Muscle-specific receptor tyrosine kinase antibody positive myasthenia gravis current status

Abstract

Muscle-specific tyrosine-kinase-antibody-positive myasthenia gravis (MuSK-MG) has emerged as a distinct entity since 2001. This disease has been reported worldwide, but with varying rates among patients with generalized acetylcholine-receptor-antibody-negative MG. MuSK-MG was detected in approximately 37% of generalized acetylcholine receptor antibody-negative MG. MuSK-MG patients were predominantly female with more prominent facial and bulbar involvement and more frequent crises. Disease onset tended to be earlier. Patients tended to have a relatively poor edrophonium response but showed prominent decrement in the repetitive nerve stimulation test in the facial muscles. Patients were more likely to display poor tolerance of, or a lack of improvement with, anticholinesterase agents. Somewhat better response was observed with steroids and plasma exchange. Most were managed successfully with aggressive immunomodulatory therapies, although a higher proportion of MuSK-MG patients had a refractory course when compared with other forms of generalized MG. I present here an up-to-date overview on MuSK-MG based on our experience at the University of Alabama at Birmingham and the existing literature.

Keywords: muscle-specific tyrosine-kinase-antibody; myasthenia gravis; seronegative myasthenia gravis.

Fig. 1

Simplitied illustration of neuromuscular junction in normal and MuSK-MG. MuSK-MG: muscle-specific tyrosine kinase antibody positive-MG.

Fig. 2

Classical repetitive nerve stimulation response in MuSK-MG. A remarkable decremental (31%) response in the orbicularis oculi (Orb oculi) muscle and normal response (3%) in the abductor digiti quinti (ADQ) muscle. Low CMAP amplitude is also noted in Orb oculi muscle. MuSK-MG: muscle-specific tyrosine kinase antibody positive-MG, CMAP: compound muscle action potential.