[Late diagnosis of a McArdle disease's case (type V glycogenosis)]. / Diagnostic tardif d'une maladie de McArdle (glycogénose de type V).

McArdle's disease is a metabolic myopathy characterized by a myophosphorylase deficiency resulting in an inability to degrade glycogen stores. We report the case of a 48 years old patient who complained since adolescence of rest and exercise myalgias and presented a chronic increased plasma creatine kinase activity. First, a maximal exercise test was performed. This test demonstrated a quasi lack of rise of respiratory exchange ratio and of blood lactate, possibly due to a glycogenolytic/glycolytic pathway deficiency. Second, a biopsy of vastus lateralis muscle was performed using Bergström needle. As expected, the analysis of mitochondrial function was normal. The in vitro screening test of the glycogenolysis/glycolysis pathway showed a lack of lactate production in presence of glycogen substrate. The study of muscular metabolism of glycogen revealed a glycogen accumulation and a decrease of active and total phosphorylase activities. These data allowed us to diagnose a type V glycogenosis, or McArdle's disease. The patient appeared heterozygous for the most frequent mutation (p.R50X).