Symptoms of McArdle’s Disease
(Type V Glycogen Storage Disease Symptoms)
The symptoms of McArdle’s disease are subtle but unmistakable. After the first ~10 seconds of strenuous physical activity following rest, skeletal muscle relies on the critical conversion of glycogen to glucose in order to drive the formation of adenosine triphosphate to provide power muscular contractions. This is the period when symptoms become apparent.
The body’s response to the inability of glycogen to metabolize within muscle to glucose for energy is an abrupt feeling of exhaustion or fatigue, with an increase in heart rate.
During particularly strenuous activities like carrying heavy objects, the failure of glycogen metabolism to provide glucose for energy quickly robs of the energy they need, and the fuel to power continued contractions is gone. Similar to the "wall" that marathon runners may hit after depleting their muscle’s glycogen stores, someone experiencing this cannot move any more, regardless of effort.
A complication of the muscle failure associated with McArdle’s disease are electrically-silent contractures which are painful and may involve significant damage to muscle tissue.
Myoglobinuria (dark urine)
As a result of damage to muscle tissue, proteins like creatine kinase and myoglobin enter the blood stream and are passed as urine, which may be darkened from the iron-containing proteins. Myoglobinuria has induced renal dysfunction in some individuals with McArdle’s disease, which is why failure and cramping episodes require medical attention.
Possibly owing to the muscle damage caused by rhabdomyolysis, fixed muscle weakness has been observed in some patients who find it difficult to improve their physical strength as a result.
The Second Wind
The second wind phenomenon has been observed in clinical studies, presenting as a feeling of improved energy levels and relief from pain accompanied by a more normal heart rate following a "warm up" period. This second wind does not completely relieve failure symptoms for intense exercise, but does appear to offer relief for moderate intensity activities like walking or biking.