GSD Type 5

Also known as McArdle disease

Defect Deficiency of myophosphorylase resulting inability to release glucose from glycogen in muscles.

Cause Gene mutation of myophosphorylase localized on chromosome 11. More that 65 mutations have been identified.

Incidence Estimated 1 per 100,000. Reported more in males than in females.

Signs & Symptoms

• Infantile form of McArdle disease is characterized by hypotonia (poor muscle tone in babies), generalized muscle weakness and progressive respiratory insufficiency (failure to breath effectively)
• Classically presents as exercise intolerance with myalgia (muscle pain), early fatigue, muscle stiffness and cramping, which are all relieved by rest. Many patients can resume activity after rest “second wind phenomenon”
• Some patients have extremely mild symptoms that display as tiredness without cramps.
• One third of the patients presents with myoglobinuria (main oxygen transporter in the muscles) following intense exercise and can lead to renal failure.
• Some patients manifests seizures.

Diagnosis

• Laboratory studies
• Imaging (31P-NMR – a special type of MRI)
• Electromyography (EMG) – an audio or visual record of the electrical activity of a skeletal muscle by means of an electrode(sensor) inserted into the muscle or placed on the skin.
• Ischemic forearm exercise test
• Muscle biopsy

Treatment

• A high protein or high fat diet may be beneficial.
• Medical Consults depending on tissue involvement ( example – renal failure)
• Sucrose may improve exercise tolerance.
• Medications – Some studies suggest that pyridoxine (vitamin B-6) may reduce the susceptibility of muscles to fatigue.