Background Assessment of muscle strength is a fundamental component of the clinical evaluation of children with juvenile dermatomyositis (JDM). This assessment is traditionally made using the 8-muscle MMT and the CMAS. However, the MMT does not cover all muscles or muscle groups affected in JDM, particularly 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. We have recently developed a new measure of muscle strength in JDM, named Hybrid MMT/CMAS (hMC), which is more comprehensive than the MMT and more feasible than the CMAS. This tool has previously demonstrated good construct validity.
Objectives To further validate the hMC by assessing its discriminant validity.
Methods The hMC is composed of all 8 items of MMT and of 3 items of the CMAS: 1) head lift; 2) sits-ups; 3) floor rise. The total score of the hMC ranges from 0 (worst) to 100 (normal). Validation analyses were performed in 167 patients included in a multinational study. Discriminant validity of the hMC was assessed by comparing its score between: 1) patients rated by the physician as being in remission or in high, moderate, or low disease activity; 2) patients rated in remission, continued activity, or flare by physician and parent; 3) parents satisfied or not satisfied with their child's disease outcome. The change in hMC score between 2 consecutive visits (median 3 months) was compared among patients defined as improved, stable, or worsened by physician and parent. Comparisons of quantitative variables among groups were made by nonparametric analysis of variance (Kruskal-Wallis test) and Mann-Whitney U Test.
Results The hMC demonstrated strong ability to discriminate between patients in high, moderate, low disease activity or remission according to the physician, between patients defined in remission, continued active disease and flare by the parent (p<0.001), between patients with (p<0.001), and between patients satisfied or not satisfied with disease outcome (p<0.001). The change in hMC score differentiated well patients rated as improved, stable, or worsened by the physician (p<0.0006) or the parent (p=0.0049).
Conclusions The hMC showed strong discriminant ability in a large multinational patient sample and is, thus, a valid tool for the assessment muscles disease activity in JDM patients in standard clinical care, observational studies, and clinical trials.
Disclosure of Interest None declared
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