Several reviews of randomized controlled trials (RCTs) have shown the efficacy of mu-opioids in reducing spontaneous neuropathic pain (NP). However, relatively little is known about their specific efficacy for evoked pain, which is a significant problem for many patients with NP. The present systematic review assesses the efficacy of opioid agonists for the treatment of evoked NP based upon published RCTs. We searched articles in any language using the MEDLINE database (1966 to December 2004), the Cochrane Central Register of Controlled Trials (4th quarter, 2004) and the reference lists of retrieved papers, employing search terms for RCTs, opioids and NP. Only RCTs in which opioid agonists were given to treat NP of any etiology, and evoked pain was assessed were included. Data were extracted by two independent investigators. Nine articles met inclusion criteria and were classified as short-term (less than 24h; n=7) or intermediate-term trials (4 weeks; n=2). Although the scarcity of retrieved data precluded formal meta-analysis of short-term trials, we found that the intensity of dynamic mechanical allodynia was significantly attenuated by opioids relative to placebo in all studies. In contrast, no consistent effects on the magnitude of static allodynia, the threshold for mechanical allodynia or the threshold or magnitude of heat allodynia were found. The threshold and magnitude of cold-induced allodynia generally responded positively to opioid treatments in patients with peripheral pain syndromes, but not central pain syndromes. Evoked pain was studied in only two intermediate-term trials, in both of which oxycodone was significantly superior to placebo. The results of the two trials were combinable for a meta-analysis that showed an overall 24 points difference in endpoint pain intensities between patients given opioids and those treated with placebo (95% CI -33 to -15; p<0.00001).
In conclusion: short-term studies show that opioids can reduce the intensity of dynamic mechanical allodynia and perhaps of cold allodynia in peripheral NP. Insufficient evidence precludes drawing conclusions regarding the effect of opioids on other forms of evoked NP. A meta-analysis of intermediate-term studies demonstrates the efficacy of opioids over placebo for evoked NP. These findings are clinically relevant because dynamic mechanical allodynia and cold allodynia are the most prevalent types of evoked pain in NP.