Background Juvenile dermatomyositis (JDM) is a multisystem vasculopathic disease characterized by muscle inflammation that causes symmetrical muscle weakness. Assessment of muscle strength is, therefore, a fundamental component of the clinical evaluation of children with JDM. This assessment is traditionally made using the 8-muscle Manual Muscle Testing (MMT) and the Childhood Myositis Assessment Scale (CMAS). However, the MMT does not cover all muscles or muscle groups affected in JDM, namely abdominal muscles. Although the CMAS is more comprehensive than the MMT, it is lengthy and, therefore, may not be feasible in a busy clinical setting or when a physical therapist is not available.
Objectives To investigate the construct validity of a new hybrid measure of muscle strength developed by merging the MMT with 3 items of the CMAS
Methods The Hybrid MMT/CMAS (hMC) is composed of all 8 items of the MMT and the following items of the CMAS: 1) head lift; 2) sits-ups; 3) floor rise. Item 3) is recoded on 0 to 9 scale. The total score of the hMC ranges from 0 (worst) to 100 (normal). The study sample included 322 children with JDM enrolled in a multinational long-term outcome survey (Ravelli et al. AC&R 2010;62:63-72), who had all measurements available. Validation procedures were conducted by comparing the correlation of the hMC, MMT and CMAS with other conventional measures of JDM activity, physical function and damage. Correlations were computed by means of the Spearman’s correlation coefficient and were considered good, moderate, or poor when the rs was > 0.7, 0.4-0.7, or < 0.4, respectively.
Results The Spearman’s correlations of hMC, MMT and CMAS with other measures of disease activity and damage are presented in the table.
Conclusions We have developed a new hybrid measure of muscle strength in JDM, which is more comprehensive than the MMT and more feasible than the CMAS. Overall, the construct validity of the hMC was comparable to that of the MMT and superior to that of the CMAS.
Disclosure of Interest None Declared
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