Investigators at the Mayo Clinic, Rochester, MN, report the sensitivity and specificity of the pediatric EMG in the diagnosis of myopathic disorders in patients <18 years referred between 2009 and 2013. Referral diagnoses included myopathy, muscle weakness, neuromuscular disorders, myositis, myalgia, myoglobinuria, myasthenia, myotonia, cramps, periodic paralysis, hypotonia, and developmental delay. Only patients with both EMG and muscle biopsy were included for analysis. Patients with neurogenic EMG and neuromuscular disorders were excluded.
Myopathic EMG was defined as short duration, low amplitude, polyphasic motor unit potentials with rapid recruitment. Of 72 patients included (age range, 6 months-18 years), 32 had myopathic EMG with biopsy- or genetically-confirmed myopathy (Group A); 12 had myopathic EMG but normal biopsy (Group B); 3 had normal EMG but biopsy or genetically confirmed myopathy, all with metabolic myopathy (Group C); 25 had normal EMG and normal or nondiagnostic biopsy (Group D). The most common diagnoses were congenital myopathy (7 cases), metabolic myopathy (6 cases), muscular dystrophy (6 cases), genetically confirmed myopathy (5 cases), myopathy, undefined (5 cases), and inflammatory myopathy (4 cases). Pediatric EMG was 91% sensitive and 67% specific in myopathic disorders. Metabolic myopathies were commonly missed by EMG. 
COMMENTARY. Earlier studies show that the EMG has the highest concordance rate with neurogenic disorders but a lower concordance with myopathic disorders, especially in very young hypotonic infants . More recent and the present studies show a higher rate of correlation in myopathic disorders but despite the exceptional expertise of the examiners, EMG frequently missed the diagnosis of a metabolic myopathy. Of 6 cases of metabolic myopathy, 3 had a normal EMG. Definitive diagnosis was made by muscle biopsy, biochemical analysis, and genetic testing of the muscle tissue. The metabolic abnormality was McArdle disease in 1 patient, mitochondrial myopathy in 1, mucolipidosis type II in 1, and undefined in 3. The authors recommend that in cases presenting with a high degree of suspicion for metabolic myopathy, biopsy should be performed despite a normal EMG.
Electrophysiological Study of Mitochondrial Disorders. In 44 unselected mitochondrial disorder patients examined at the University of Pisa, Italy, motor nerve conduction studies were abnormal in 36.4%, consistent with a sensori-motor axonal multifocal neuropathy, mainly affecting the lower limbs. EMG evidence of myopathy was present in 54.5% of patients. Peripheral nerve and muscle involvement is common but not uniformly present in mitochondrial disorders .