Assessment of a disease-specific muscular impairment rating scale in myotonic dystrophy

Objective: To document the intra/interrater reliability and the construct validity of the Muscular Impairment Rating Scale (MIRS) in assessing patients with myotonic dystrophy type 1 (DM1). The MIRS is a ordinal five-point rating scale, established in accordance with the clinically recognized distal to proximal progression of the muscular involvement in DM1, based partly on a manual muscle testing (MMT) of 11 muscle groups. Methods: To assess the reliability of the MIRS, 55 patients with DM1 were examined by three different observers, one of them evaluating each patient twice. Intra- and interobserver reliability of the MIRS was measured using Cohen's weighted kappa. To assess the construct validity of the MIRS, correlations were made with the Functional Status Index (FSI) and eight timed functional tasks. Results: The intraobserver reliability of the MIRS was excellent (weighted kappa = 0.84), and the interobserver reliability was interpreted as a substantial agreement (weighted kappa = 0.77 to 0.79). The correlation coefficients between MMT scores and MIRS grades were all highly significant (r(s) = -0.81 to -0.88, p < 0.001). The FSI showed a significant progressive increase of the total median dependence score in activities of daily living from 0 in MIRS grade 1 to 39 in MIRS grade 5 (p < 0.001). The time needed to perform the eight functional tasks was also found to significantly increase in relation with the progression of the MIRS grades. Conclusion: The MIRS is a quick, simple, and reliable measurement of muscular impairment in DM1. The FSI questionnaire and the timed motor activities supported its construct validity. The MIRS is useful to monitor major stages of DM1 progression, to study the natural history of the disease, and to identify homogeneous groups of patients for clinical trials.