The phosphorylase enzyme plays a vital role in the breakdown of glycogen into glucose. In the absence of phosphorylase in muscles, glucose can not be released from the glycogen stored in skeletal muscles to create energy. People with Type V GSD experience problems performing and completing most exercises, especially anaerobic exercises. Because they lack the enzyme to metabolize glycogen, which is the main source of energy for anaerobic activity, their body struggles to find other sources of energy to complete a given activity or exercise. Under these circum- stances, the body breaks down muscle when trying to attain energy. This causes many symptoms such as muscle pain, muscle cramping, muscle fatigue, and muscle tenderness. With the breakdown of muscle (rhabdomyolysis) and the release of the red protein myoglobin, myoglobinuria may develop, as evidenced by dark-red or red-brown urine. Serum creatine kinase levels will be greatly elevated.
The physical exam of patients with Type V glycogen storage disease is normal. They complain of painful muscle cramps after exercise. These persons are commonly muscular; they do not have large livers, and are normal in height. Their liver phophorylase activity is normal, and they do not have hypoglycemia. A muscle biopsy will show increased concentrations of glycogen, and a deficiency of the phosphorylase enzyme.
At present, there is no specific treatment for this type of GSD. It is very important for patients to learn how to safely get into 'second wind' (achieved in about 10 minutes) and to take regular aerobic exercise beyond that point. Anaerobic activity and exercise must be avoided as it can cause considerable muscle breakdown resulting in a great deal of myoglobin in the urine. Large amounts of myoglobin may precipitate in the kidneys and cause temporary kidney failure. Some patients have developed significant muscle problems (myopathies) later in life.