Diagnosis of Glycogen Storage Disease

Initial Clinical and Laboratory Evaluation

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, 2
For GSD Type I, key laboratory findings include hypoglycemia with lactic acidosis, hyperuricemia, hypercholesterolemia and hypertriglyceridemia, elevated AST/ALT, and neutropenia suggests GSD Ib specifically 3, 4, 1
For GSD Types III, VI, and IX, hypoglycemia typically occurs only with fasting, with normal blood lactate and uric acid, hyperketosis present, higher transaminases, and elevated creatine kinase in GSD IIIa due to muscle involvement 3, 4

The American College of Medical Genetics recommends molecular genetic testing as the first-line diagnostic approach, with full gene sequencing detecting up to 100% of mutations in homogeneous populations, and ~94% in mixed populations 2
Targeted mutation analysis is useful when ethnicity is known, and can identify up to 100% in specific ethnic groups 5, 2
Common mutations by ethnicity include GSD Ib Japanese: c.352T>C, and GSD Ib Caucasian: c.1015G>T, c.1042_1043delCT 1
Enzymatic analysis requires snap-frozen tissue, and normal G6Pase activity is 3.50 ± 0.8 μmol/min/g tissue, with GSD Ia having <10% of normal activity 5, 2

The diagnostic algorithm involves suspecting GSD based on clinical presentation, obtaining initial labs to characterize type, and ordering molecular genetic testing, with liver biopsy only if genetic testing is inconclusive or tissue is available for enzymatic confirmation 1, 5, 2, 3
Initial labs should include blood glucose, lactate, uric acid, lipid panel, AST/ALT, creatine kinase, and complete blood count to check for neutropenia 3, 4

Detection limitations of standard sequencing include missing large deletions/duplications, and promoter region mutations not being detected, requiring additional analysis such as deletion/duplication analysis or targeted array 1, 2, 3

Initial laboratory findings consistent with GSD I include hypoglycemia, lactic acidosis, hyperuricemia, hypercholesterolemia, and hypertriglyceridemia 6
The lactic acidosis in GSD I results from impaired glucose-6-phosphatase activity, preventing proper glycogen breakdown and leading to accumulation of lactate 7
A glucagon stimulation test in GSD I patients can worsen metabolic acidosis and is not recommended for diagnosis due to risk of acute decompensation 6
When glucagon stimulation is performed in GSD I, there is a significant increase in blood lactate but little or no increase in blood glucose concentration 8

GSD type I (von Gierke's disease) consistently presents with lactic acidosis 7
GSD type III (debrancher enzyme deficiency) shows absence of hyperlactatemia and has normal blood lactate and uric acid levels 6, 8
GSD types VI (hepatic phosphorylase deficiency) and IX (phosphorylase kinase deficiency) typically have normal blood lactate levels, though postprandial elevations can occur 6
GSD type 0 (glycogen synthase deficiency) presents with postprandial hyperlactatemia rather than fasting lactic acidosis 6

When evaluating a patient with suspected GSD, the presence of lactic acidosis strongly suggests GSD type I rather than other types 6, 7
Molecular genetic testing can confirm the diagnosis of specific GSD types without the need for invasive liver biopsy 7
Disorders of gluconeogenesis can also present with hyperlacticacidemia and should be considered in the differential diagnosis 6

A normal beta-hydroxybutyrate level does not rule out glycogen storage disease, and may suggest GSD Type I, which characteristically shows modest ketone elevation despite hypoglycemia, whereas other GSD types typically present with marked hyperketonemia 9, 10, 11

The American College of Medical Genetics and Genomics recommends considering GSD Type I as the primary diagnosis when beta-hydroxybutyrate is normal or only modestly elevated, and checking for elevated lactate and uric acid, which are characteristic of GSD I 9, 10, 11
The American College of Medical Genetics and Genomics suggests that hypoglycemia with modest ketosis and elevated lactate is pathognomonic for GSD I, while hypoglycemia with marked ketosis and normal lactate suggests GSD III, VI, or IX 9, 11

If BHB is markedly elevated, consider GSD Types 0, III, VI, or IX, and check lactate and uric acid, which should be normal in these types 9, 10, 11
The combination of hypoglycemia with modest ketosis and elevated lactate is a distinguishing feature that helps differentiate GSD I from other types 9, 11

The American College of Medical Genetics and Genomics emphasizes that the absence of significant ketosis during hypoglycemia is a red flag for GSD Type I, and should prompt immediate evaluation for lactic acidosis, hyperuricemia, and hepatomegaly 9, 10, 11