Development and validation of a muscular dystrophy-specific functional rating scale

Abstract

Objective: To develop a reliable and valid new rating scale for measuring the functional impact of muscular dystrophy. Design: Prospective and longitudinal investigation. Setting: Three academic medical centres in Taiwan and the Muscular Dystrophy Association of Taiwan. Measures: The Brooke Scale, the Vignos Scale, the Barthel Index, muscular strength, contracture severity, and predicted forced vital capacity (FVC%). Methods: Scale development was in three stages. In stage I, a preliminary pool of 53 items was generated from patient interviews (n-/25), literature review, existing functional rating scales and expert opinion. In stage II, these items were administered to 85 patients with muscular dystrophy. The resulting data were analysed to construct a rating scale (the Muscular Dystrophy Functional Rating Scale, MDFRS) that encompassed four unidimensional constructs: mobility, basic activities of daily living, arm function and impairment. In stage III, the measurement properties of this rating scale were assessed in 121 muscular dystrophy patients different from those examined with the preliminary instrument. Results: Internal consistency reliability was excellent for all domains of the final 33-item scale, with values of Cronbach’s alpha ranging from 0.84 to 0.97. Intraclass correlation coefficients for test-retest and inter-rater reliability were 0.99 for all domains of the MDFRS. The MDFRS showed moderate to high correlations with a range of functional rating scales measuring similar aspects and impairment parameters (Spearman’s rho=0.65-0.91; P<0.001, each). Confirmatory factor analysis supported a unitary construct of the four-dimensional MDFRS. The MDFRS had small floor and ceiling effects in the study samples. Sensitivity to change was confirmed by large standardized response means for the MDFRS total score. Conclusions: The MDFRS is a reliable and valid disease-specific measure of functional status for patients with muscular dystrophy.