Deficits in Muscle Mass, Muscle Density, and Modified Associations With Fat in Rheumatoid Arthritis

Objective. To quantify muscle outcomes, independent of fat mass, in rheumatoid arthritis (RA) patients compared to healthy controls. Methods. Quantitative computed tomography scans measured calf muscle and fat cross-sectional area (CSA) and muscle density (an index of intramuscular adipose tissue), and isometric dynamometry was used to measure ankle muscle strength in 50 participants with RA ages 18-70 years and 500 healthy controls. Multivariable linear regression models assessed muscle deficits in RA after adjusting for group differences in adiposity and assessing for an altered muscle-fat association. Associations between RA disease characteristics and fat-adjusted muscle outcomes were also assessed. Results. Compared to controls, RA subjects had significantly greater body mass index (BMI) and fat area, and lower muscle area, muscle density, and muscle strength (P < 0.001 for all). Strength deficits were eliminated with adjustment for the smaller muscle area. The magnitude of muscle deficits, relative to controls, was significantly greater (P < 0.03 for interaction) in participants with lower fat area and BMI. Among those in the lower tertiles of adiposity, RA subjects demonstrated more significant deficits compared to controls with similar adiposity. In contrast, among those in the highest tertile for adiposity, RA was not associated with muscle deficits. Among RA, greater Sharp/van der Heijde scores were associated with lower muscle CSA and muscle density. Greater disease activity and disability were associated with low muscle density. Conclusion. Deficits in muscle area and muscle density are present in RA patients compared to controls and are most pronounced in subjects with low fat mass. Greater joint destruction is associated with greater muscle deficits.