Pressure pain threshold mappings of the infraspinatus muscle in chronic unilateral shoulder pain patients do not reflect generalized hypersensitivity

Objectives: Increased mechanical sensitivity has been observed on the unaffected side in chronic pain conditions, suggesting generalized or widespread hypersensitivity. However, this cannot be considered as a universal response since this hypersensitivity is inconsistent across muscle pain pathologies. The aim of this study was to assess generalized hypersensitivity in chronic unilateral shoulder pain, using pressure pain threshold (PPT) mappings of the infraspinatus muscle. The proposed evaluation is based on the assessment of PPT on a limited subset of sites, reducing potential habituation or sensitization effects. Methods: Twenty-nine patients with unilateral shoulder pain (USP) and twenty-seven healthy volunteers were recruited. PPT was assessed using a manual pressure algometer. Six sites distributed over the infraspinatus muscle were assessed, and three repetitions were performed at each site. Mappings were derived using two-dimensional interpolation. Results: Lower PPT values were found in the symptomatic side in comparison with the asymptomatic side at all assessment sites (estimated difference: 1.42 ± 0.10 kgf/cm², p < 0.001), but there were no differences among the asymptomatic side of USP patients and any of the sides in healthy volunteers (largest estimated difference: 0.17 ± 0.28 kgf/cm², p = 0.927). Furthermore, the medial region of the infraspinatus muscle showed higher mechanical sensitivity in both healthy volunteers and USP patients. Conclusions: These results suggest that USP does not induce generalized hypersensitivity, in contrast with previously reported findings. Physiotherapists could take these results into account for the assessment and treatment of patients with USP.