Metabolic myopathies affect the metabolism of carbohydrates, in particular glucose and fats, which are the two main energy sources of skeletal muscles. The disorders include disturbances in glycolysis and glycogen degradation, ß-oxidation of fatty acids, and respiratory chain defects. The clinical symptoms of this heterogeneous group of diseases are variable and include muscle hypotonia, muscle weakness, stiffness and/or cramps, or acute rhabdomyolysis. Triggers and age of manifestation vary. Neonates usually show hypotonia and multisystemic involvement (liver and brain) while late manifesting forms often involve exercise intolerance, hyperCKemia, myoglobinuria, and may be with or without progressive muscle weakness.
Incidence and prevalence are still largely unknown, although mitochondrial disorders are the most common cause of metabolic myopathies with a prevalence of 1:8,000. The most common glycogenoses include Pompe disease at 1:40,000 and McArdle disease at 1:100,000. Carnitine palmitoyl transferase 2 (CPT 2) deficiency represents the most common lipid storage disease at 1:300,000.
Differential diagnoses include other genetic and acquired disorders; therefore, a broad diagnostic approach is required. Gene panel diagnostics can contribute to accurate classification.
Lilleker et al. 2018 Pract Neurol 18:14 / Angelini 2015, Biochim Biophys Acta 1852:615 / Olpin 2015, J Clin Pathol 68:410