Image analysis quantification of substance P immunoreactivity in the trapezius muscle of patients with fibromyalgia and myofascial pain syndrome

Objective. Substance P (SP), a neurotransmitter stored within the afferent nociceptive fibers, is likely to be involved in the pathogenesis of musculo skeletal pain. We investigated SP immunoreactive (SP-ir) nerve fibers in th e upper trapezius of patients with fibromyalgia (FM) and myofascial pain sy ndrome (MPS) by immunochemistry. Methods. Trapezius muscle obtained from tender points of 9 women with prima ry FM, from trigger points of 9 women with regional myofascial pain, and fr om 9 control women were immunostained with anti-SP sera. Quantitative evalu ation was performed by computerized image analysis. Results. No significant differences in the number of SP-ir areas were detec ted between groups (one way ANOVA: p = 0.2); in contrast, mean optical dens ity (OD) of SP-ir showed a significant difference comparing the groups (one way ANOVA: p < 0.0001). Mean OD of the immunostaining for SP was statistic ally greater in trapezius muscle of patients with MPS (0.594 +/- 0.096) com pared to specimens from patients with FM (0.436 +/- 0.140) (p < 0.05) and c ontrols (0.314 +/- 0.105) (p < 0.05); mean OD of immunostaining for SP was greater in FM specimens than in controls (p < 0.05). Conclusion. Our results point to a peripheral hyperactivity of the peptider gic nervous system in FM as well as in MPS. These findings support the noti on of pathogenetic involvement of the afferent nervous system in the develo pment and perception of myofascial pain.