Clinical phenotype of muscle-specific tyrosine kinase-antibody positive myasthenia gravis

Antibodies to muscle-specific receptor tyrosine kinase (MUSK-Ab) are detected in approximately 40% of generalized acetylcholine receptor (AChR) antibody-negative myasthenia gravis (MG). Based on a growing number of clinical series, a MUSK-Ab-positive phenotype is emerging. While these clinical patterns are not wholly distinct from either AChR-Ab-positive or seronegative (both AChR-Ab and MUSK-Ab-negative) MG, they are still helpful in identifying these patients. MuSK-Ab-positive MG patients are predominantly female with more prominent cranial and bulbar involvement and more frequent crises than other MG populations. Disease onset tends to be earlier, generally by the third or fourth decade. MUSK-Ab-positive patients are more likely to display poor tolerance of or a lack of improvement with anticholinesterase agents. The yield of repetitive nerve stimulation with conventional limb muscles is lower in these patients, but at least three-quarters demonstrate abnormalities on recording of facial-innervated muscles. Similarly, single-fiber electromyography of distal limb muscles tends to have a lower yield of abnormality in MUSK-Ab-positive patients than either AChR-Ab-positive or seronegative MG, whereas jitter is increased in nearly all MUSK-positive patients when proximal limb or cranial musculature is studied.