Glycogen storage disease type 5
Glycogen storage type V (McArdle's disease), is a severe form of glycogen storage disease characterized by exercise intolerance.
Prevalence is unknown. This autosomal recessive disease is caused by mutations in the gene PYGM (11q13), resulting in muscle phosphorylase deficiency.
The onset occurs during childhood. Patients have a syndrome of muscular exercise intolerance with myalgia, cramps, fatigue and muscle weakness. In half of the patients has detected a massive increase in creatine kinase and rhabdomyolysis with myoglobinuria after exercise, potentially it involves an acute renal failure. In many patients a phenomenon of recovery it is observed with relief of myalgia and fatigue after a few minutes of rest. The clinical picture is usually very classical, but some patients may show very mild forms. In a few cases it described the appearance at a very early age with hypotonia, generalized muscle weakness and progressive respiratory failure.
The diagnosis is based on biological findings that reveal a lack of elevated blood lactate levels during testing on the forearm ischemia, excessive deficit glycogen phosphorylase activity in muscle biopsy. The molecular diagnosis is based on the finding of pathogenic mutations in the PYGM gene.
The treatment is based on controlled physical training in order to develop mitochondrial oxidation capacities in muscles, and programmed glucose intake according to exercising periods. Diets high in protein have yielded variable results.
The prognosis is favorable when the massive rhabdomyolysis is avoided. However, myoglobinuria can cause potentially fatal renal failure.