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Key Features

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  • Glycogen is a highly branched polymer of glucose that is stored in liver and muscle

  • Different enzyme defects affect its biosynthesis and degradation

  • Types 0, I, III, VI, and IX manifest with hypoglycemia in infants

  • Types II, V, and VII manifest with rhabdomyolysis or muscle weakness

  • Types IV and IX manifest with hepatic cirrhosis

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Clinical Findings

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  • Hepatic forms of the glycogenoses cause

    • Growth failure

    • Hepatomegaly

    • Severe fasting hypoglycemia

  • Glycogen synthase deficiency (GSD; glycogen storage disease type 0)

    • Causes hypoglycemia, usually after about 12 hours of fasting

    • Can cause mild postprandial hyperglycemia and hyperlactatemia

  • Glycogenosis type IV, brancher enzyme deficiency, usually presents with progressive liver cirrhosis, as do some rare forms of phosphorylase kinase deficiency

  • The gluconeogenesis disorder fructose-1,6-bisphosphatase deficiency presents with major lactic acidosis and delayed hypoglycemia on fasting

  • Skeletal myopathy with weakness or rhabdomyolysis may be seen in

    • Muscle phosphorylase deficiency (type V)

    • Phosphofructokinase deficiency (type VII)

    • Acid maltase deficiency (type II; Pompe disease)

  • Infantile form of Pompe disease also has hypertrophic cardiomyopathy and macroglossia

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Diagnosis

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  • Initial tests include glucose, lactate, triglycerides, cholesterol, uric acid, transaminases, and creatine kinase

  • Functional testing includes responsiveness of blood glucose and lactate to fasting

  • An ischemic or nonischemic exercise test is helpful for myopathic forms

  • Most glycogenoses can be diagnosed by molecular analysis

  • Other diagnostic studies include enzyme assays of leukocytes, fibroblasts, liver, or muscle

  • Disorders diagnosable from analysis of red blood cells include deficiency phosphorylase kinase (type IX) in half the cases

  • Pompe disease can usually be diagnosed by assaying acid maltase in a blood spot with confirmation in fibroblasts

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Treatment

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  • Treatment is designed to prevent hypoglycemia and avoid secondary metabolite accumulations such as elevated lactate in glycogenosis type I

  • In GSD1, the special diet must be strictly monitored with restriction of free sugars and measured amounts of uncooked cornstarch, which slowly releases glucose in the intestinal lumen

  • Immunomodulation is used for patients whose treatment response declines due to antibodies to the recombinant enzyme

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