PURPOSE OF REVIEW: Some of the 20% of myasthenia gravis patients who do not have 
antibodies to acetylcholine receptors (AChRs) have antibodies to muscle specific 
kinase (MuSK), but a full understanding of their frequency, the associated 
clinical phenotype and the mechanisms of action of the antibodies has not yet 
been achieved. Moreover, some patients do not respond well to conventional 
corticosteroid therapy. Here we review recent clinical and experimental studies 
on MuSK antibody associated myasthenia gravis, and summarize the results of 
newer treatments for myasthenia gravis.
RECENT FINDINGS: MuSK antibodies are found in a variable proportion of AChR 
antibody negative myasthenia gravis patients who are often, but not exclusively, 
young adult females, with bulbar, neck, or respiratory muscle weakness. The 
thymus histology is normal or only very mildly abnormal. Surprisingly, limb or 
intercostal muscle biopsies exhibit no reduction in AChR numbers or complement 
deposition. However, patients without AChR or MuSK antibodies appear to be 
similar to those with AChR antibodies and may have low-affinity AChR antibodies. 
A variety of treatments, often intended to enable corticosteroid doses to be 
reduced, have been used in all types of myasthenia gravis with some success, but 
they have not been subjected to randomized clinical trials.
SUMMARY: MuSK antibodies define a form of myasthenia gravis that can be 
difficult to diagnose, can be life threatening and may require additional 
treatments. An improved AChR antibody assay may be helpful in patients without 
AChR or MuSK antibodies. Clinical trials of drugs in other neuroimmunological 
diseases may help to guide the treatment of myasthenia gravis.