Clinical comparison of muscle-specific tyrosine kinase (MuSK) antibody-positive and -negative myasthenic patients

We assayed cryopreserved sera from 38 acetylcholine receptor (AChR) anti body-negative patients with myasthenia gravis (MG) who were followed clinically for muscle-specific tyrosine kinase (MuSK) antibodies and analyzed and compared their clinical characteristics. None of 13 sera from patients with purely ocular MG were positive. Sera from 10 of 25 patients (40%) with generalized MG were positive for MuSK antibodies. The age at onset of myasthenic symptoms was significantly earlier in MuSK anti body-positive patients (P = 0.02). MuSK antibodies were present in AChR anti body-negative patients of either gender, with virtually identical prevalence in women (41.2%) and men (37.5%). The distribution of weakness more commonly involved neck muscles in MuSK antibody-positive patients, and limb muscles in MUSK anti body-negative patients. Patients responded to immunosuppressive treatment regardless of whether MuSK antibody was present. We conclude that MuSK antibodies are present and diagnostically useful in a subset of myasthenic patients without AChR antibodies. Although the distribution of weakness differs somewhat depending on whether MuSK antibodies are present, responses to anticholinesterase and immunosuppressive treatments are similar.