The assay of circulating autoantibody against acetylcholine receptor (AChRAb) is known to be an important diagnostic test in myasthenia gravis (MG). In 10 to 20% of MG patients, the anti-AChR antibodies are not detected by the radioimmunoassay (RIA) . The nature of the pathogenetic mechanisms and clinical features of seronegative MG may be different from those of seropositive MG. The authors performed this study to understand the differences of clinical features between the seronegative and seropositive MGs.
Total 157 MG patients were enrolled, who were diagnosed by the typical clinical features of MG, and appropriate laboratory studies, such as Tensilon test , Jolly test, single fiber EMG, and/or anti-AChR antibody test. The antiAChR antibody was considered to be negative when the titer was below 0.2 nmol per liter by the RIA technique using the AChR purified from TE-671. The seronegative MGs were found in 22(14.0%) out of 157 subjects. Those 22 seronegative MGs consisted of 12 males and 10 females, and their mean age of onset was 28.3 years. Ocular type MGs were found in 13 (59. 1%) out of 22 seronegative MGs, which shows high incidence compared with that in seropositive MGs. There was no thymoma. Thymic hyperplasia was detected in one patient who underwent thymectomy. They did rarely show other systemic autoimmune disorders as only one subject had diffuse nontoxic goiter. Tensilon
test and Jolly test were positive in 94.1% and 56% of seronegative MG patients respectively. Nine patients showed good response to mestinone and 9 patients needed immunosuppressive treatment with prednisolone.
The most typical clinical characteristics in seronegative MG are the higher proportion of the ocular type MG, low frequency of the thymic abnormalities and associated autoimmune diseases , compared with those in the seropositive MGs. Therefore we suggest that the seronegative MGs have much different clinical characteristics from the seropositive MGs, and their pathogenetic mechanisms might be different from that of seropositive MGs.
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