Sensitivity and specificity of single-fibre EMG in the diagnosis of ocular myasthenia varies accordingly to clinical presentation
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Background and purpose
Single-fibre electromyography (SF-EMG) is considered as the most sensitive test for the diagnosis of MG. However, previous studies had limitations, such as a retrospective design, non-consecutive sampling, incorporation bias or were performed in small or mixed populations. Our aims were to determine the diagnostic sensitivity and specificity of SF-EMG of the orbicularis oculi in OMG and the utility of this test in relation to patients’ clinical presentation.
Materials and methods
We studied 232 consecutive patients referred to the SF-EMG laboratory for a suspected OMG. Stimulated SF-EMG was performed on the orbicularis oculi muscle.
OMG was diagnosed in 165 cases and other disorders (OD) in 67. SF-EMG showed a sensitivity of 0.79 (95% CI 0.73–0.85) and a specificity of 0.80 (95% CI 0.71–0.90). False negative results were associated with mild symptoms and with isolated diplopia. Comparison of the diagnostic yield among patients with different clinical presentations showed a similar diagnostic accuracy of SF-EMG in patients with ptosis and in patients with ptosis and diplopia, significantly higher than in patients with isolated diplopia (P < 0.0001). Twenty-two patients with OMG presenting with isolated ptosis or diplopia, who initially tested negative, were re-tested in relation to a worsening of their symptoms showing a positivisation in 91% of cases.
SF-EMG on the orbicularis oculi muscle is very sensitive in patients with ptosis. In contrast, in patients with isolated diplopia SF-EMG does not exclude OMG. Therefore, the interpretation of the results of the test should take into account the patients’ clinical presentation.
KeywordsOcular myasthenia SF-EMG Orbicularis oculi muscle Sensitivity Specificity
Compliance with ethical standards
Conflicts of interest
The authors declare that they have no conflict of interest.
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