To our knowledge, this study is the first to assess the potential use of blob analysis in FM, introducing a novel ultrasonographic method for more objective muscle assessment. Considering previous diagnostic approaches and studies in FM, this research offers a quantitative imaging perspective to further investigate muscle alterations.

Diagnosing FM remains challenging owing to the symptom overlap with conditions such as myofascial pain syndrome and the non-specific nature of its clinical presentation [25]. Although terms, such as the WPI, SSS, and generalized pain were introduced with the revised 2016 criteria, it should be acknowledged that this assessment is still primarily based on subjective patient-reported outcomes [26]. Chronic muscle pain, stiffness, and fatigue are hallmark features of FM, making muscle structure evaluation a compelling target for understanding its pathophysiology. Initially, the diagnosis of FM largely relied on the assessment of tender points through pressure evaluation. The evaluation of tender points in patients with FM using ultrasonography can provide clinicians with reliable data, aiding in both diagnosis and monitoring, especially in conditions where the assessment of muscle pain is prioritized.

Although no specific laboratory or imaging tests currently exist for FM diagnosis, various studies have investigated structural and functional muscle changes using different methodologies [27, 28]. Henriksson et al. performed one of the early systematic studies investigating muscle alterations in FM, examining structural changes in the muscle tissue of patients diagnosed with FM [29]. Biopsy samples obtained from the tender points of the trapezius muscle revealed a focal loss of nicotinamide adenine dinucleotide (NADH) diaphorase activity, presenting a characteristic moth-eaten appearance. Additionally, electron microscopy demonstrated mitochondrial abnormalities, Z-band streaming in myofibrils, and cytoplasmic bodies. Another study by Kalyan–Raman et al. confirmed the presence of moth-eaten fibers in type I fibers of the trapezius muscle. Electron microscopy revealed selective type II fiber atrophy and subsarcolemmal deposition of glycogen and mitochondria [30]. Although these muscle biopsy studies provide valuable insights into the structural and metabolic alterations associated with FM, their invasive nature and limited practicality hinder their routine use in clinical practice.

A literature review revealed that research on microcirculatory dysfunction in FM suggests a significant link between impaired blood flow and development of muscular pain. Lund et al.identified irregular tissue oxygen pressure distribution in patients with FM compared to healthy controls, indicating inadequate oxygen delivery to tender point regions, potentially due to localized ischemia, suggesting that compromised microcirculation may trigger pain at specific sites by contributing to intramuscular nociceptor sensitization and amplifying pain perception [31]. In a study conducted by Sanderson et al., which evaluated the histological response of skeletal muscle to ischemia using electron microscopy, several changes were observed in both electron and light microscopy [32]. Concomitant fibroplastic activity was observed throughout this process, along with the production of new collagen fibers. These findings suggest that ischemic changes may lead to disruptions in the fundamental muscle structure.

Muscle dysfunction and central sensitization are interconnected in FM. Peripheral sensitization driven by tissue alterations lowers nociceptor thresholds and amplifies afferent fiber sensitivity [33]. Although inflammation is typically absent, nociceptors and elevated levels of substance P have been detected in FM muscle tissue [34, 35]. These peripheral changes create persistent nociceptive input, feeding into central sensitization and sustaining pain perception [36, 37]. Consistent with this, one of the key findings of our study was the significant positive correlation between blob analysis results and CSI scores in patients with FM, supporting the hypothesis that nociceptive stimuli, along with structural changes in muscle tissue, may contribute to central sensitization.

Ultrasound is becoming an essential part of rheumatology practice and research, offering a feasible and effective imaging technique for real-time recognition of anatomical changes. It has emerged as a critical tool in the early diagnosis of rheumatic diseases such as rheumatoid arthritis (RA), gout, and Sjögren’s syndrome, leading to its integration into their diagnostic classification criteria [38, 39]. In RA, ultrasound facilitates the early detection of synovitis, identifying subclinical inflammation that may not be apparent during clinical examination, thereby allowing for timely diagnosis and treatment initiation [40].

Similarly, in spondyloarthritis (SpA), ultrasound can detect early structural changes such as enthesitis before radiographic alterations become evident, making it a crucial diagnostic tool, particularly in patients without established axial involvement [37]. As recommended by the European Alliance of Associations for Rheumatology (EULAR), ultrasound significantly enhances the early identification of peripheral joint and enthesis involvement in SpA, complementing clinical evaluation.

Given the diagnostic advantages of ultrasound in these inflammatory rheumatic diseases, its potential application in non-inflammatory conditions such as FM remains largely unexplored. This study highlights the potential of advanced ultrasonographic techniques, particularly blob analysis, in assessing structural muscle alterations in FM, proposing a novel imaging-based approach for its evaluation. Accordingly, the use of quantitative ultrasonographic techniques has expanded in musculoskeletal imaging, as demonstrated in various studies [41,42,43]. In one such study, it was highlighted as a valuable, noninvasive tool for diagnosing chronic thyrotoxic myopathy by reliably assessing muscle parameters, such as thickness, cross-sectional area, and echointensity, offering significant insights into muscle quality and pathology [41]. In another study, B + M-mode ultrasound was used to assess swallowing mechanics in stroke patients with dysphagia, identifying key predictors such as tongue and geniohyoid muscle movements and the hyoid-larynx approximation, demonstrating its value in clinical screening [42].

