Three Clusters of Different Properties Characterize Women with Chronic Trapezius Myalgia

Objectives: A correlative study on data from 14 women with unilateral chronic shoulder pain was undertaken. Methods: Data were obtained on evoked pain and pressure-pain thresholds [PPTs] changes upon muscle exertion and biopsy findings on capillary density and muscular pathology. The PPTs were measured in the trapezius muscle, before and after a static abduction endurance test of the shoulder [electronic algometer]. Holding time and pain intensity was registered. Capillarization and ragged red fibers, cytochrome-c-oxidase negative fibers, and moth-eaten muscle fibers were analyzed in the same trapezius muscles. Results: Principal component analysis was used for multivariate analysis, showing a model with three statistically significant components. The first component explained 33 percent of the variation. Pressure-pain threshold changes were positively correlated with capillarization, and negatively correlated with prevalence of moth-eaten fibers and cytochrome-c-oxidase negative fibers. The second component explained 23 percent of the variation, and reflected the correlations between holding time, differences in pain and PPTs, i.e. between various aspects of perceived pain after exertion. The third component explained 19 percent of the variation. The pain difference correlated positively with the prevalence of cytochrome-coxidase negative fibers and ragged-red fibers; subjects with high prevalence of these two fiber types presented increased pain. In summary, our results suggest that not only capillarization and histopathological findings of the trapezius muscle, but also centrally modulated pain intensity and PPT changes after muscle exertion are associated. Conclusions: Three clusters of different properties were revealed in women with trapezius myalgia, highlighting the multifactorial mechanisms responsible. These components may have prognostic value.