What is the diagnostic workup for glycogen storage disease?

Diagnostic Workup for Glycogen Storage Disease

When hepatomegaly and hypoglycemia coexist, initiate a comprehensive metabolic workup with critical blood samples drawn at the time of hypoglycemia, followed by definitive diagnosis through either genetic testing or tissue biopsy with enzymatic analysis. 1

Initial Clinical Assessment

Key Clinical Features to Identify

• Hepatomegaly with or without hypoglycemia is the hallmark presentation requiring immediate workup 1
• Timing and severity of hypoglycemia helps differentiate GSD types: severe hypoglycemia 3-4 hours after feeding suggests GSD I, while less severe hypoglycemia suggests GSD III 1
• Muscle involvement: proximal and distal weakness, elevated CK, exercise intolerance, or cardiomyopathy suggests GSD III or other muscle GSDs 1
• Morning symptoms (tachycardia, dizziness) represent counter-regulatory responses to nocturnal hypoglycemia and are pathognomonic for glucose homeostasis disorders 2

Critical Distinguishing Features

• Massive splenomegaly points away from GSD toward Gaucher or Niemann-Pick disease 1
• Absence of hepatomegaly suggests GSD 0 (glycogen synthase deficiency) 1
• Diaphragmatic weakness with respiratory distress is highly suggestive of Pompe disease (GSD II), not other GSDs 1

Primary Laboratory Evaluation

Draw critical blood samples at the time of hypoglycemia whenever possible 1

Essential First-Line Tests

• Blood glucose (ideally during hypoglycemia) 1
• Blood lactate: elevated in GSD I, normal in GSD III 1
• Uric acid: elevated in GSD I, normal in GSD III 1
• Hepatic profile with liver function studies (AST, ALT): transaminases often 2× upper limits of normal and frequently >500 IU/L in GSD III 1, 2
• Serum lipid profile (cholesterol, triglycerides): hyperlipidemia common in GSD I and III 1
• Plasma CK: elevated in GSD III with muscle involvement 1
• Plasma total and free carnitine 1
• Plasma acylcarnitine profile 1
• Plasma amino acids 1
• Urinalysis 1
• Urine organic acids 1

Secondary Tests When Diagnosis Unclear

• Insulin, growth hormone, cortisol: to exclude endocrine causes of hypoglycemia 1
• Free fatty acids 1
• Beta-hydroxybutyrate and acetoacetate: elevated (hyperketotic) in GSD III during hypoglycemia, distinguishing it from hypoketotic fatty acid oxidation disorders and hyperinsulinism 1, 2
• Review newborn screening results: to exclude fatty acid oxidation disorders and galactosemia 1

Functional Testing

Glucagon Stimulation Test

Administer glucagon 2 hours after a carbohydrate-rich meal versus after overnight fast 1

• Normal response post-meal: blood glucose increases normally in GSD III 1
• Blunted response after fasting: no change in blood glucose in GSD III, distinguishing it from normal and helping differentiate from GSD I 1

Additional Specialized Testing

• Electromyography (EMG) and nerve conduction studies: show myopathy (small, short duration motor units) and mixed myopathy/neuropathy pattern in GSD III 1
• Forearm exercise testing: demonstrates blunted lactate increase in GSD III (rarely performed clinically) 1

Definitive Diagnostic Testing

Genetic Testing (Preferred First-Line Approach)

Perform genetic sequencing immediately when GSD is suspected, as biopsies are not necessary if genetic testing is available 1, 2

• Gene panels including G6PC, SLC37A4, and AGL genes should be ordered for suspected hepatic GSDs 2
• Identification of pathogenic mutations on both alleles confirms diagnosis 1
• Genetic testing has largely replaced the need for invasive tissue biopsies in modern practice 1

Tissue Biopsy (When Genetic Testing Inconclusive or Unavailable)

Liver and/or muscle biopsy with proper tissue processing is required for enzymatic analysis and histology 1

Critical Biopsy Requirements

• Tissue processing: specimens must be processed for light microscopy, electron microscopy, AND snap-frozen (15 mg) in liquid nitrogen in the operating room for biochemical analysis 1
• Adequate sample size: 30-40 mg of tissue or four cores of liver tissue required for complete diagnostic studies 1
• Enzymatic analysis: reliable enzymatic analysis in the United States is only available on frozen muscle and liver biopsy samples 1

Histopathologic Findings

GSD III liver biopsy shows 1:

• Vacuolar accumulation of non-membrane-bound glycogen in cytoplasm
• Markedly increased glycogen content (3-5× normal levels)
• Structurally abnormal glycogen with shorter outer branches (decreased G-1-P to glucose ratio)—this is a key distinguishing feature
• Fibrosis ranging from minimal periportal to micronodular cirrhosis (present in GSD III, not in GSD I)
• Less frequent lipid vacuoles compared to GSD I
• Periodic acid-Schiff positive, diastase-sensitive material

Diagnostic Algorithm by Clinical Presentation

Hepatomegaly + Hypoglycemia + Elevated Transaminases

1. Draw critical blood samples during hypoglycemia including glucose, lactate, uric acid, ketones, liver enzymes 1, 2
2. Check lactate and uric acid: normal suggests GSD III; elevated suggests GSD I 1, 2
3. Assess ketone status: hyperketotic suggests GSD III; hypoketotic suggests fatty acid oxidation disorder or hyperinsulinism 1, 2
4. Order genetic testing for AGL (GSD III) or G6PC/SLC37A4 (GSD I) 2

Muscle Weakness + Elevated CK ± Hepatomegaly

1. Measure CK, AST, ALT, and assess for hypoglycemia 1
2. Perform EMG/nerve conduction studies: myopathy pattern suggests GSD III 1
3. Check for cardiac involvement: echocardiogram to assess for cardiomyopathy in GSD IIIa 2
4. Order genetic testing for AGL gene 2

Common Pitfalls to Avoid

• Do not assume all hepatomegaly with hypoglycemia is GSD I: GSD III can present similarly but has normal lactate and uric acid 1
• Do not perform liver biopsy without proper tissue processing: snap-freezing in liquid nitrogen is essential for enzymatic analysis 1
• Do not overlook cardiac screening in GSD III: regular echocardiograms are necessary to detect cardiomyopathy in GSD IIIa 2
• Do not delay dietary intervention: immediate treatment is essential to prevent hypoglycemia, lactic acidosis, seizures, and permanent brain damage 2

Long-Term Surveillance Requirements

• Ultrasound surveillance every 6-12 months to monitor for hepatic adenomas (occur in 16-75% of GSD patients) 2
• Alpha-fetoprotein and ultrasound screening for hepatocellular carcinoma risk, though AFP may remain normal in GSD III 2
• Regular echocardiograms and CK monitoring for GSD IIIa to detect cardiomyopathy and progressive myopathy 2