Is MuSK myasthenia gravis linked to IgG4-related disease?

Immunoglobulin (Ig) G4-related disease (IgG4-RD) is an immune-mediated inflammatory condition that affects a wide variety of sites, including the nervous system, where it can involve the meninges or the pituitary gland, and cause perineural mass lesions. A large subset of acetylcholine receptor antibody (Ab)-negative myasthenia gravis (MG) patients has muscle-specific tyrosine kinase (MuSK) Abs, generally of the IgG4 subclass. There has not been any association found between IgG4-RD and MuSK MG yet. We report the first case of MuSK MG associated with lymphadenopathy with histopathology consistent with IgG4-RD. A 54-year-old woman with MuSK MG developed eight compression fractures related to steroid therapy. Eighteen months after initial presentation she was found to have retroperitoneal lymphadenopathy with biopsy findings consistent with IgG4-RD. She was started on rituximab with clinical improvement and ability to taper immunomodulatory agents for the first time. Our case raises number of questions regarding a potential link between MuSK MG and IgG4-RD which may shed further light on the pathophysiology and management of these diseases.