Smoking, greater disease activity leading predictors of muscle density declines in RA
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Declines in muscle density among patients with rheumatoid arthritis are linked to greater disease activity, smoking and low insulin-like growth factor-1 levels, according to data published in Arthritis Care & Research.
“While prior cross-sectional studies may suggest a relationship between muscle density (muscle quality), disease activity and physical functioning, they do not establish temporal relationships and may suffer from reverse causality,” Joshua F. Baker, MD, MSCE, of the Philadelphia VA Medical Center and the University of Pennsylvania, and colleagues wrote. “In other words, an apparent association between low muscle density and poor physical function could conceivably result from the presence of common risk factors.”
They added, “We hypothesized that higher disease activity at baseline would be associated with long-term declines in muscle density and that lower muscle density at baseline would be associated with greater decline in physical functioning independent of other factors including body composition.”
To identify the predictors of changes in muscle density, as well as examine whether low muscle density predicts changes in strength and physical function, Baker and colleagues conducted a longitudinal study of patients with RA aged 18 to 70 years, recruited from the University of Pennsylvania and the Philadelphia VA Medical Center. Participants underwent whole-body dual-energy X-ray absorptiometry (DXA) and peripheral quantitative CT scans to analyze lean and fat mass, as well as muscle density. In addition, the researchers used dynamometry hand, knee and lower leg strength.
Disability and physical function were examined using the Health Assessment Questionnaire (HAQ) and the Short Physical Performance Battery (SPPB). The researchers performed assessments at baseline and follow-up, which occurred between 2 and 3 years from baseline for most participants. Muscle density was assessed in 107 participants at baseline, with 79 returning for follow-up. Baker and colleagues used regression analysis to examine the links between patient characteristics, muscle density and decreases in strength and function.
According to the researchers, female sex, higher disease activity, smoking and lower insulin-like growth factor-1 (IGF1) levels were associated with decreasing muscle density. A greater muscle density ZScore at baseline was associated with less annual worsening of HAQ, SPPB and 4meter walk time. Greater muscle density was also associated with a lower risk for clinically important worsening of HAQ (OR = 1.9; 95% CI, 1.06-3.42) and walking speed (OR = 2.87; 95% CI, 1.05-7.89).
“The main advance of this study over prior studies was the observation that people with RA who have fat infiltration on their muscles have greater declines over time in their physical functioning,” Baker told Healio Rheumatology. “Further, the study also confirmed prior cross-sectional studies suggesting that RA disease activity plays a role in the development of fat infiltration of the muscles over time. Prior studies in this area were cross-sectional but it is difficult understand cause and effect in cross-sectional studies.”
According to Baker, these findings are clinically important because they support the notion that fat infiltration of muscle is an important problem in patients with RA that contributes to loss of physical functioning over time.
“Treatments that target improvements in muscle quality might therefore have an important impact on how patients experience their disease,” he added. “In the short term, we hope that these findings help encourage providers to prescribe exercise, as it is one of the few interventions that consistently demonstrates an impact on skeletal muscle function.” – by Jason Laday
Disclosure: The researchers report no relevant financial disclosures.
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