Description, Causes and Risk Factors:
Type V glycogen storage disease, affecting muscle, caused by deficiency of muscle phosphorylase or myophosphorylase.
A gene of chromosome 11q13 encodes myophosphorylase. It exists in two forms phosphorylase A is the active form and phosphorylase B is the inactive form. Phosphorylase B kinase is the enzyme that converts inactive form to the active form and is itself activated by a protein kinase. Deficiencies of either enzyme result in exercise intolerance.
The condition is caused by mutations in the PYGM gene (11q13), leading to myophosphorylase deficiency. Mutation p.R50X may account for 40-50% of the alleles in Caucasian populations.
According to the most recent publications, 95 different mutations have been reported. The forms of the mutations may vary between ethnic groups. For example, the Arg49Stop mutation is most common in North America and Europe, the R49X mutation is most common in Dutch patients, and the Y84X mutation is most common among central Europeans.
The exact method of protein disruption has been elucidated in certain mutations. For example, R138W is known to disrupt to pyridoxal phosphate binding site. In 2006, another mutation (c.13_14delCT) was discovered which may contribute to increased symptoms in addition to the common Arg50Stop mutation.
Genetic transmission of myophosphorylase deficiency is by the autosomal recessive inheritance. The gene encodes on chromosome 11q13. Reports of autosomal dominant inheritance may represent manifesting heterozygotes. Phosphorylase activity is deficient only in muscle. The deficiency blocks the first step of glycogenolysis, and muscle glycogen is unavailable to produce glucose for energy. Liver phosphorylase concentrations are normal and hypoglycemia does not occur.
Prevalence is unknown. Onset occurs in childhood. Prognosis is favorable when severe rhabdomyolysis is avoided. However, myoglobinuria may lead to potentially life-threatening renal failure.
The severity of symptoms varies with the percentage of enzyme activity. Children with only mild deficiency states have few or no symptoms until adolescence. Aching becomes increasingly prominent and then, after an episode of vigorous exercise severe cramps occur in exercised muscle. Myoglobinuria is sometimes present. The pain can last for hours. Thereafter, exercise leads to repeated bouts of cramps that cause a decrease in the overall level of activity. Pain begins soon after initiating vigorous exercise and myoglobulinuria several hours later. Some patients exercise through the pain by slowing down just before the time of fatigue. Once passing that point, exercise may continue unimpeded. The second wind phenomenon is probably due to an increase in cardiac output, the use of blood glucose and free fatty acids as a substrate for muscle metabolism, and the recruitment of more motor units.
Examination is generally unrevealing. Usually muscle mass, muscle strength, and tendon reflexes are normal. Only adult patients develop weakness, and even then tendon reflexes are normal.An alternate presentation of myophosphorylase deficiency is a slowly progressive proximal weakness beginning during childhood or adult life. Affected individual may never report cramps with exercise or myoglobulinuria. Tendon reflexes are present until late in the course of disease.
EMG results are usually normal. The serum creatine kinase (SCK) concentration is increased and myoglobin may appear in the urine coincidentally with the cramps.
Salient features of muscle biopsy specimens are histochemical evidence of sub-sarcolemmal vacuoles containing glycogen and the absence of phosphorylase. Muscle fiber degeneration and regeneration are present immediately after an episode of cramps and myoglobulinuria.
Definitive diagnosis requires the biochemical demonstration of decreased myophosphorylase activity.
Treatment is based on controlled physical training in order to develop mitochondrial oxidation capacities in muscles, and programmed glucose intake according to exercising periods. Diets with high protein intake have yielded variable results.
Creatine supplementation may increase muscle fraction. Moderate aerobic conditioning improves exercise capacity. Ingestion of oral sucrose 30-40 minutes before aerobic exercise may also improve exercise tolerance. Patients usually learn to live with their disorder by controlling their level of exercise.
NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
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