How is glycogen storage disease (GSD) diagnosed?

Diagnosis of Glycogen Storage Disease

Molecular genetic testing through full gene sequencing is now the first-line diagnostic approach for glycogen storage disease, eliminating the need for invasive liver biopsy in most cases. 1

Initial Clinical and Laboratory Evaluation

Key Laboratory Findings by GSD Type

For GSD Type I (most common hepatic form):

• Hypoglycemia with lactic acidosis (distinguishing feature) 1, 2
• Hyperuricemia 1
• Hypercholesterolemia and hypertriglyceridemia 1
• Elevated AST/ALT (normalizes with treatment) 1
• Neutropenia suggests GSD Ib specifically, though can occur in Ia 1

Critical pitfall: Avoid glucagon stimulation testing in suspected GSD I—it causes significant lactate elevation without glucose rise and risks acute metabolic decompensation. 1, 2

For GSD Types III, VI, and IX:

• Hypoglycemia typically less severe, occurs only with fasting 1
• Normal blood lactate and uric acid (key differentiator from Type I) 1, 2
• Hyperketosis present 1
• Higher transaminases (may exceed 500 U/L in Type III) 1
• Elevated creatine kinase in GSD IIIa due to muscle involvement 1

Definitive Diagnostic Testing

Molecular Genetic Testing (First-Line)

Testing strategy: 1

• Sequence G6PC gene first for suspected GSD Ia (responsible for ~80% of GSD I cases) 1
• Sequence SLC37A4 gene if neutropenia present (GSD Ib, ~20% of GSD I cases) 1
• Full gene sequencing detects up to 100% of mutations in homogeneous populations, ~94% in mixed populations 1
• Targeted mutation analysis useful when ethnicity known (can identify up to 100% in specific ethnic groups) 1

Common mutations by ethnicity: 1

• GSD Ib Japanese: c.352T>C (37-50% of cases) 1
• GSD Ib Caucasian: c.1015G>T (19-21%), c.1042_1043delCT (27-31%) 1

Enzymatic Analysis (Alternative/Confirmatory)

When liver biopsy performed: 1

• Requires snap-frozen tissue (~15 mg) in liquid nitrogen immediately in operating room 1
• Normal G6Pase activity: 3.50 ± 0.8 μmol/min/g tissue 1
• GSD Ia: <10% of normal activity (most cases <1.0 μmol/min/g) 1
• G6P translocase (GSD Ib) requires fresh unfrozen tissue—most labs don't offer this test 1

Histopathology findings in GSD I: 1

• Hepatocytes distended with glycogen and fat, uniformly distributed 1
• Total glycogen content much lower than GSD III, IV, VI, IX 1
• No fibrosis typically present 1

Critical point: Biopsies are not necessary when GSD suspected because gene sequencing is available and non-invasive. 1

Diagnostic Algorithm

1. Suspect GSD based on clinical presentation: Hepatomegaly with fasting hypoglycemia, elevated transaminases, hyperlipidemia 3, 4

2. Obtain initial labs to characterize type: 1

   • Blood glucose, lactate, uric acid, lipid panel
   • AST/ALT, creatine kinase
   • Complete blood count (check for neutropenia)

3. Presence of lactic acidosis strongly suggests GSD Type I (vs. Types III, VI, IX which have normal lactate) 2

4. Order molecular genetic testing: 1

   • G6PC sequencing first (unless neutropenia present)
   • SLC37A4 if neutropenia or G6PC negative
   • Consider targeted mutation analysis if specific ethnicity

5. Liver biopsy only if: 1

   • Genetic testing inconclusive
   • Tissue available for enzymatic confirmation
   • Must ensure proper snap-freezing protocol

Additional Diagnostic Considerations

Whole exome sequencing increasingly used when clinical presentation unclear or multiple GSD types possible, particularly effective for accurate sub-typing. 5

Detection limitations: 1

• Standard sequencing misses large deletions/duplications
• Promoter region mutations not detected
• May require deletion/duplication analysis (quantitative PCR, MLPA, targeted array) 1