Blob analysis, as a quantitative assessment tool, has been utilized in studies to characterize echointensity and structural alterations in musculoskeletal tissues [44]. Kumbhare et al. conducted a quantitative ultrasound study of the upper trapezius muscle in 15 healthy individuals and 17 patients with myofascial pain. This study demonstrated that the blob area and blob count were effectively differentiated between the two groups. These differences may indicate structural changes such as taut bands, abnormal fiber alignment, or intramuscular fat deposits. In line with these previous findings, the blob alterations observed in our study support the reliability of blob analysis as a valuable method for assessing differences between healthy and pathological muscle tissues.

One of the most important findings of our study was the identification of pain-VAS as a significant predictor of total blob size/mm² in the regression analysis. This suggests that among the various parameters derived from blob analysis, total blob size/mm² may hold particular clinical relevance. Its strong association with pain severity implies that it could serve not only as a supportive diagnostic indicator but also as a valuable parameter in monitoring disease progression or evaluating treatment response in patients with FM. Consequently, incorporating such quantitative parameters into clinical workflows may enhance the precision and consistency of patient monitoring in FM. Although several correlations between ultrasonographic parameters and clinical measures were statistically significant, the relatively modest R values suggest that the effect sizes are small to moderate. Therefore, these findings should be interpreted cautiously, especially in light of the multifactorial and heterogeneous nature of fibromyalgia.

Few studies have evaluated muscle structure in FM using ultrasonography. Umay et al. assessed muscle thickness and cross-sectional areas in patients with FM and reported significant reductions in these parameters, except for the deltoid muscle, when compared to healthy individuals [45]. Their findings suggested that the muscle atrophy observed in FM cannot be solely attributed to physical inactivity, as structural muscle changes are evident even in the early stages of the disease. Research involving both newly diagnosed and established patients with FM indicates that these muscle alterations are more likely driven by disease-specific mechanisms than by inactivity alone. Furthermore, the lack of a significant correlation between physical mobility and muscle measurements supports the hypothesis that structural changes in the FM muscles are primarily associated with the pathophysiological processes of the disease itself, rather than being secondary to reduced physical activity [46]. In line with these findings, Mirza et al. recently demonstrated that FM patients showed significantly reduced thickness of lumbar multifidus muscles. These structural changes were closely associated with impaired muscle endurance, increased fatigue, and decreased functional mobility, further supporting the role of musculoskeletal involvement in FM [47].

Therefore, studies focusing on muscle assessment in FM highlight the significant role of evaluating muscle structures in understanding the pathophysiology, diagnosis, and follow-up stages of the disease. Future research on FM, incorporating ultrasonography with larger sample sizes, diverse muscle group analyses, histopathological validations, and advanced imaging techniques, has the potential to enhance the objective and reliable assessment of tender points, offering clinicians a more effective diagnostic tool than traditional methods such as palpation and patient-reported scales.

Quantitative ultrasonography has emerged as an advanced and promising technique for detailed muscle analysis that offers valuable insights into disease-specific structural changes. Considering the frequent involvement of the trapezius muscle in FM and its association with tender points, this study focused on evaluating this muscle group. The major strength of our study is its contribution to the development of an objective and clinically applicable biomarker, which could enhance diagnostic accuracy, facilitate disease monitoring, and guide more targeted therapeutic interventions in FM.

There are several limitations to this study that should be considered when interpreting the results. Due to the cross-sectional nature of the study, it is not possible to determine whether the observed muscle alterations are a consequence of FM or if they might precede its development. Clarifying this relationship requires future longitudinal research to explore changes over time. The sample included only female participants, which limits the applicability of our findings to the broader FM population, including male patients. Although the condition is more commonly diagnosed in women, it also occurs in men, and their exclusion prevents a more comprehensive understanding of possible sex-based differences in muscle characteristics.

This research was confined to the assessment of the upper trapezius muscle. Since FM is a condition characterized by widespread musculoskeletal pain, evaluating additional muscle groups could offer more holistic insight into the extent and nature of muscle involvement. While we identified significant associations between ultrasound-based muscle changes and clinical measures such as pain and central sensitization, it is important to note that these findings do not establish a direct or disease-specific relationship. FM is widely believed to involve dysfunction of the nervous system, rather than being a primary muscle disorder. Mechanisms involving altered neural processing, autonomic regulation, and small fiber neuropathy are thought to play critical roles in the symptomatology of the disease, and may secondarily affect muscle tissue.

Although the potential influence of commonly prescribed medications such as pregabalin, duloxetine, and amitriptyline was acknowledged, we did not adjust for their effects in the statistical analysis. These agents could impact muscle physiology or alter pain perception, and future studies should aim to control for medication use in order to isolate disease-related changes more precisely.

Finally, quantitative ultrasonography analyses have the potential to provide measurable assessments for FM, thereby supporting its diagnosis, follow-up, and treatment. Central sensitization plays a significant role in the clinical symptoms and muscle structural changes within the complex mechanisms of FM.