Basal heat pain thresholds predict opioid analgesia in patients with postherpetic neuralgia

BACKGROUND: A variety of analgesics have been studied in the treatment of postherpetic neuralgia, with several medications demonstrating some degree of efficacy. However, existing trials have documented large individual differences in treatment responses, and it is important to identify patient characteristics that predict the analgesic effectiveness of particular interventions. Several animal studies have indicated that reduced basal nociceptive sensitivity, in the form of relatively high heat pain thresholds, is associated with greater opioid analgesia, but this finding has not been applied to human studies of opioid treatment for chronic pain. METHODS: Using data from a previously published crossover trial of opioids and tricyclics in postherpetic neuralgia, the authors evaluated baseline thermal pain thresholds, assessed at a body site contralateral to the affected area, as a predictor of treatment responses. RESULTS: During opioid treatment, a greater reduction in pain and higher ratings of pain relief were observed in patients with relatively higher heat pain thresholds at baseline. Baseline pain thresholds did not predict responses to tricyclics or placebo. Interestingly, other individual-difference variables such as age and baseline pain intensity also significantly predicted opioid responses (i.e., higher baseline pain and younger age were related to greater opioid-associated pain reduction, with nearly 20% of the variance in opioid analgesia explained by these two factors). CONCLUSIONS: These findings, which will require replication, suggest that pretreatment assessment of heat pain sensitivity might prove useful in identifying those patients most likely to respond to opioids.