[Myasthenia gravis with anti-acetylcholine receptor antibody or anti-muscle specific kinase antibody].
Patients with myasthenia gravis (MG) are divided into 3 groups: anti-acetylcholine receptor antibody-positive MG (AChR MG), anti-muscle specific kinase antibody-positive MG (MuSK MG), and AChR-and MuSK-negative MG (double seronegative MG; double SNMG). A recent study on the detection of low-affinity antibodies binding to AChR showed the presence of AChR antibodies in about 70% of double SNMG patients. There is accumulating evidence that double SNMG patients are similar to AChR-MG patients with respect to clinical features and thymic pathology. Since most MG patients are thought to belong to the AChR-MG or MuSK-MG group of patients, this article reviews the pathophysiology, clinical features, and treatments in these 2 groups of MG patients. The pathophysiology of AChR-MG is closely related to the abnormal thymic pathology, such as thymic hyperplasia or presence of thymoma, and thy(mo)mectomy is recommended in patients with generalized AChR-MG. On the contrary, little thymic abnormality in patients with MuSK-MG discourages thymectomy; however, MuSK-MG patients do respond to thymectomy and therefore studies to define the indications of thymectomy in MuSK-MG patients are required. The responses to cholinesterase inhibitors are poor in patients with MuSK-MG, and these patients tend to show hyperactivity to the Tensilon test, such as fasciculation of facial muscles and stuffy sensation in the throat. The adverse responses to a small dose of intravenous edrophonium chloride injection may support the clinical diagnosis of MuSK-MG. Further, only small doses of acetylcholinesterase inhibitors are administered to patients with MuSK-MG in order to avoid cholinergic hyperactivity. In general, both types of MG patients respond well to treatments with immunosuppressants, including steroids, but some patients with MuSK-MG show persistent bulbar symptoms.