Respiratory Muscle Strength in Rheumatoid Arthritis

Introduction: Rheumatoid arthritis (RA) is known to affect the musculoskeletal system and, consequently, may lead to sarcopenia, but the role of respiratory muscle involvement in RA patients is unclear. Methods: This prospective, exploratory, single-center, matched-pair analysis study was designed to compare respiratory muscle strength and handgrip strength in RA patients and controls. Results: RA patients with low disease activity as estimated from the Disease Activity Score 28 (2.3 ± 1.2) and without signs of interstitial lung disease (n = 36, 72% female, 28% smoker, mean age 48 + 15 years, mean forced vital capacity 3.9 ± 1.0 L, 98% ± 11% predicted) and control subjects (n = 36, 72% female, 11% smoker, mean age 48 + 14 years, mean forced vital capacity 4.1 ± 1.1 L, 98% ± 16% predicted) were well balanced. Maximal inspiratory mouth pressure (PImax, primary endpoint) tended to be lower in RA patients, but this was statistically not significant (-0.9 kPa; 95%CI = -2.11/0.32). However, RA patients more frequently had PImax values below the lower limit of normal (OR 1.74 kPa; 95% CI 0.65/4.77). RA patients had lower handgrip strength (-5.97 kg; 95%CI = -9.43/-2.50). In addition, PImax was correlated to handgrip strength both in RA patients (R = 0.51, p = 0.0017) and controls (R = 0.48, p = 0.0029) and to the 6-minute walking distance (RA-patients: R = 0.30, p = 0.075; controls: R = 0.52, p = 0.0012). Conclusions: Even though the primary endpoint has not been reached, an impairment of respiratory muscle strength in RA cannot be excluded at least in a subset of patients. Further studies also involving RA patients with more disease activity are needed.