The heterogeneity of symptoms among patients with fibromyalgia (FM) makes the development of standardized diagnostic criteria challenging. No imaging technique has reliably shown FM-related muscle changes to aid clinical assessment. This study aimed to quantitatively analyze the upper trapezius muscle in FM patients using B-mode ultrasonography and blob analysis and to examine its correlation with clinical parameters. A total of 34 female FM patients and 34 healthy controls were included in this cross-sectional study. B-mode ultrasonography was used to image the dominant-side upper trapezius muscle, and MATLAB-based blob analysis was performed to assess blob size, blob count, and echointensity. These measurements were correlated with disease severity indices, including the Central Sensitization Inventory (CSI), Visual Analog Scale (VAS) for pain, Fibromyalgia Impact Questionnaire (FIQ), 36-Item Short Form Survey (SF-36), and Beck Depression and Anxiety Inventories (BDI, BAI). FM patients had significantly higher total blob size (p < 0.001) and blob size per mm² (p < 0.001) than controls. Echointensity was significantly increased in the FM group (p = 0.009). Total blob size showed a moderate positive correlation with CSI scores (p = 0.002). Regression analysis indicated that pain-VAS was a significant predictor of total blob size per mm² (p < 0.001). Blob analysis demonstrated quantifiable muscle alterations in FM, supporting its potential role as an objective assessment tool. Given the correlation between muscle echotexture and FM severity, quantitative ultrasonography may contribute to a better understanding of FM pathophysiology.
