The Association of methylprednisolone dosing to cessation of myotonia in a patient with myotonic dystrophy type 1

•The symptoms of myotonia disappeared four weeks after starting methylprednisolone.•The first symptoms of myotonia returned about a month after the last dose.•Myotonia severity was expressed as relaxation time after voluntary contraction.•The relaxation time improved from 1.3 s to normal value of 0.2 s.