Background/Purpose: Fibromyalgia (FM) is a chronic nonarticular pain syndrome of unknown etiology characterized by diffuse muscular pain, fatigue and mood disturbances. Previous studies indicated the absence of skeletal muscle degeneration, regeneration or inflammation. Altered muscle fiber size distribution and decreased capillary density were the only abnormalities reported.

From the clinical point of view some FM symptoms (fatigue, pain, myalgias, trigger points, and stiffness) suggest involvement of the skeletal muscle.

The objective of this study is to determine if there are structural and functional abnormalities in the skeletal muscle in patients with primary FM assessed through a non-invasive multi-modality approach.

Methods: Female patients older than 18 years with primary FM diagnosis (fulfilling 2016 revision to the 2010/2011 ACR criteria), and a group of healthy controls matched by age and gender. Skeletal muscle morpho-structure and function were assessed by the following tests/evaluations: body mass index (BMI), total fat mass and total muscle mass calculated by direct segmental multi-frequency bioelectrical impedance analysis. Cross-sectional measurements of rectus femoris muscle area and tissue echogenicity evaluated by ultrasound and post-processing pixel analysis. Maximum handgrip strength by digital dynamometry, gait speed by 6-meter time walk test, and Fibromyalgia Health Assessment Questionnaire (FHAQ).

Results: Ninety-four FM patients and 140 healthy controls were included, mean age was 51.8 years +/- 10.8 vs 50.2 +/- 11.3, respectively (p = 0.27). FM patients compared with controls had similar BMI (27.9 kg/m² +/- 4.9 vs 26.8 +/- 4.5, p = 0.14), higher total body fat mass (27.8 kg +/- 9.2 vs 25.1 +/- 7.6, p = 0.04); the rectus femoris muscle area was also higher for FM patients (44.6 cm² +/- 11.4 vs 41.7 +/- 13.9, p = 0.05); regarding ultrasound tissue echogenicity, FM patients demonstrated higher mean brightness in rectus femoris (157.2 pixels +/- 19.4 vs 149.9 +/- 22.4, p = 0.01); lesser handgrip strength (22.0 kg +/- 6.6 vs 26.2 +/- 5.4 p = 0.0001); slower gait speed (1.14 m/s +/- 0.2 vs 1.3 +/- 0.2 p = 0.0001); and major impairment in daily activities by FHAQ  (47% vs 2.90% p = 0.0001).

Conclusion: This non-invasive multimodal evaluation discloses the presence of structural and functional skeletal muscle abnormalities in women with FM. These changes can be attributed to sedentary and hypoactive lifestyle with consequent higher total body fat mass, muscular fat infiltration, and functional impairment of activities of daily living. Alternatively, these abnormalities may also be an expression of a small-fiber polyneuropathy (myoneurovascular dysregulation). Further studies are required to elucidate its underlying pathological process.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/is-fibromyalgia-associated-with-structural-or-functional-abnormalities-in-skeletal-muscle/