Selective response to rituximab in a young child with MuSK-associated myasthenia gravis.

Neuromuscular junction disorders in children are either genetic, such as congenital myasthenic syndrome, or autoimmune with circulating antibodies most commonly against acetylcholine receptors. There is limited experience recognizing and treating children with myasthenia associated with muscle-specific tyrosine kinase antibodies. We report a seven-year-old child with intermittent esotropia since age 3 months, and two years of progressive and severe diplopia, dysarthria, dysphagia, and facial weakness. Acetylcholine receptor antibodies and genetic testing for congenital myasthenic syndrome were negative. Muscle specific tyrosine kinase antibodies were significantly elevated. Ophthalmoplegia and bulbar weakness were refractory to treatment with acetylcholinesterase inhibitors, corticosteroids and IVIg but completely resolved following treatment with rituximab. Her neurologic examination remained normal at the most recent follow-up, 15 months after initiation of rituximab. Children with MuSK myasthenia, like adults, can respond to rituximab despite long standing disease and failure to improve on other immunosuppressant medications.