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Myasthenia Gravis Impairment Index: Responsiveness, meaningful change, and relative efficiency
Carolina Barnett1, Vera Bril2, Moira Kapral2
1From the Division of Neurology (C.B., V.B.), Department of Medicine (M.K.), University of Toronto and University Health Network; Institute of Health Policy, Management and Evaluation (M.K., A.V.K., A.M.D.), and Department of Physical Therapy and Rehabilitation Science Institute (A.M.D.), University of Toronto; Division of Neurosurgery (A.V.K.), Sick Kids Hospital; and Division of Health Care and Outcomes (A.M.D.), Krembil Research Institute, University Health Network, Toronto, Canada. c.barnetttapia@utoronto.ca.
Objective:
To study responsiveness and meaningful change of the Myasthenia Gravis Impairment Index (MGII) and its relative efficiency compared to other measures.
Methods:
We enrolled 95 patients receiving prednisone, IV immunoglobulin (IVIg), or plasma exchange (PLEX) and 54 controls. Patients were assessed with the MGII and other measures-including the Quantitative Myasthenia Gravis Score, Myasthenia Gravis Composite, and Myasthenia Gravis Activities of Daily Living-at baseline and 3-4 weeks after treatment. Statistical markers of responsiveness included between-groups and within-group differences, and we estimated the relative efficiency of the MGII compared to other measures. Patient-meaningful change was assessed with an anchor-based method, using the patient's impression of change. We determined the minimal detectable change (MDC) and the minimal important difference (MID) at the group and individual level.
Results:
Treated patients had a higher change in MGII scores than controls (analysis of covariance p < 0.001). The ocular domain changed more with prednisone than with IVIg/PLEX (effect size 0.67 and 0.13, analysis of covariance p = 0.001). The generalized domain changed more with IVIg/PLEX than with prednisone (effect size 0.50 and 0.22, analysis of covariance p = 0.07). For the total MGII score, the individual MDC95 was 9.1 and the MID was 5.5 for individuals and 8.1 for groups. Relative efficiency ratios were >1 favoring the MGII.
Conclusions:
The MGII demonstrated responsiveness to prednisone, IVIg, and PLEX in patients with myasthenia. There is a differential response in ocular and generalized symptoms to type of therapy. The MGII has higher relative efficiency than comparison measures and is viable for use in clinical trials.
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