Title: Severity of muscle weakness predicts disability in scleroderma

Background/Purpose: The Health assessment questionnaire disability index (HAQ-DI) is a self-administered questionnaire that is a patient reported outcome which provides a measure of disability responsive to change in clinical trials in scleroderma. The purpose of this study was to determine if the degree of muscle weakness in scleroderma associates with HAQ-DI scores.     

Methods: We queried a large, well characterized cohort of patients with scleroderma to determine whether the presence of muscle weakness associates with abnormal HAQ-DI scores. A total of 2481 patients with scleroderma were enrolled in the database from 1990 until 2010. 1723 patients were included in the analyses since these patients had a complete muscle severity assessment. We defined muscle weakness using the Medsger Muscle Severity Score. A value of 0 indicates no weakness, and a value ranging from 1-4 characterizes the patient at various degrees of weakness. The maximum score recorded from all clinic visits was used. The outcome, HAQ-DI, at their most recent visit was used in this analysis.  

Multivariate linear regression was used to calculate effect estimates. Muscle severity scores were modeled as a categorical variable, with an effect estimate for each category 1 to 4, with zero as the reference category. Linear regression on the original data, log-transformed data and inverse-transformed data was performed, but in all cases the residuals showed strong non-normality.  A generalized linear model using the gamma distribution with identity link was seen to improve model fit.

Results: 407/1733 patients were weak based on their maximum muscle severity score. The mean age was 52.4 ± 13.7 years old among the weak patients. Age and gender were not different in those who were weak vs. not weak. There were also more weak scleroderma patients who were limited subtype rather than diffuse subtype (p<0.0001).

After controlling for age, sex, disease duration, signs of synovitis, disease subtype, level of gastrointestinal severity score (0-4), and lung function (most recent forced vital capacity (<70%)), the severity of weakness was significantly positively associated with higher scores of  the HAQ-DI. For every unit of change in the Muscle Severity score at the Score level of >0 and <4, there was a positive association with the disability score (see Table).   

Summary: This study demonstrates that a history of muscle weakness is positively associated with self-reported disability as measured by the HAQ-DI.

Conclusion: Muscle weakness is an independent cause of disability among patients with scleroderma.