A reappraisal of diagnostic tests for myasthenia gravis in a large Asian cohort

BACKGROUND Myasthenia gravis (MG) is a chronic autoimmune neuromuscular disease characterized by weakness of bodily skeletal muscles. Office-based diagnostic tests such as repetitive nerve stimulation (RNS), single fiber electromyography (SFEMG), and the ice test, are used to refine the differential clinical diagnosis of this disease. Evaluating the clinical sensitivity and specificity of these tests, however, may be confounded by lack of a gold standard, non-blinding, incorporation bias, use of non-representative populations and retrospective data. OBJECTIVE In this study comprising a large Asian cohort of 127 patients recruited from a Neuro-ophthalmology clinic, we minimized aforementioned confounders and tested the diagnostic value of 3 office-based tests against 2 reference standards of MG by virtue of clinical features, antibody assay and response to treatment. RESULTS Regardless of the reference standard used, the ice and SFEMG tests displayed a higher sensitivity (86.0 to 97.3%) compared to the RNS test (21.3 to 30.6%). Conversely, the specificity of the ice (31.3%) and SFEMG (21.7% and 17.2%) tests were reduced compared to the RNS test (82.6% and 84.4%). The combined use of the ice test and SFEMG, improved the specificity of MG diagnosis to 63.6% and 64.3%, without affecting the sensitivity of those tests. CONCLUSION Our findings indicate, in an Asian population, high sensitivity of the SFEMG test and suggest the ice test as a valid, affordable and less technically demanding approach to diagnose MG with ocular involvement. Both ice test and SFEMG alone, however, yielded poor specificity. We suggest that the combination of SFEMG and ice test provides a more reliable diagnosis of MG.