Gabapentin versus tricyclic antidepressants for diabetic neuropathy and post-herpetic neuralgia: discrepancies between direct and indirect meta-analyses of randomized controlled trials.

BACKGROUND: Previous systematic reviews concluded that tricyclics antidepressants are superior to gabapentin for neuropathic pain, but were based on indirect comparisons from placebo-controlled trials.

PURPOSE: To evaluate gabapentin versus tricyclic antidepressants for diabetic neuropathy and post-herpetic neuralgia, using direct and indirect comparisons.

DATA SOURCES: MEDLINE (1966 to March Week 4 2008), the Cochrane central register of controlled trials (1st quarter 2008), and reference lists.

STUDY SELECTION: We selected randomized trials directly comparing gabapentin versus tricyclic antidepressants or comparing either of these medications versus placebo.

DATA EXTRACTION: Studies were reviewed, abstracted, and quality-rated by two independent investigators using predefined criteria.

DATA SYNTHESIS: We performed a meta-analysis of head-to-head trials using a random effects model and compared the results to an adjusted indirect analysis of placebo-controlled trials.

RESULTS: In three head-to-head trials, there was no difference between gabapentin and tricyclic antidepressants for achieving pain relief (RR 0.99, 95% CI 0.76 to 1.29). In adjusted indirect analyses, gabapentin was worse than tricyclic antidepressants for achieving pain relief (RR = 0.41, 95% CI 0.23 to 0.74). The discrepancy between direct and indirect analyses was statistically significant (p = 0.008). Placebo-controlled tricyclic trials were conducted earlier than the gabapentin trials, reported lower placebo response rates, had more methodological shortcomings, and were associated with funnel plot asymmetry.

CONCLUSIONS: Though direct evidence is limited, we found no difference in likelihood of achieving pain relief between gabapentin and tricyclic antidepressants for diabetic neuropathy and post-herpetic neuralgia. Indirect analyses that combine data from sets of trials conducted in different eras can be unreliable.