If exercise is continued despite the pain, a severe muscle spasm or contracture will develop. This results in muscle damage leading to myoglobinuria, a dark discolouration of the urine.
Many people with the condition remember painful symptoms from early childhood but the disorder is rarely diagnosed before adulthood. Some people notice a worsening of their symptoms in middle-age and this may be accompanied by some muscle wasting, especially over the shoulders and back. Most people live normal lives and learn to improve their exercise tolerance using a “second wind”.
McArdle disease does not affect life expectancy. Some people do notice a slow deterioration in their symptoms over many years but by far the majority of people remain independent and able to walk.
What is a second wind?
There are two types of exercise: aerobic exercise includes walking, gentle swimming, jogging and cycling. Anaerobic exercise is more intense or sustained exercise and includes running, walking uphill and carrying loads. The first few minutes of any exercise are usually anaerobic.
Normally, during anaerobic exercise, muscle phosphorylase converts glycogen (stored starch) to glucose (sugar). The glucose enters a metabolic pathway known as the glycolytic pathway, which ultimately produces ATP (energy). During aerobic exercise, the main energy source comes from free fatty acids carried in the blood stream. These fatty acids enter a different metabolic pathway, called oxidative phosphorylation, which takes place in the mitochondria (these are the power houses of the cell) the end result is the production of ATP (energy)
A second wind is like a key opening the door, enabling exercise without pain for people with McArdle disease. After a few minutes of exercise when pain occurs, if the individual rests for a moment or two, exercise can be continued without severe pain. The second wind probably results from a switch in metabolic pathway from the glycolytic pathway to oxidative phosphorylation.
Which muscles are affected?
Any skeletal muscle is affected. Usually walking brings on symptoms in the legs and carrying heavy shopping affects the arms. Sawing, digging or squatting may affect the back muscles and some people develop pain eating chewy foods.