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Updated: Dec 29 2021

Glycogen Storage Disorders

  • Snapshot
    • A 16-year-old boy presents to his primary care physician with a complaint of fatigue and muscle cramps. He recently tried out for his high school basketball team and found himself exhausted and unable to run after performing a set of high intensity sprints. He notes that after resting briefly, he is able to get a “second wind.” After practice, he noticed that his urine was a brown color. Laboratory results are significant for an elevated creatine kinase level of 1,423 U/L (normal < 200 U/L). There was no elevation of lactate levels on forearm nonischemic testing. (McArdle disease)
  • Introduction
    • Overview
      • glycogen storage disorders (GSD) result from errors of glycogen metabolism caused by mutations in genes for enzymes involved in glycogen synthesis and degradation
    • Glycogen Storage Diseases
      GSDDefective Enzyme
      Characteristics

      Von Gierke disease (type 1)

      • Hepatic glucose-6-phosphatase
      • Autosomal recessive
      • Usually presenting at 3-6 months of age
      • Hepatomegaly
      • Signs of hypoglycemia
        • seizure
      • "Doll-like" face
      • Poor growth
      • ↑ uric acid
        • due to impaired renal clearance and increased production
        • gout
      • ↑ lactic acid
      • ↑ triglycerides
        • xanthomas
        • pancreatitis

      Pompe disease (type 2)

      • Lysosomal α(1,4)-glucosidase
      • Autosomal recessive
      • Infantile form
        • cardiomegaly
        • generalized muscle hypotonia
        • respiratory distress
      • Juvenile and adult form
        • skeletal myopathy
        • delayed-gross motor development
        • limb-girdle weakness
        • respiratory issues

      Cori disease (type 3)

      • Glycogen debranching enzyme
      • Autosomal recessive
      • Most cases involve the muscle and liver
      • Hepatomegaly
      • Mild hypoglycemia
        • intact gluconeogenesis

      McArdle disease (type 5)

      • Muscle glycogen phosphorylase
      • Autosomal recessive
      • Affects theskeletal muscle
      • No elevation of blood lactate during forearm exercise test
      • ↑ creatine kinase
      • Muscle cramp, fatigue, myalgias, and myoglobinuria
      • "Second wind" phenomenom
        • rapid relief of fatigue and myalgia
        • secondary to increase blood flow, improved free fatty acid delivery, and liver glucose utilization
  • Epidemiology
    • Incidence
      • overall incidence of all forms of glycogen storage diseases in the US, Europe, and Canada is approximately 1 in 20,000-40,000
      • von Gierke disease (GSD type 1) most common type
        • ~25% of all patients with GSD
    • Demographics
      • age of onset may range from the first months of life to later decades of life
      • highest incidence of Cori disease (GSD type 3) in non-Ashkenazi Jews in northern Africa
  • ETIOLOGY
    • Pathophysiology
      • glycogen storage disorders result from abnormal glycogen metabolism and accumulation of glycogen within cells of certain tissues (i.e., liver and skeletal muscle)
    • Genetics
      • inheritance pattern
        • autosomal recessive
  • Studies
    • Von Gierke disease (type 1)
      • serum glucose and electrolyte levels
        • fasting hypoglycemia (serum glucose < 2.5 mmol/L)
      • serum uric acid level
        • hyperuricemia (uric acid > 5.0 mg/dL)
      • serum triglyceride level
        • hypertriglyceridemia (triglyceride level > 250 mg/dL)
      • serum cholesterol level
        • hypercholesterolemia (cholesterol level > 200 mg/dL)
    • Pompe disease (type 2)
      • acid α-glucosidase testing
        • initial test
        • gives definitive diagnosis
      • laboratory testing
        • ↑ creatine kinase, lactic acid, and aspartate aminotransferase
    • Cori disease (type 3)
      • serum glucose and electrolyte levels
        • fasting hypoglycemia (serum glucose < 2.5 mmol/L)
      • laboratory testing
        • ↑ creatine kinase and aspartate aminotransferase
    • McArdle disease (type 5)
      • serum creatine kinase levels
        • ↑ serum CK at rest
        • further ↑ in CK after intensive exercise
      • myophosphorylase enzyme activity assay
        • ↓↓ myophosphorylase levels
      • genetic testing
        • identification of pathogenic variants in the PYGM gene
        • gives definitive diagnosis
  • Treatment
    • Von Gierke disease (type 1)
      • dietary management
        • frequent oral glucose/cornstarch consumption
        • avoid fructose and galactose
          • prevent lactic acidosis
    • Pompe disease (type 2)
      • enzyme replacement therapy
        • alglucosidase alfa
    • Cori disease (type 3)
      • symptomatic treatment
    • McArdle disease (type 5)
      • sucrose consumption before exercise may improve symptoms
  • Prognosis
    • GSD type 1
      • good when dietary and medical treatments are implemented
    • GSD type 2
      • poor prognosis without treatment
    • GSD type 3
      • good prognosis with many patients surviving into adulthood
    • GSD type 5
      • variable prognosis
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