Intravenous immunoglobulin improves many antibody-mediated autoimmune disorders, but its mode of action is unknown. We investigated its effects on muscle strength and on the serum titer of the calcium-channel autoantibodies that are likely to be pathogenic in the Lambert-Eaton myasthenic syndrome (LEMS). In a randomized, double-blind, placebo-controlled crossover trial, serial indices of limb, respiratory, and bulbar muscle strength and the serum titer of calcium-channel antibodies in nine patients were compared over an 8-week period, using the area-under-the-curve approach, following infusion on two consecutive days of immunoglobulin at 1 g/kg body weight/day (total dose 2.0 g/kg body weight) or placebo (equivalent volume of 0.3% albumin). Calcium-channel antibodies were measured by radioimmunoassay using 125I-omega-conotoxin MVIIC. Direct anti-idiotypic actions of immunoglobulin were tested in this assay. Immunoglobulin infusion was followed by significant improvements in the three strength measures (p = 0.017 to 0.038) associated with a significant decline in serum calcium-channel antibody titers (p = 0.028). Improvement peaked at 2 to 4 weeks and was declining by 8 weeks. Mean serum titers were unchanged at 1 week, however, and direct anti-idiotypic neutralization by immunoglobulin was not demonstrable in vitro. We conclude that immunoglobulin causes a short-term improvement in muscle strength in LEMS that probably results from the induced reduction in calcium-channel autoantibodies. The reduction is not due to a direct neutralizing action of the immunoglobulin, but a delayed anti-idiotypic action cannot be excluded. Improvement following intravenous immunoglobulin in other autoantibody-mediated disorders may similarly be associated with decline in levels of pathogenic autoantibodies.

We report a 60-year-old male with thymoma-associated myasthenia gravis with anti-MuSK antibodies. In October 2010, he had diplopia, ptosis, and dysphagia. He was diagnosed to have MG in February 2011. The neurological examination disclosed external ophthalmoplegia, bilateral ptosis, mild dysphagia, and fatigability. Repetitive nerve stimulation of the right facial nerve showed CMAP decrement greater than 10%. Patients showed an improvement in ptosis after administration of edrophonium.Anti-acetylcholine receptor antibody was negative, and anti-muscle specific receptor tyrosine kinase antibody was 66.8 nmol/l (cut-off value: 0.05 nmol/l). Prednisolone (50 mg every other day) were started. Contrast-enhanced chest MRI showed a mediastinal mass suggestive of thymoma. Extended thymectomy was performed in March 2011. Histological examination disclosed a type B1 thymoma. After resection of the tumor, the symptoms of MG improved with prednisolone (100 mg every other day). This is a rare case of MG with anti-MuSK antibodies and thymoma, which has been reported previously only in 2 cases.

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