This paper is aimed to present the validation of the myasthenia gravis disability assessment (MG-DIS), a MG-specific patient-reported disability outcome measure. Consecutive MG patients were enrolled, followed-up for 12 months and administered the SF-36, the WHO disability assessment schedule (WHODAS 2.0) and the preliminary 31-item MG-DIS addressing impairments and activity limitations. Factor structure and metric properties were assessed. In total, 109 patients were enrolled: 76 were females, mean age 50, mean MG duration 10.4 years, 86 were AChR-positive. The MG-DIS was reduced to 20 items, explaining 70.6 % of the original questionnaire variance, four subscales (generalized impairment-related problems; bulbar function-related problems; mental health and fatigue-related problems; vision-related problems) and an overall disability index. The MG-DIS has good metric properties (Cronbach's alpha ranging between .808 and .930), is stable, showed to be more sensitive than the WHODAS 2.0 and SF-36 to detect group differences and longitudinal changes and was well correlated with the MG-composite (.642). The MG-DIS includes items representing ocular, generalized, bulbar and respiratory symptoms, and is therefore well-built around MG-specific features. MG-DIS can be used in clinical trials as well as in observational or epidemiological studies to characterize patients' disability level and address the amount of improvement in disability. Further studies are needed to explore the possibility of a shorter disability scale.
Keywords: Disability evaluation; Factor analysis; Myasthenia gravis; Patient-reported outcome; Validation study.