Treatment of MuSK-Associated Myasthenia Gravis

Approximately 5-8 % of myasthenia gravis (MG) patients test positive for antibodies against muscle- specific tyrosine kinase (MuSK) receptors. Except in extremely rare reports, all are acetylcholine receptor (AChR) antibody-negative. While MuSK myasthenia gravis (MMG) patients have distinct clinical phenotypes and may differ from AChR-positive patients in diagnostic testing and response to treatment, goals for the treatment of MMG are similar to those in non-MMG. Priority of treatment should be directed toward reducing weakness as much and as quickly as possible. This is particularly true in patients with bulbar or respiratory weakness in order to avoid progression to respiratory failure. After this initial phase, medications should be slowly tapered to the minimum effective dose. Considering the natural history of MMG, a small proportion of patients can be completely taken off treatment at some point, but the vast majority will require treatment for life. Response to acetylcholinesterase inhibitors (ACEi) is usually poor, and the likelihood of side effects is relatively high. However, considering the benign nature of this line of treatment and the potential for rapid response, an initial trial of ACEi is reasonable. Unless clearly contraindicated by other medical conditions, we recommend initiating corticosteroid treatment for all MMG patients, starting at a dose of 1.5-2 mg /kg/ day of prednisone, followed by gradual and slow taper to the minimum effective dose. A steroid-sparing agent such as azathioprine - and, less often, mycophenolate mofetil or cyclosporine - may be added. When prednisone is used in combination with another immunosuppressive agent, reducing and then tapering off prednisone may be tried after maximum improvement is achieved. It should be emphasized that response to immunosuppressive medications can be delayed for months, although most patients eventually show marked and sustained response. Cyclophosphamide may be used sparingly in select patients who do not respond to the above medications. Rituximab has shown promising results in MMG, and should be considered in severe and refractory cases or in situations where other options are contraindicated or not tolerated by patients. Acute exacerbations may be treated by plasma exchange, which most reports indicate is superior to IVIg, although IVIg may still be used. To date, there is no convincing evidence for the role of thymectomy in MMG.