Glycogen Storage Disease Type V

Also known as: McArdle disease, myophosphorylase deficiency, GSD5

Gene: PYGM

Protein: glycogen phosphorylase, muscle form

Clinical Characteristics

  • Exercise intolerance with isometric or sustained aerobic exercise:
    • Rapid fatigue
    • Muscle pain and cramps
    • “Second wind” phenomenon, or relief of pain and fatigue after a few minutes of rest
  • Onset in second or third decade of life
  • Muscle weakness:
    • Nonprogressive muscle weakness in one third of affected individuals
    • Progressive weakness in some affected individuals in the sixth or seventh decade of life
  • Myoglobinuria in about 50% of affected individuals, can result in acute renal failure
  • Supportive lab findings:
    • Increased resting serum creatine kinase
    • No increase in plasma lactate on the forearm exercise test
    • Cycle test decrease in heart rate from seven to 15 minutes of stationary cycling
  • Rare severe form with rapid progression and onset shortly after birth

Inheritance pattern: Autosomal recessive

What Can Be Learned From This Test

Testing is performed by sequencing the entire coding region of PYGM. This will detect point mutations, small deletions and small insertions. It will not detect a partial or whole gene deletion or duplication. Partial sequencing of exons 1 and 5 can be carried out; please note on submission form if this tiered approach is being requested.

A negative test result does not rule out a genetic cause of myopathy. There are many other genetic forms of myopathy, and this test will only detect mutations associated with glycogen storage disease type V, caused by deficiency of glycogen phosphorylase, muscle form.

Sample Requirements

Draw 2ml–4ml of blood in EDTA/purple-top tube (minimum of 1ml–2ml for infants).

Turnaround time: 10-14 business days

CPT Codes and Cost

Exon Sequencing (1 & 5 Only)

  • Code: 81401
  • Cost: $250

Full Gene Sequencing

  • Code: 81406
  • Cost: $750

Known Variant Testing

  • Code: 81479
  • Cost: $225

Additional Information