Independent Association of Severity of Muscle Weakness With Disability as Measured by the Health Assessment Questionnaire Disability Index in Scleroderma

Objective: To determine whether the presence and degree of muscle weakness in scleroderma is associated with disability. Methods: The study included a cohort of 1,718 scleroderma patients who had available data on muscle strength and disability. The primary independent variable was muscle weakness as defined by the maximum Medsger muscle severity score and the outcome was disability as measured by the last recorded Health Assessment Questionnaire disability index (HAQ DI) score. Univariate regression analyses were performed to assess the association of HAQ DI scores with the Medsger muscle severity score and other scleroderma characteristics. A multivariate regression analysis was performed to determine whether an association existed between the degree of muscle weakness and disability, while controlling for confounders. Results: In 1,718 patients with scleroderma, 22.8% (392 of 1,718) had muscle weakness, as defined by a Medsger muscle severity score of ≥1. This subset was more likely than those without weakness to have diffuse cutaneous scleroderma (55.6% versus 35.1%; P < 0.0001), higher modified Rodnan skin thickness scores (mean ± SD 16.3 ± 13.7 versus 10.3 ± 10.6; P < 0.00001), shorter disease duration (mean ± SD 5.21 ± 6.75 versus 6.22 ± 7.67 years; P = 0.02), synovitis (17.7% versus 11.4%; P = 0.001), forced vital capacity <70% (46.2% versus 30.6%; P = 0.0001), and higher creatine kinase values (mean ± SD 441 ± 1,211 versus 151 ± 255; P = 0.00001). Both univariate and multivariate analyses revealed that for every unit of increase in the Medsger muscle severity score, there was a clinically significant (minimum clinically important difference ± 0.14) increase in the mean HAQ DI score at last followup visit. Conclusion: The presence of muscle weakness associates with several features of worse disease burden and independently associates with disability as measured by the HAQ DI.