Evaluation of the scales assessing the severity of myasthenia gravis

Abstract

Objective To evaluate the reliability and validity of the 4 myasthenia gravis (MG) scales widely used for assessing the grades of disease severity in Chinese MG patients. Methods Sixty MG patients were examined by a neurologist with the following four MG scales: Quantitative Myasthenia Gravis Score (QMGS), Myasthenia Gravis Composite (MGC), Myasthenic Muscle Scale (MMS), Absolute and Relative Score of MG (ARS-MG). The whole assessment process was videotaped. MG Activities of Daily Living (MG-ADL) score was acquired after the examination. The patients were examined by the same neurologist with the same MG scales within 24 hours after the first examination. The original videotapes of each patient were assessed by five other neurologists independently. Interobserver reliability of each item of the four MG scales was assessed with weighted Kappa test. Internal consistency of the scales was assessed with Cronbach ɑ. Test-retest reliability and interobserver reliability among the five neurologists were assessed with intraclass correlation coefficience (ICC). The construct validity was assessed with factor analysis. Criterion validity was assessed by Spearman correlation analysis. Results Interobserver reliability of items of the 4 MG scales was moderate (k 0.310-0.891, P<0.01). All 4 MG scales showed good internal consistency (Cronbach ɑ 0.701-0.734) with high test-retest reliability (ICC 0.887-0.948, P<0.01) and interobserver reliability (ICC 0.853-0.917, P<0.01). Factor analysis showed good construct validity (accumulative dedication rate 66.49%-72.64%). There was moderate to strong correlation between QMGS (as criterion) and the other three scales (r 0.792-0.840, P<0.01), and moderate correlation between MG-ADL (as criterion) and the four scales (r 0.693-0.766, P<0.01). Conclusion The 4 widely used MG severity scales can effectively assess the grades of disease severity of MG with good reliability and validity. Key words: Myasthenia gravis; Evaluation studies; Reproducibity of results; Validation studies