LES MYOPATHIES METABOLIQUES

Metabolic myopathies are genetic disorders of muscle metabolism. The disorders of glycogen metabolism are probably well known because they were the first to be recognized. In recent years, new entities have been recognized: glycogen type II can manifest as infant, childhood or an adult form, glycogenosis type III has different chemical and clinical subtypes, glycogenesis type V may express itself as an infant or an adult form. In addition, defects in glycolytic enzymes (PGK, PGAM, and LDH M-type) can cause recurrent myoglobinuria. Disorders of lipid metabolism in muscle sometimes have a dramatic onset: systemic carnitine deficiency, can present with acute muscle weakness and a picture of hepatic encephalopathy; muscle carnitine deficiency may lead to prominent muscle dysfunction; carnitine-palmityl transferase deficiency is usually manifested by recurrent myoglobinuria. Other lipid disorders are associated with either beta oxydation defects or mitochondrial enzymes dysfunction. In this field numerous examples of genetic heterogeneity have been described manifesting as clinical or biochemical variants. Although the biochemical error is known, the precise pathogenetic relationship between the genetic error and the clinical manifestation remains to be elucidated in many of these disorders.