Development and validation of the Myasthenia Gravis Impairment Index

OBJECTIVE: We aimed to develop a measure of myasthenia gravis impairment using a previously developed framework and to evaluate reliability and validity, specifically face, content, and construct validity. METHODS: The first draft of the Myasthenia Gravis Impairment Index (MGII) included examination items from available measures enriched with newly developed, patient-reported items, modified after patient input. International neuromuscular specialists evaluated face and content validity via an e-mail survey. Test–retest reliability was assessed in stable patients at a 3-week interval and interrater reliability was evaluated in the same day. Construct validity was assessed through correlations between the MGII and other measures and by comparing scores in different patient groups. RESULTS: The first draft was assessed by 18 patients, and 72 specialists answered the survey. The second draft had 7 examination and 22 patient-reported items. Field testing included 200 patients, with 54 patients completing the reliability studies. Test–retest reliability of the total score was good (intraclass correlation coefficient 0.92; 95% confidence interval 0.79–0.94), as was interrater reliability of the examination component (intraclass correlation coefficient 0.81; 95% confidence interval 0.79–0.94). The MGII correlated well with comparison measures, with higher correlations with the MG–activities of daily living (r = 0.91) and MG-specific quality of life 15-item scale (r = 0.78). When assessing different patient groups, the scores followed expected patterns. CONCLUSIONS: The MGII was developed using a patient-centered framework of myasthenia-related impairments and incorporating patient input throughout the development process. It is reliable in an outpatient setting and has demonstrated construct validity. Responsiveness studies are under way.