What tests are needed to diagnose glycogen storage disease?

Diagnostic Testing for Glycogen Storage Disease

Initial Laboratory Panel

When hepatomegaly with or without hypoglycemia is present, immediately order a comprehensive metabolic panel including blood glucose (ideally drawn during hypoglycemia), blood lactate, uric acid, hepatic profile with AST/ALT, plasma CK, serum lipid profile, plasma total and free carnitine, plasma acylcarnitine profile, plasma amino acids, urinalysis, and urine organic acids. 1, 2

Critical First-Line Tests

• Blood glucose should be measured at the time of hypoglycemia when possible, as the timing and severity help differentiate GSD types—severe hypoglycemia 3-4 hours after feeding suggests GSD I, while less severe hypoglycemia suggests GSD III 2
• Lactate and uric acid are essential discriminators: elevated lactate plus elevated uric acid with normal carnitine strongly suggests GSD I rather than fatty acid oxidation disorders 2
• Plasma CK is crucial because muscle involvement (weakness, exercise intolerance, elevated CK) suggests GSD III or other muscle GSDs 1, 2
• Beta-hydroxybutyrate and acetoacetate distinguish hypoketotic hypoglycemia of GSD I from the hyperketotic hypoglycemia of GSD III, VI, or IX 1, 2

Secondary Evaluation Tests

• Insulin, growth hormone, cortisol, and free fatty acids should be measured when the diagnosis remains unclear after initial testing 1
• Acylcarnitine profile is mandatory (not just total/free carnitine) to exclude fatty acid oxidation disorders, which present similarly but show markedly abnormal acylcarnitine patterns 2

Genetic Testing

Molecular genetic testing with full gene sequencing of G6PC, SLC37A4, and AGL genes should be performed immediately for suspected hepatic GSDs, as this has replaced the need for invasive tissue biopsy in most cases. 1, 2, 3

• Gene panels are now readily available and provide definitive diagnosis without the risks of biopsy 1, 4
• Identification of pathogenic mutations on both alleles confirms the diagnosis 1

Tissue Biopsy (When Genetic Testing Is Inconclusive)

Liver and/or muscle biopsy should only be pursued when genetic testing fails to provide a diagnosis, and requires meticulous tissue processing with snap-freezing in liquid nitrogen for enzymatic analysis. 1

Biopsy Processing Requirements

• 30-40 mg of tissue or four cores are required for comprehensive analysis 1
• Tissue must be processed for light microscopy, electron microscopy, and snap-frozen (~15 mg) in the operating room in liquid nitrogen for biochemical analysis 1
• In the United States, reliable enzymatic analysis is only available on frozen liver and muscle biopsy samples 1

Biochemical Analysis on Tissue

• GSD III shows extremely elevated glycogen content (3-5 times normal) with structurally abnormal glycogen (shorter outer branches, indicated by decreased glucose-1-phosphate to glucose ratio) 1
• This structural abnormality is the key distinguishing feature from GSD II, V, VI, and IX where glycogen content may be elevated but structure is normal 1
• GSD I shows glycogen content at the upper limit of normal with normal structure, much lower than GSD III 1

Histopathology Findings

• GSD III liver biopsy demonstrates vacuolar accumulation of non-membrane-bound glycogen in the cytoplasm, less frequent lipid vacuoles than GSD I, and presence of fibrosis ranging from minimal periportal fibrosis to micronodular cirrhosis 1, 2
• GSD I liver biopsy shows distention of liver cells by glycogen and fat with uniform glycogen distribution, mosaic pattern with pale staining, nuclear hyperglycogenation, and large frequent lipid vacuoles without fibrosis 1

Specialized Testing for Muscle Involvement

When muscle weakness, exercise intolerance, or cardiomyopathy is present, perform electromyography, nerve conduction studies, and consider forearm exercise testing. 1, 2

• EMG and nerve conduction studies show evidence of myopathy (small, short duration motor units) and a mixed pattern of myopathy and neuropathy in GSD III 1
• Forearm exercise testing demonstrates a blunted increase in lactate in GSD III, though this is rarely performed clinically 1
• Uric acid levels may be elevated in the setting of muscle exertion 1

Critical Diagnostic Pitfalls to Avoid

• Never rely on plasma carnitine alone to diagnose or exclude GSD—always order the complete acylcarnitine profile as the pattern of individual acylcarnitines provides the most diagnostic information 2
• Do not assume all hepatomegaly with hypoglycemia is GSD without checking beta-hydroxybutyrate, as this can miss fatty acid oxidation disorders requiring completely different management 2, 5
• Biopsies are not necessary when GSD is suspected because gene sequencing is now available for individual disorders and panels of relevant genes 1
• Failure to measure lactate and uric acid together misses the diagnostic pattern of GSD I (both elevated) versus GSD III (normal lactate) 2

Long-Term Surveillance Testing

Once GSD is diagnosed, establish regular monitoring with ultrasound every 6-12 months for hepatic adenomas, alpha-fetoprotein for hepatocellular carcinoma screening, and echocardiograms with CK monitoring for GSD IIIa to detect cardiomyopathy and progressive myopathy. 2, 3