Laboratory Studies
Initial laboratory workup should include serial measurements of blood glucose levels, correlated with the time of the last feeding. Because the entire pathway of gluconeogenesis is intact in patients with glycogen-storage disease (GSD) type III (GSD III), they can usually maintain their blood glucose concentrations at acceptable values for several hours after a meal. Even a moderately prolonged fasting study is inadvisable because blood glucose values may fall precipitously and without warning.
A complete panel of liver function studies, including prothrombin levels, is essential. Transaminases levels, which are routinely higher in infants and children, usually fall during pubescence and often return to reference range values. Alterations in prothrombin levels occur only in patients with significant fibrosis and/or cirrhosis.
Obtain a lipid profile. Modest elevations in very low-density lipoprotein cholesterol and triglyceride levels sometimes occur. [22]
Evaluate blood and urine for ketones, especially after a brief fast. Fasting ketosis is prominent. [27]
Obtain blood lactate levels after a brief fast. These levels are occasionally elevated, although rarely to more than a moderate extent.
Blood uric acid levels should also be obtained after a brief fast. These levels, too, are occasionally elevated, although rarely to more than a moderate extent.
Always obtain serum creatine kinase levels, even in infants and children, but remember that patients with GSD IIIb have no muscle involvement so their creatine kinase levels are within reference ranges. Because significant muscle involvement does not usually begin until the second or third decade of life, even in patients with GSD IIIa, reference range creatine kinase levels do not exclude debrancher activity deficiency in muscles. However, most patients with GSD IIIa have significantly elevated creatine kinase levels. No correlation is noted between the levels of serum creatine kinase and the extent of the myopathy.
Differentiation of GSD III from other types of GSD, particularly GSD I, can be difficult without direct enzyme activity or genetic analysis owing to similar metabolic disturbances. However, GSD III is typically characterized by significantly higher levels of serum transaminases and lower triglyceride, uric acid, and lactic acid levels. [28, 21]
To confirm GSD III, laboratory test results must demonstrate abnormal glycogen (ie, short outer branches) and a debrancher enzyme activity deficiency in liver and muscle tissues. Normal debrancher activity in muscle precludes a diagnosis of GSD IIIa or IIId.
An alternative method measures debrancher activity—and even the absolute quantity of enzyme protein—in skin fibroblasts or lymphocytes. This method, however, has not been as reliable as measuring debrancher activity in liver and muscle tissues.
Molecular analysis of the AGL gene using DNA isolated from peripheral blood is now clinically available in numerous laboratories and is diagnostic when mutations are detected. Direct genotype-phenotype correlations are still typically lacking for GSD IIIa, which shows significant genetic heterogeneity, while two common mutations account for a significant percentage of GSD IIIb cases. [29, 30, 31] Although the frequency of pathologic AGL gene deletions and duplications in GSD III is unknown, it is recommended that deletion and duplication analysis be performed when sequencing is unrevealing.
Imaging Studies
Abdominal ultrasonographic examinations can provide reliable estimates of the liver's size, an important assessment because patients' livers usually become smaller with aging. Abdominal ultrasonography also helps monitor the liver to detect adenomas and hepatocellular carcinomas. [32]
Perform a pelvic ultrasonographic examination of female patients to detect polycystic ovaries. These are common in all forms of GSD III, yet, remarkably, they do not seem to interfere with patients' fertility.
Perform abdominal CT scanning of patients who develop cirrhosis because scans may provide early detection of hepatocellular carcinoma.
Baseline and serial electrocardiography and echocardiography should be considered for GSD IIIa owing to increased incidence of progressive cardiac hypertrophy with risk for arrhythmia and heart failure associated with hypertrophic cardiomyopathy. [33, 34]
Other Tests
Electromyography is essential for early detection of myopathic changes. The technique also permits monitoring the rate of progression of myopathy. Nerve conduction studies also help evaluate patients for possible myopathic changes.
Glucagon administration 2 hours after a meal rich in carbohydrates usually induces a normal rise of blood glucose levels. Administering the same glucagon dose after a 6-hour to 8-hour fast rarely affects blood glucose levels. Administration of glucagon to patients with GSD III is entirely safe because the hormone does not induce the occasionally dangerous rises in blood lactate that may occur when patients with GSD I receive the drug.
Oral administration of galactose or fructose (1.75 g/kg) usually induces a normal rise in blood glucose levels. No elevation in blood lactate levels occurs as a result of these carbohydrate challenges in patients with GSD III, while levels almost invariably rise in patients with GSD I.
Although GSD I and GSD III may be almost indistinguishable during infancy and childhood, challenge tests involving glucagon, galactose, or fructose administration are not recommended to differentiate between these conditions because these tests may cause sudden, marked, and potentially dangerous lactic acid elevations.
Histologic Findings
Accumulated glycogen in the livers of patients with GSD III causes extensive distention of hepatocytes. Fat rarely accumulates in their livers, a finding that distinguishes the histologic appearance of the liver in GSD III from the appearance in GSD I.
In addition, fibrous septa usually form in the livers of patients with GSD III but not in the livers of patients with GSD I. The extent of fibrosis ranges from minimal periportal fibrosis, to bridging fibrosis, to micronodular cirrhosis. [35] This fibrosis is not progressive in most patients, although it occasionally progresses to severe cirrhosis, a condition apparently most common in Japanese patients.
Hepatic adenomas are frequent, with a possible prevalence as high as 25% in French patients. While malignant transformation of the adenomas is unreported, 2 patients with end-stage cirrhosis developed hepatocellular carcinomas.
No extensive descriptions of histopathologic findings in skeletal and cardiac muscle are available, probably because myopathy or cardiomyopathy diagnoses are usually based on findings from electromyography, nerve conduction studies, electrocardiography, and echocardiography rather than histologic studies. The histopathologic findings consist of vacuoles within the myocytes. Vacuolization extent varies and does not correlate with myopathy extent. The vacuoles are periodic acid-Schiff positive, consistent with the limit dextrin produced by the action of phosphorylase on glycogen in the absence of debrancher.
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Schematic illustration of the degradation of glycogen by the concerted action of the enzymes phosphorylase and debranching enzyme. First, phosphorylase removes glucose moieties (linked to their neighbors via alpha1,4 glucosidic bonds and depicted as the 7 black circles) from the unbranched outer portions of the glycogen molecule until only 4 glucosyl units (depicted as the 3 green circles and the 1 red circle) remain before an alpha1,6 branch point. The transferase component of debranching enzyme then transfers the 3 (green) glucose residues from the short branch to the end of an adjacent branch of the glycogen molecule. The glucosidase component of debranching enzyme then removes the glucose moiety (depicted as the red circle) remaining at the alpha1,6 branch point. In the process, the branch point formed by the alpha1,6 glucosidic bond is removed, hence the name debrancher.Unlike phosphorylase, which removes glucose moieties from glycogen in the form of glucose-1-phosphate, debrancher releases 1 free glucose moiety from each branch point. After the cleavage of the branch site, phosphorylase attacks unbranched portions of the glycogen molecule until the enzyme is stymied by the appearance of another branch point, at which point debranching enzyme once again is called into play. Eventually, large portions of the glycogen molecule are degraded to free glucose by the action of the amylo-alpha1,6-glucosidase activity of debranching enzyme and to glucose-1-phosphate by the action of phosphorylase.
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Schematic representation of a portion of a molecule of glycogen. Open circles represent the glucose moieties connected to each other via alpha1,4 linkages. Solid circles represent the glucose moieties connected to their neighbors via alpha1,6 linkages. Thus, each solid circle represents a branch point in the molecule.
