Seronegative myasthenia with antibodies against muscle-specific tyrosine kinase

About 10-20% of patients with generalized myasthenia gravis do not have acetylcholin receptor (AChR) antibodies and are referred to as having "seronegative" myasthenia. Recently antibodies to muscle-specific kinase (MuSK), which mediates aggregation of AChR, were detected in half of these patients. We investigated six patients with seronegative myasthenia and identified two characteristic cases with MuSK antibodies. A 56-year-old woman developed ptosis and double vision followed by bulbar symptoms and respiratory insufficiency. She showed moderate response to treatment with pyridostigmine and no response to prednisolone, azathioprine, or intravenous immunoglobulin but good response to several plasma exchanges. A 46-year-old man with exercise-induced weakness of the upper limbs and mild bulbar and respiratory symptoms had a milder course for 9 years. These cases high-light the clinical spectrum of MuSK-positive myasthenia and the differences to AChR-positive myasthenia with predominant bulbar symptoms and often severe symptoms with respiratory insufficiency. Finally, they illustrate the therapeutic difficulties with less response to pyridostigmine and immunosuppressive drugs.