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 musculoskeletal pain. We investigated SP immunoreactive (SP-ir) nerve fibers in the upper trapezius of patients with fibromyalgia (FM) and myofascial pain syndrome (MPS) by immunochemistry.

METHODS

Trapezius muscle obtained from tender points of 9 women with primary FM, from trigger points of 9 women with regional myofascial pain, and from 9 control women were immunostained with anti-SP sera. Quantitative evaluation was performed by computerized image analysis.

RESULTS

No significant differences in the number of SP-ir areas were detected between groups (one way ANOVA: p = 0.2); in contrast, mean optical density (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 statistically greater in trapezius muscle of patients with MPS (0.594 +/- 0.096) compared to specimens from patients with FM (0.436 +/- 0.140) (p < 0.05) and controls (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 peptidergic nervous system in FM as well as in MPS. These findings support the notion of pathogenetic involvement of the afferent nervous system in the development and perception of myofascial pain.