A glycogen storage disease (GSD, also glycogenosis and dextrinosis) is a metabolic disorder caused by enzyme deficiencies affecting either glycogen synthesis, glycogen breakdown or glycolysis (glucose breakdown), typically in muscles and/or liver cells.GSD has two classes of cause: genetic and acquired. Genetic GSD is caused by any inborn error of metabolism (genetically defective enzymes) involved in these processes. [Source: Wikipedia ]