In the case of primary carnitine deficiency, the only symptom may be heart failure; however, this disorder responds well to carnitine supplementation. See Medical Management .
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The main symptom of most of the metabolic myopathies is difficulty performing some types of exercise, a situation known as exercise intolerance, in which the person becomes tired very easily.
The degree of exercise intolerance in the metabolic myopathies varies greatly between disorders and even from one individual to the next within a disorder. For instance, some people may run into trouble only when jogging, while others may have trouble after mild exertion such as walking across a parking lot or even blow-drying their hair.
In general, people with defects in their carbohydrate-processing pathways tend to become very tired at the beginning of exercise but may experience a "second wind," or renewed feeling of energy after 10 or 15 minutes. On the other hand, those with carnitine palmityl transferase deficiency (CPT deficiency)  may experience fatigue only after prolonged exercise.
A person with exercise intolerance also may experience painful muscle cramps and/or injury-induced pain during or after exercising.
The exercise-induced cramps (actually sharp contractions that may seem to temporarily “lock” the muscles) are especially noted in many of the disorders of carbohydrate metabolism and, rarely, in myoadenylate deaminase deficiency . The injury-induced pain is caused by acute muscle breakdown, a process called rhabdomyolysis, which may occur in any metabolic muscle disorder and is particularly noted in CPT deficiency.
Episodes of rhabdomyolysis usually occur when a person with a metabolic myopathy “overdoes it” (sometimes unknowingly). These episodes, often described as “severe muscle pain,” may occur during exercise or several hours afterward. In those with carbohydrate-processing disorders, rhabdomyolysis may be triggered by aerobic exercise (such as running or jumping) or isometric exercise (like pushing or pulling heavy objects, squatting or standing on tiptoes).
In people with CPT deficiency, rhabdomyolysis is usually brought on by prolonged, moderate exercise, especially if an affected person exercises without eating. Rhabdomyolysis also may be triggered in CPT deficiency by illness, cold, fasting, stress or menstruation.
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Although the metabolic muscle diseases characterized by exercise intolerance typically don’t involve muscle weakness, some chronic or permanent weakness can develop in response to repeated episodes of rhabdomyolysis and to the normal loss of strength that occurs with aging.
The degree of muscle weakness that develops in these disorders is extremely variable and may depend on such factors as genetic background and the number of episodes of rhabdomyolysis experienced.
The diseases involving exercise intolerance don’t usually progress to the degree that a wheelchair or any other mechanical assistance is needed. However, in acid maltase deficiency , carnitine deficiency  and debrancher enzyme deficiency , progressive muscle weakness, rather than exercise intolerance, is the primary symptom.
People with these deficiencies may, over time, develop mobility problems.
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Myoglobinuria refers to rust-colored urine caused by the presence of myoglobin (a muscle protein). When overexertion triggers acute muscle breakdown (rhabdomyolysis), muscle proteins like creatine kinase and myoglobin are released into the blood and ultimately appear in the urine. Myoglobinuria can cause severe kidney damage if left untreated.
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Acid maltase deficiency  and debrancher enzyme deficiency  tend to weaken the respiratory muscles (those that operate the lungs), meaning that a person with one of these disorders may require supplemental oxygen or mechanical respiratory assistance at some point.
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