Glycogen Storage Disease Type V - Laboratory Tests

Laboratory Tests

There are some laboratory tests that may aid in diagnosis of GSD-V. A muscle biopsy will note the absence of myophosphorylase in muscle fibers. In some cases, acid-Schiff stained glycogen can be seen with microscopy.

PYGM genetic sequencing may be done to determine the presence of gene mutations, determining if McArdle's is present. This type of testing is considerably less invasive than a muscle biopsy. The test involves bidirectional sequencing of the coding regions of all 20 PYGM exons plus about 50 Bp of non-coding flanking DNA on each side. Because the disease consists of two gene mutations and because the test can be performed to identify carries of the disease, the test has two tiers. Tier 1 involves sequencing of exons 1 and 5. If two likely causative mutations are detected in patients in Tier 1 or one mutation carriers in Tier 1, then the testing stops. Otherwise, testing continues with Tier 2 involving sequencing the remaining 18 exons. These tests require a simple blood draw.

The physician can perform an ischemic forearm exercise test as described above. Some findings suggest a nonischemic test could be performed with similar results. The nonischemic version of this test would involve not cutting off the blood flow to the exercising arm. Findings consistent with McArdle’s disease would include a failure of lactate rise in venous blood and exaggerated rise of ammonia levels. These findings would indicate a severe muscle glycolytic block. Ammonia arises from the impaired buffering of ADP, which leads to an increase in AMP concentration resulting in an increase in AMP deamination.

Physicians may check creatine kinase resting levels, which are moderately increased in 90% of patients. In some, the level is increased in multitudes (a person's normal CK without GSD-V rests between 60 and 400IU/L, while a person with the syndrome can have a level of 5,000IU/L at rest and achieve 35,000 IU/L and more when overexerted). This piece of data helps distinguish McArdle's from carnitine palmitoyltransferase deficiency, or CPT-II, a lipid-based metabolic disorder. Also, serum electrolytes, endocrine studies (such as thyroid function, parathyroid function and growth hormone levels) will be completed. Urine studies are required only if rhabdomyolysis is suspected. Urine volume, urine sediment and myoglobin levels would be ascertained. If rhabdomyolysis is suspected, serum myoglobin, creatine kinase, lactate dehydrogenase, electrolytes and renal function will be checked.