Researchers at the EMG Laboratory, Boston Children's Hospital, MA assessed the spectrum of disorders associated with electrophysiologic myotonia in a pediatric electromyography laboratory. Of 2234 patients tested from 2000-2011 and screened retrospectively, 11 patients had myotonic discharges alone; 8 exhibited both myotonic discharges and myopathic motor unit potentials; and 54 demonstrated myopathic motor unit potentials alone. The diagnoses of patients with myotonic discharges alone included myotonia congenita (Thomsen disease) in 3, paramyotonia congenita in 2, congenital myopathy in 1, and Pompe disease (acid maltase deficiency) in 1; one mimicking SMA type 1 in infancy had Prader-Willi syndrome. The diagnoses of patients with both myotonic discharges and myopathic motor unit potentials included congenital myopathy in 2 and non-Pompe glycogen storage diseases in 3. The presence of myopathic potentials helps in the differential diagnosis. 
COMMENT. Channelopathies (myotonia congenita and paramyotonia congenita), glycogen storage diseases (primarily Pompe disease), and congenital myopathies are most commonly associated with EMG myotonic discharges in childhood.