To evaluate the rate and underlying causes of misdiagnosis in patients with myasthenia gravis (MG).

A total of 106 patients were referred for a second opinion after receiving an MG diagnosis. Of these, 23 (21.7%) were found to have been misdiagnosed. Twelve of 23 (52.2%) were seronegative; low-titre acetylcholine receptor antibodies (AChR-Abs) were detected in 10/23 (43.5%), in 7/10 by radioimmunoassay (RIA) and in 3/10 by ELISA, muscle-specific kinase antibodies (MuSK-Abs) were detected in one; in all patients positivity was not confirmed by fixed cell-based assay and in 7/9 who repeated RIA. In 6 subjects, a prior electrodiagnostic study was deemed positive: 4 patients had abnormal repetitive nerve stimulation (RNS) and single-fiber electromyography showed minimal or equivocal abnormalities in two. Repeated RNS did not confirm previous results in 4/4. Median duration of misdiagnosis was 39 months [1-196]. All but 4 subjects had received anticholinesterase therapy, immunosuppressive treatment was started in 15/23 (65.2%) and 2 underwent thymectomy.

MG misdiagnosis is not rare and often leads to years of unnecessary treatment. The main factors contributing to MG misdiagnosis are atypical clinical presentations, false positive antibody results and neurophysiological testing performed with insufficient expertise. These findings highlight the importance of experienced clinical judgment in MG diagnosis, along with the appropriate use of antibody testing and neurophysiological studies within the right clinical context.