Morphine responsiveness of chronic pain: double-blind randomised crossover study with patient-controlled analgesia

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There is controversy about whether the lack of response of some chronic pain to opioid treatment is absolute or relative. It is widely believed that nociceptive pain is responsive to opioids whereas neuropathic pain tends not to be. We have used a method of patient-controlled analgesia (PCA) with simultaneous nurse-observer measurement of analgesia, mood, and adverse effects to address these issues. Ten patients with chronic pain were given morphine at two concentrations (10 and 30 mg/ml) by PCA in two separate sessions in a double-blind randomised crossover study. Before the study a clinical judgment was made as to whether each pain was nociceptive or neuropathic. Seven patients showed good analgesic responses (more than 70 mm pain relief on a visual-analogue scale) of pain at rest, two patients poor responses (less than 30 mm pain relief), and one a moderate response with both concentrations (30-70 mm pain relief). The response to morphine was consistent (greater and faster relief with the higher concentration) in nine patients. Two patients had pain on movement that responded moderately to low-concentration morphine and well to the higher concentration. All patients with pains judged to be nociceptive showed good analgesic responses compared with half of those with neuropathic pain. There was no evidence that analgesic responses in patients with neuropathic pain were due to changes in mood. This PCA method is a quick and efficient tool to determine the consistency of the analgesic response. Such consistency can guide the clinician as to whether continued or higher-dose opioid treatment will produce good analgesia. An inconsistent response points to the use of other pain-relieving strategies.