Development of validated disease activity and damage indices for the juvenile idiopathic inflammatory myopathies - II. The Childhood Myositis Assessment Scale (CMAS): A quantitative tool for the evaluation of muscle function

Objective. To develop, validate, and determine the measurement characterist ics of a quantitative tool for assessing the severity of muscle involvement in children with idiopathic inflammatory myopathies. Methods. The Childhood Myositis: Assessment Scale (CMAS) was developed from 2 existing observational functional assessment tools to assess Hnuscle fun ction in the areas of strength and endurance across a wide range of ability and ages. The 14 ordinal items included were chosen to assess primarily ax ial and proximal muscle groups and are ranked with standard performance and scoring methods. Following the development of the CMAS, a training video a nd written instructions were developed and reviewed by the physicians parti cipating in this study. Subsequently, utilizing a randomized block design, 12 physicians independently scored 10 children (9 with dermatomyositis, 1 w ith polymyositis; ages 4-15 years) twice in one day (morning and afternoon) on the CMAS, A pediatric physical therapist performed quantitative manual muscle strength testing (MMT) twice on each child (morning and afternoon), including the neck, trunk, and proximal and distal extremity muscle groups. Results. The CMAS has a potential range of 0-51, with higher scores indicat ing greater muscle strength and endurance. The observed mean for the 10 pat ients was 36.4 (median 44, SD 14.1, observed range 5-51), The total score f or the CMAS correlated with the physician's global assessment (by visual an alog scale) of disease activity, the MMT score, serum creatine kinase level , and the Juvenile Arthritis Functional Assessment Report score, The score on the CMAS was not correlated with patient age. Interrater reliability (Ke ndall's coefficient of concordance) ranged from 0.77 to 1.0 for individual items (all P < 0.001), and overall, it was 0.95 (P < 0.001). Intrarater rel iability for the individual physicians was measured by correlation of the C MAS scores for each patient on 2 separate evaluations and ranged from 0.97 to 0.99, with an overall correlation for all physicians of 0.98 (all P < 0. 001), Conclusion. The CMAS demonstrated an acceptable range of observed scores, e xcellent convergent validity, and excellent inter- and intrarater reliabili ty. The CMAS is validated to quantitatively assess muscle function in the a reas of strength and endurance in children with idiopathic inflammatory myo pathies, It can be used in routine clinical care as well as therapeutic tri als.