The high frequency and clinical feature of seronegative myasthenia gravis in Southern China.

Anti-acetylcholine receptor antibodies (anti-AChR-Ab) are responsible for the failure of neuromuscular junction in myasthenia gravis (MG). Some anti-AChR-Ab-seronegative MG patients have anti-muscle-specific tyrosine kinase antibodies (anti-MuSk-Ab). Here, the anti-AChR-Ab was tested in 250 MG outpatients from Southern China. While anti-MuSk-Ab was tested in 66 patients who had no anti-AChR-Ab in blood serum, but none of them was positive. The antibodies were measured by a radioimmunoprecipitation assay. The frequency of anti-AChR-Ab was 51.2 %. The percentage of anti-AChR-Ab in ocular type was lower than generalized type (44.9 vs. 66.2 %, P = 0.002). Seronegative MG was characterized by a lower percentage of thymoma than seropositive patients (P = 0.013). It seemed to be less severe in seronegative MG than seropositive MG in these 250 patients. In ocular type, seronegative MG mainly manifesting blepharoptosis but seldom diplopia or eyeball fixation related to ocular movement disability (P = 0.016). While in generalized type, seronegative MG was characterized by a lower percentage of bulbar muscle involvements than seropositive patients (P = 0.005). Logistic regression analysis revealed that bulbar weakness was affected by the existence of anti-AChR antibodies (OR = 3.524, P = 0.015). Besides, seronegative MG tended to be characterized by a lower percentage of neck extensor involvement, but this did not reach significance. The percentage of anti-AChR antibodies was much lower than other countries. Seronegative MG has characteristic clinical features that are different from features of the remaining seropositive MG. This emphasises the predictive value of anti-AChR antibodies analysis in MG patients.