist administered by the same route (whether oral or parenteral), in conjunction with adjuvant analgesics such as tricyclic antidepressants. The most commonly used alternative oral opioids are phenazocine and methadone; transdermal fentanyl is an additional option.