Laboratory and Imaging Studies
Laboratory studies
The following laboratory studies are recommended:
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Obtain a creatine kinase level in all cases of suspected glycogen storage disease (GSD).
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Obtain a lipid profile, owing to changes in glucose metabolism.
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Because hypoglycemia may be found in some types of GSD, fasting glucose measurement is indicated. Hypoglycemia is concerning and may lead to hypoglycemic seizures.
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Urine studies are indicated because myoglobinuria may occur in some GSDs.
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Hepatic failure occurs in some GSDs. Liver function studies are indicated.
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Biochemical assay is required for definitive diagnosis.
Imaging studies
Imaging may reveal hepatic adenoma, which may become malignant.
Other Tests
Ischemic forearm test
The ischemic forearm test is an important tool for the diagnosis of muscle disorders. The basic premise is an analysis of the normal chemical reactions and products of muscle activity. Obtain consent before the test.
The following steps are included in the ischemic forearm test:
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Instruct the patient to rest. Position a loosened blood pressure cuff on the arm, and place a venous line for blood samples in the antecubital vein.
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Obtain blood samples for the following tests: creatine kinase, ammonia, and lactate. Repeat in 5-10 minutes.
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Obtain a urine sample for myoglobin analysis.
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Immediately inflate the blood pressure cuff above systolic blood pressure and have the patient repetitively grasp an object, such as a dynamometer. Instruct the patient to grasp the object firmly, once or twice per second. Encourage the patient for 2-3 minutes, at which time the patient may no longer be able to participate. Immediately release and remove the blood pressure cuff.
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Obtain blood samples for creatine kinase, ammonia, and lactate immediately and at 5, 10, and 20 minutes.
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Collect a final urine sample for myoglobin analysis.
Interpretation of ischemic forearm test results
With exercise, carbohydrate metabolic pathways yield lactate from pyruvate. Lack of lactate production during exercise is evidence of pathway disturbance, and an enzyme deficiency is suggested. In such cases, muscle biopsy with biochemical assay is indicated.
Healthy patients demonstrate an increase in lactate of at least 5-10 mg/dL and ammonia of at least 100 µg/dL. Levels will return to baseline. If neither level increases, the exercise was not strenuous enough and the test result is not valid.
Increased lactate at rest (before exercise) is evidence of mitochondrial myopathy. Failure of lactate to increase with ammonia is evidence of a GSD resulting in a block in carbohydrate metabolic pathways. Not all GSDs have a positive ischemic test. Failure of ammonia to increase with lactate is evidence of myoadenylate deaminase deficiency. The results of the ischemic forearm test are normal in patients with GSD type Ib.
Electromyelography
Electromyelographic findings may be suggestive of myopathy, although abnormal spontaneous activity may be present. Electrical myotonia without clinical myotonia may be present.
Myotonic discharges may be found in the paraspinal muscles. Fibrillation potentials, positive sharp waves, and complex repetitive discharges may also be found. Myopathic findings of polyphasic responses, decreased duration of potentials, and decreased amplitude usually are present.
Electrocardiography
The electrocardiogram demonstrates a pan-lead, short PR interval and elevated QRS complexes in the infantile form.
Endoscopy
Given the association of inflammatory bowel disease, endoscopic procedures may be necessary.
Liver biopsy
Liver histology is characterized by hepatocytes distended by glycogen and fat. Associated fibrosis is minimal.
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Metabolic pathways of carbohydrates.
