Effect Sizes in Experimental Pain Produced by Gender, Genetic Variants and Sensitization Procedures

BACKGROUND Various effects on pain have been reported with respect to their statistical significance, but a standardized measure of effect size has been rarely added. Such a measure would ease comparison of the magnitude of the effects across studies, for example the effect of gender on heat pain with the effect of a genetic variant on pressure pain. METHODOLOGY/PRINCIPAL FINDINGS Effect sizes on pain thresholds to stimuli consisting of heat, cold, blunt pressure, punctuate pressure and electrical current, administered to 125 subjects, were analyzed for 29 common variants in eight human genes reportedly modulating pain, gender and sensitization procedures using capsaicin or menthol. The genotype explained 0-5.9% of the total interindividual variance in pain thresholds to various stimuli and produced mainly small effects (Cohen's d 0-1.8). The largest effect had the TRPA1 rs13255063T/rs11988795G haplotype explaining >5% of the variance in electrical pain thresholds and conferring lower pain sensitivity to homozygous carriers. Gender produced larger effect sizes than most variant alleles (1-14.8% explained variance, Cohen's d 0.2-0.8), with higher pain sensitivity in women than in men. Sensitization by capsaicin or menthol explained up to 63% of the total variance (4.7-62.8%) and produced largest effects according to Cohen's d (0.4-2.6), especially heat sensitization by capsaicin (Cohen's d = 2.6). CONCLUSIONS Sensitization, gender and genetic variants produce effects on pain in the mentioned order of effect sizes. The present report may provide a basis for comparative discussions of factors influencing pain.