Glycogen Storage Disorders — Investigations

Pathway of glycogen synthesis and glycogenolysis. Enzyme defects at each step define the different GSD types.

Overview

First-Line Biochemical Investigations (All Types)

Provocation / Dynamic Tests

Glucagon stimulation test

• Administer glucagon (0.02–0.03 mg/kg IV/IM); measure glucose at 0, 15, 30, 45, 60 min
• Normal: glucose rises ≥30 mg/dL
• GSD I: no rise in blood glucose; lactate rises markedly
• GSD III, VI, IX: blunted or absent rise after prolonged fasting, but partial response post-prandially (distinguishes from GSD I)

Galactose tolerance test (oral or IV)

• Galactose → glucose-1-phosphate → glucose-6-phosphate → glucose (via glucose-6-phosphatase)
• GSD I: failure of blood glucose to rise after galactose administration is diagnostic (glucose-6-phosphatase cannot complete the final step); lactate rises instead
• Still used to confirm GSD I before enzyme assay

Epinephrine tolerance test (historical)

• IM epinephrine: normal response = blood glucose rises 35–45 mg/dL within 40–60 min
• GSD I: no glucose rise; GSD III: no rise after fasting, partial rise after carbohydrate feeding
• Rarely used today; replaced by molecular testing

Ischaemic forearm exercise test (McArdle / GSD V & VII)

• Patient squeezes hand dynamometer 30× under tourniquet (ischaemic conditions)
• Blood sampled for lactate and ammonia before and 1, 2, 3, 5, 10 min after
• Normal: both lactate and ammonia rise (2–5× baseline lactate)
• GSD V (McArdle) / GSD VII (Tarui): ammonia rises normally, but lactate fails to rise (blocked glycolysis in muscle)
• Caution: the non-ischaemic (aerobic) version is safer and preferred now

Enzyme Assays (Tissue-Specific)

Molecular Genetic Testing

• Gene-targeted testing: single-gene sequencing or multigene panel (clinically preferred first-line where genotype is strongly suspected)
• Comprehensive testing: whole-exome sequencing (WES) or whole-genome sequencing (WGS) when panel testing is non-diagnostic
• Common mutations are population-specific (e.g. GSD Ia: p.Arg83Cys in Caucasian/Jewish; c.648G→T splicing in Japanese; GSD Ib: c.1042_1043delCT in Caucasian)
• Used for prenatal diagnosis and carrier detection

Tissue Biopsy

Imaging

Newborn Screening (GSD II — Pompe)

• Dried blood spot (DBS) acid α-glucosidase (GAA) enzyme activity assay — several countries include GSD II in neonatal screening
• Confirmed by GAA gene sequencing ± lymphocyte/fibroblast enzyme assay
• Rationale: enzyme replacement therapy (ERT) with alglucosidase alfa is most effective when started before symptom onset

Type-Specific Investigation Highlights

GSD I (von Gierke) — Key Lab Fingerprint

• Hypoglycaemia + lactic acidosis + hyperuricaemia + hypertriglyceridaemia + hepatomegaly
• No glucose response to glucagon or galactose
• Confirm: liver biopsy → G6Pase assay (intact microsomes: Ia; + disrupted microsomes required: Ib); G6PC1 / SLC37A4 sequencing

GSD II (Pompe) — Key Lab Fingerprint

• Infantile: cardiomegaly, hypotonia, elevated CK, very high GAA deficiency on DBS
• Late-onset: progressive proximal myopathy, respiratory failure, CK elevated, normal/mild CK in some
• Confirm: DBS GAA activity → GAA gene sequencing; muscle biopsy shows lysosomal glycogen

GSD III (Cori/Forbes) — Distinguishing from GSD I

• Elevated transaminases (GSD I has normal AST/ALT), elevated CK (muscle involvement in IIIa)
• Hyperglycaemic response to galactose (unlike GSD I)
• Lower uric acid and lactate than GSD I
• Confirm: debranching enzyme (AGL) assay in erythrocytes or liver/muscle; AGL gene sequencing

GSD IV (Andersen) — Key Findings

• Hepatosplenomegaly → cirrhosis → liver failure
• Abnormal glycogen (amylopectin-like) on biopsy (PAS+, poorly diastase-digestible)
• Confirm: branching enzyme (GBE1) assay in leukocytes/fibroblasts; GBE1 sequencing

GSD V (McArdle) — Key Lab Fingerprint

• Elevated resting CK; myoglobinuria after exercise
• Ischaemic forearm test: ammonia rises, lactate fails to rise
• "Second-wind" phenomenon
• Confirm: absent phosphorylase on muscle biopsy histochemistry; PYGM sequencing

GSD VII (Tarui) — Key Lab Fingerprint

• Like GSD V but also haemolytic anaemia (RBCs share the PFK defect)
• Hyperbilirubinaemia + reticulocytosis + pigmenturia
• Glucose does not relieve symptoms (unlike GSD V)
• Confirm: PFK assay in RBCs + muscle biopsy; PFKM sequencing

Summary Algorithm

Suspected GSD
     │
     ├── Hepatic features (hepatomegaly, hypoglycaemia)
     │         ├── + Lactic acidosis + ↑↑ uric acid + ↑↑ TG → GSD I → galactose test → liver biopsy/G6PC gene
     │         ├── + ↑ transaminases + ↑ CK ± cardiomyopathy → GSD III → AGL enzyme/gene
     │         ├── + Cirrhosis/liver failure → GSD IV → GBE enzyme/gene
     │         └── Mild, asymptomatic hepatomegaly → GSD VI / IX → phosphorylase/kinase assay
     │
     └── Muscle features (exercise intolerance, cramps, rhabdomyolysis)
               ├── + Cardiomyopathy + hypotonia (infant) → GSD II → DBS GAA assay
               ├── + Forearm test: ↑↑ NH₃, flat lactate + "second wind" → GSD V → PYGM gene
               └── + Haemolytic anaemia + flat lactate forearm test → GSD VII → PFK assay

• Tietz Textbook of Laboratory Medicine, 7th Edition — Table 35.1, GSD types I–VII
• Henry's Clinical Diagnosis and Management by Laboratory Methods — Tables 17.6 & 17.7
• Goldman-Cecil Medicine (International Edition) — Chapter 191 (Kishnani)
• Sleisenger & Fordtran's GI and Liver Disease — Chapter 77