Abstract
Muscle-specific tyrosine kinase (MuSK) myasthenia gravis (MG) is a rare, frequently more severe, subtype of MG with different pathogenesis, and peculiar clinical features. The prevalence varies among countries and ethnic groups, affecting 5–8% of all MG patients. MuSK-MG usually has an acute onset affecting mainly the facial-bulbar muscles. The symptoms usually progress rapidly, within a few weeks. Early respiratory crises are frequent. The disease may lead to generalized muscle weakness up to muscle atrophy. The main bulbar involvement, the absence of significant thymus alterations, and the association with HLA class II DR14, DR16, and DQ5 alleles have been confirmed. Atypical onset, such as ocular involvement, lack of symptom fluctuations, acetylcholinesterase inhibitors failure, and negative results of electrophysiologic testing, if not specifically performed in the mainly involved muscle groups, makes MuSK-MG diagnosis challenging. In most cases, steroids are effective. Conventional immunosuppressants are not commonly able to replace steroids in maintaining a satisfactory long-term control of symptoms. However, the majority of MuSK-MG patients are refractory to treatment. In these cases, the use of rituximab showed promising results, resulting in sustained symptom control.
Highlights
In 2001, serum antibodies against muscle-specific tyrosine kinase (MuSK-Abs) were identified for the first time as cause of myasthenia gravis (MG) [1], opening the way to the description of a distinct peculiar subtype of MG disease [2,3,4,5]
SFEMG plays an important role in diagnosing musclespecific tyrosine kinase (MuSK)-MG, and we underline the importance to focus it on the mainly affected muscles to precociously detect alterations
All authors have both approved the submitted version of the manuscript and agreed to be personally accountable for their own contributions
Summary
In 2001, serum antibodies against muscle-specific tyrosine kinase (MuSK-Abs) were identified for the first time as cause of myasthenia gravis (MG) [1], opening the way to the description of a distinct peculiar subtype of MG disease [2,3,4,5]. NMJ transmission efficacy is strictly related to the “safety factor,” which refers to the ability of the NMJ to remain effective under several conditions. This is possible mainly because each nerve impulse releases more transmitter than is required to excite the muscle fiber, ensuring that the transmission does not fail [6]. MuSK-Abs interfere with MuSK–LRP4 complex and, AChR clustering is inhibited [11]. The aim of this mini-review is to report on the epidemiological and major clinical features, diagnostic approach, and treatment of MuSK-MG subtype. No significant thymus alterations have been reported in MuSK-MG patients as related to the disease [9, 12, 13]
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