Glycogen storage diseases

Abstract Glycogen is a highly branched glucose polymer with a compacted structure found predominantly in liver and muscle. Liver glycogen is important in the maintenance of euglycaemia during fasting; muscle glycogen is an immediate source of glucose for energy production during exercise. Genetic disorders affecting proteins that regulate glycogen metabolism and transport, as well as those which catalyse its biosynthesis and breakdown, cause marked accumulation of glycogen in diverse tissues, and pathological glycogen often has an abnormal macromolecular structure. Depending on the enzyme system involved, diseases of glycogen metabolism principally affect liver and muscle. Clinical features are related to pathological glycogen in tissues and/or failure to release glucose. Glycogen storage is associated with organomegaly and tissue injury. Fasting hypoglycaemia occurs where hepatic breakdown of glycogen is impaired. Glycogen diseases that affect muscle usually present with rhabdomyolysis, exercise intolerance, and muscle pain or weakness. Formerly, diseases of glycogen metabolism were diagnosed by showing excess storage of glycogen in the tissue of interest, accompanied by reduced activity of particular glycogen-metabolizing enzymes. Currently, where available, molecular analysis of genomic DNA is the preferred method for providing a definitive diagnosis. The mainstay of treatment of glycogen diseases affecting the liver is dietary, including pre-emptive management of hypoglycaemia that is readily provoked by fasting. Dietary interventions may also ameliorate some of the glycogen diseases that affect muscle, and weakness and pain after exertion can be improved by graduated exercise programmes in some patients.