A case of refractory anti–muscle–specific tyrosine kinase myasthenia gravis responding to plasma exchange

Abstract

I report a 50–year–old man with generalized anti–muscle–specific tyrosine kinase (MuSK) antibody–positive myasthenia gravis (MG). He first experienced diplopia and heaviness in his neck muscles 2 years prior. He was diagnosed with generalized MuSK–MG, with high anti–MuSK antibody levels (80.5nmol/L) and negative anti–acetylcholine receptor antibodies. Repetitive median nerve stimulation significantly decreased the compound thumb muscle action potentials. His myasthenic symptoms consisted of moderate myasthenia in all extremities, with bilateral ptosis and diplopia but no bulbar symptoms. His moderate myasthenic symptoms were resistant to oral corticosteroids with repetitive simultaneous intravenous immunoglobulin (IVIG), and methylprednisolone pulses following cyclophosphamide injection for almost 2 years. He underwent an extended endoscopic thymectomy owing to a suspected hyperplastic thymus in the right anterior mediastinumfrom contrast–enhanced chest computed tomography (CT). Complete histopathological examination revealed almost fatty tissue, including an involuted, but not hyperplastic, thymus. His myasthenic symptoms deteriorated after thymectomy. Simultaneous IVIG and steroid pulse therapy caused a transient decrease in anti–MuSK antibody titers but did not improve his myasthenic symptoms. Subsequent repetitive PE at 1– to 3–month intervals caused a steady decline in the anti–MuSK antibody titers from the preoperative titer of 32.6 to 0.7–1.8nmol/L. His myasthenic symptoms gradually improved, and bilateral ptosis and diplopia disappeared.

Although a hyperplastic thymus was suspected on contrast–enhanced CT, an almost fatty tissue containing an involuted thymus was excised. Thymectomy did not improve his myasthenic symptoms, as recently reported. In refractory MuSK–MG, repetitive plasmapheresis is more effective than simultaneous IVIG and steroid pulse therapy in decreasing anti–MuSK antibodies and ameliorating myasthenic symptoms. I report this as a suggestive case for treating refractory MuSK–MG patients.