Background The incidence of concurrent myasthenia gravis (MG) and neuromyelitis optica spectrum disorder (NMOSD) is higher than what chance predicts, yet it remains unclear why MG and NMOSD appear concurrently. Objective The purpose of the present study was to examine the clinical features of the concurrence of these diseases. Methods Clinical details were analyzed retrospectively. Results Three (0.5%) out of 631 MG patients had confirmed (n = 2) or suspected (n = 1) NMOSD. Two of these patients were women. All showed early-onset MG (EOMG) that preceded NMOSD and were positive for acetylcholine receptor antibody (AChR-Ab). Two patients were tested for aquaporin 4 antibody (AQP4-Ab) and were positive. Two patients were treated with a thymectomy that preceded NMOSD. Two patients had decreased frequency of regulatory T (Treg) cells. We identified in the literature 46 patients with both MG and NMOSD. Our results of female predominance, EOMG, MG preceding NMOSD, and positive AChR-Ab are consistent with previous descriptions. Conclusions This is the first report to examine the frequency of NMOSD in Japanese patients with MG. The reduction and/or dysfunction of Treg cells may be one cause of NMOSD development in MG.
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