Muscle-Specific Receptor Tyrosine Kinase Antibody-Positive and Seronegative Myasthenia Gravis

Abstract

To summarize current understanding of muscle-specific receptor tyrosine kinase antibody (MuSKAb)-positive and seronegative myasthenia gravis (MG). We reviewed the current literature on MuSK and seronegative MG, and placed lighter emphasis on seronegative MG studies published prior to the discovery of MuSKAb. MuSKAb are detected in approximately 40% of generalized acetylcholine receptor antibody (AChRAb)-negative MG, but the rate of sero-positivity differs across the globe. MuSK MG patients are predominantly female, have prominent cranial and bulbar involvement, and tend to have a higher rate of crises than those with other forms of MG. Disease onset tends to be earlier, with most patients presenting by the third or fourth decade. The yield of repetitive nerve stimulation on conventional limb muscles is lower in both MuSK MG and seronegative ocular MG. Including cranial muscles increases the yield. Single-fiber electromyography of distal limb muscles tends to have a lower rate of abnormality in MuSK MG than in either AChRAb-positive or seronegative MG. MuSK MG patients are more likely to display poor tolerance ofor a lack of improvement with anticholinesterase agents; this is not a feature of seronegative myasthenia. Both MuSK and seronegative MG patients are managed successfully with immunomodulatory therapies, but a higher proportion of MuSK MG patients have a refractory course. Evidence for a favorable response to thymectomy in both MuSK and seronegative MG is limited. MuSK and seronegative MG are distinct entities. Clinical characteristics and response to symptomatic andimmunomodulatory treatments show meaningful differences for these two populations when compared to AChRAb-positive MG.