[A case of myasthenia gravis with anti-MuSK antibodies showing a dramatic improvement with plasma exchange].

A 49-year-old woman with seronegative myasthenia gravis (SNMG) was admitted to our hospital with severe respiratory failure, proximal muscle weakness and bulbar palsy. Permanent tracheostomy and continuous mechanical ventilation were performed. At a previous hospital, she was diagnosed as SNMG on the basis of the positive waning during 3 Hz repetitive stimulation of the ulnar nerve, although no acetylcholine receptor antibodies (Ab) were detected by serological examination. Before admission to our hospital, she was treated with corticosteroids, intravenous immunoglobulin and tryptophan column immuno-adsorption therapy without clinical improvement. At our hospital, serological examination detected muscle-specific receptor tyrosine kinase (MuSK) Ab and plasma exchange was performed as treatment. Plasma exchange and subsequent immunomodulating therapy with corticosteroids and tacrolimus showed a dramatic clinical improvement with a marked decline of MuSK Ab level in the serum. These results suggested that plasma exchange should be considered as first choice to treat patients with refractory MuSK Ab-positive MG.