Type Ib Glycogen Storage Disease Workup

Updated: Apr 11, 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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  • Obtain a creatine kinase in all cases of suspected glycogen storage disease (GSD).

  • Obtain a lipid profile due to changes in glucose metabolism.

  • 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 GSDs.

  • Hepatic failure occurs in some GSDs. Liver function studies are indicated.

  • Biochemical assay is required for definitive diagnosis.

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Imaging Studies

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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 ischemic test.

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

    • Ischemic forearm test is normal in GSD type Ib.

  • Electromyelography

    • Aminoff reports electromyelographic findings 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 be found.

    • Myopathic findings of polyphasic responses, decreased duration of potentials, and decreased amplitude usually are present.

  • ECG demonstrates a pan-lead, short PR interval and elevated QRS complexes in the infantile form.

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Procedures

Given the association of inflammatory bowel disease, endoscopic procedures may be necessary.

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Histologic Findings

Liver histology is characterized by hepatocytes distended by glycogen and fat. Associated fibrosis is minimal.

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