[Titer of anti-muscle-specific receptor tyrosine kinase (MuSK) antibody correlated with symptomatic improvement in response to corticosteroid therapy in a patient with anti-MuSK antibody-positive myasthenia gravis: a case report].

A 54-year-old woman was admitted to our hospital because of diplopia, dysphagia, dropped head, and muscle weakness with easy fatigability. A neurological examination showed bilateral ptosis, ocular motility disorder, dysphagia, and weakness of the neck extensor muscles. Edrophonium and repetitive nerve stimulation tests of the thenar muscles showed positive results. The serum titer of anti-acetylcholine receptor antibody was negative. A thymoma was not detected in her chest CT. Finally, she was diagnosed with anti-MuSK antibody-positive myasthenia gravis based on the high serum titer of anti-MuSK antibody (239 nmol/l). Her symptoms improved after administration of prednisolone. However, the symptoms were aggravated when the prednisolone dosage was reduced, and the titer of anti-MuSK antibody rose at the same time. We evaluated the possible association between changes in the severity of her clinical symptoms and the titer of the antibody during prednisolone therapy. It was revealed that the titer of the antibody was correlated to the severity of clinical symptoms expressed by a QMG (Quantitative Myasthenia Gravis) score. These findings indicate that monitoring the titer of anti-MuSK antibody can be useful for assessing disease activity as well as decision making during treatment.