A de novo Mutation in the SCN4A Gene Causing Sodium Channel Myotonia.

We describe the case of a six year old boy with findings consistent with myotonia congenita: muscular hypertrophy, stiffness when commencing movements and typical warm-up signs. The most prominent symptom was myotonia of the eyelid muscles with apparent swelling around the eyes. Even though the pronounced warm-up phenomena in our patient suggested a chloride channel-associated myotonia congenita, the myotonia of his eyelid muscles indicated an involvement of sodium channels.