Type III Glycogen Storage Disease Workup

Updated: May 25, 2017
  • Author: Wayne E Anderson, DO, FAHS, FAAN; Chief Editor: George T Griffing, MD  more...
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Workup

Laboratory Studies

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  • Because hypoglycemia may be found in some types of GSD, fasting glucose is indicated. Hypoglycemia is concerning and may lead to hypoglycemic seizures.

  • Urine studies are indicated because myoglobinuria may occur in some cases of GSD.

  • Hepatic failure occurs in some cases of GSD. Liver function studies are indicated.

  • The presence of dextrin is unique to Cori disease.

  • With a biochemical assay, debrancher enzyme activity is reduced or absent.

  • Hyperlipidemia is a common finding.

  • Fasting ketonemia is noted with the rapid metabolism of fatty acids.

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Imaging Studies

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  • Imaging may reveal hepatomegaly.

  • Cardiomegaly may be present, but heart failure is not typical of GSD II.

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Other Tests

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  • Ischemic forearm test

    • The ischemic forearm test is an important tool for 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.

    • 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.

    • Obtain blood samples for the following tests: creatine kinase, ammonia, and lactate. Repeat in 5-10 minutes.

    • Obtain a urine sample for myoglobin analysis.

    • 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.

    • Obtain blood samples for creatine kinase, ammonia, and lactate immediately and at 5, 10, and 20 minutes.

    • 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 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 result on ischemic test.

    • Failure of ammonia to increase with lactate is evidence of myoadenylate deaminase deficiency.

    • In Cori disease, the ischemic forearm test result is positive.

  • Electromyography

    • Electromyography patterns are diverse and vary from patient to patient.

    • The myopathic finding of polyphasic responses is found, but amplitude and duration may be either decreased, as expected, or increased in some cases.

    • Spontaneous abnormal activity (fibrillation potential and positive sharp waves) may be found.

    • Myotonic discharges are observed in some cases.

    • A study by Mogahed et al assessed the prevalence of neuromuscular and cardiac involvement in 28 children with GSD III and reported that 61% of the patients had myopathic changes detected by electromyography and that the children with myopathic changes were significantly older and all but one had elevated creatine phosphokinase. [11]

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Histologic Findings

Muscle biopsy is periodic acid-Schiff positive with basophilic deposits in all tissues, including the CNS.

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