Laboratory Studies
The initial laboratory workup of glycogen-storage disease type I (GSD I) should include measuring blood glucose levels with electrolytes. If the blood glucose concentration is low, measurement of electrolyte levels may permit calculation of an increased anion gap, which suggests lactic acidemia.
Measurement of liver function, plasma uric acid levels, and urinary creatinine clearance are essential.
Perform a CBC count and differential. In patients with glycogen-storage disease type Ia, the WBC count is usually within reference ranges because the defect does not affect leukocyte function. In contrast, glycogen-storage disease Ib causes chronic granulocytopenia due to the impaired function of the neutrophils, particularly in relation to gram-positive organisms.
Perform a coagulation profile to include bleeding time tests.
Imaging Studies
Ultrasonography of liver and kidneys may be useful. Abdominal ultrasonography can enable reasonable estimates of liver and kidney size. These estimates are especially useful during treatment of patients with chronic glycogen-storage disease because the relative size of the liver is expected to diminish with growth of the child. Ultrasonography may also be useful late in the clinical course to provide a baseline for evaluating nodules.
Bone-density studies may be useful in middle childhood to identify patients who may have diminished bone density in adolescence.
Other Tests
Glucagon administration produces no hyperglycemic response, although it markedly increases the plasma lactic acid level.
Oral (PO) administration of galactose (1.75 g/kg) causes no change in blood glucose levels, although plasma lactic acid levels markedly increase.
Procedures
Always directly measure glucose-6-phosphatase activity in the liver before and after detergent treatment of the homogenate, especially when activity appears to be within the reference ranges before treatment.
Open biopsy was usually needed to obtain sufficient tissue for this assay; however, complete sequencing of the gene now permits molecular diagnosis without the need for invasive procedures.
Histologic Findings
Although histology should be routinely performed on liver samples obtained during surgery, neither light nor electron microscopy permit differentiation of the underlying reasons for glycogen storage.
Clinically significant lipid storage occurs with both types of glycogen-storage disease type I, which does little to clarify the picture.
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Microsome is shown in relation to the substrate, glucose-6-phosphate, which has been released from cytosolic glycogen. This substrate is transferred across the microsomal membrane by the protein translocase, where by glucose-6-phosphatase acts on it to release free glucose and inorganic phosphate. Patients with glycogen-storage disease type Ia are genetically deficient in glucose-6-phosphate activity, while those affected with glycogen-storage disease type Ib lack translocase.
