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Metabolic Myopathies

Medical Management

Avoiding rhabdomyolysis and myoglobinuria

When overexertion triggers acute muscle breakdown (rhabdomyolysis), muscle proteins are released into the blood and ultimately appear as rust-colored urine myoglobinuria). This condition can cause severe kidney damage, and should be dealt with as an emergency (see below). You may require intravenous fluids to avoid kidney failure.

Because rhabdomyolysis (acute muscle breakdown) is painful and can cause extensive kidney damage, many people with metabolic muscle diseases try to avoid triggering these episodes by modifying their physical activities. Each person must learn his or her activity limitations. Your MDA clinic director can help you work out a lifestyle plan to optimize your health and abilities.

There’s emerging evidence that people with some carbohydrate-processing disorders, such as McArdle disease, may benefit from light exercise. Researchers believe that people who are physically fit are better able to use alternative fuel sources to make energy. Because overexertion can trigger muscle breakdown, you should only undertake an exercise program under the supervision of a doctor who’s familiar with your disorder.

It’s unclear whether regular exercise is beneficial in the fat-metabolizing disorders, such as carnitine palmityl transferase deficiency. Because of their rarity, the characteristics of several of these diseases aren’t well known.

A small percentage of adults with metabolic disorders may experience painful muscle cramps that have no obvious triggers; painkillers and meditation techniques may be effective under these circumstances.

Cardiac care

People with debrancher enzyme deficiencycarnitine deficiency and acid maltase deficiency may develop significant heart problems. If you’re at risk for cardiac problems, a cardiologist who’s familiar with your disorder should monitor your heart function.

Carnitine supplements are usually given for carnitine deficiency and can be very effective in reversing heart failure in this disorder.

Dietary changes

Some people with metabolic disorders have benefited from dietary changes. There’s evidence that those with carbohydrate-processing problems may be helped by a high-protein diet, while those with difficulty processing fats may do well on a diet high in carbohydrates and low in fat.

Please consult your doctor before undertaking any special diets. Your MDA clinic director can help you design a specific plan suited for your metabolic disorder and your individual needs.

For a look at diet considerations in acid maltase deficiency (Pompe disease), carnitine palmityl transferace deficiency, and phosphorylase deficiency (McArdle disease), see What Not to Eat: Some consensus, much controversy about diet in three metabolic diseases.

Emergencies and anesthesia

The metabolic muscle diseases are so rare that emergency room staffs are frequently unfamiliar with them. As a result, they may not treat episodes properly (with fluids and pain medications) or may give the patient food or anesthesia that could trigger further problems.

People with these disorders may want to consider carrying a treatment “protocol” listing their doctor’s phone number, their current medications and dietary requirements, and guidelines for handling emergency situations. A MedicAlert bracelet also can be worn.

People with metabolic muscle disorders may be at higher risk for a potentially fatal reaction to certain common general anesthetics (typically combinations of halothane and succinylcholine). This reaction, called malignant hyperthermia, can be avoided in planned surgeries by using lower-risk anesthetics. However, it’s a good idea to wear a MedicAlert bracelet stating this susceptibility in case of an emergency.

Exercise intolerance

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 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. In CPT deficiency, rhabdomyolysis also may be triggered by illness, cold, fasting, stress or menstruation.

Because rhabdomyolysis is painful and can cause extensive kidney damage, many people with metabolic muscle diseases try to avoid triggering these episodes by modifying their physical activities or diet. Your MDA clinic director can help you work out a lifestyle plan to optimize your health and abilities.

Pompe treatment

In April 2006, the U.S. Food and Drug Administration (FDA) approved Myozyme, a synthetic form of the acid maltase enzyme, manufactured by Genzyme of Cambridge, Mass., for the treatment of infantile-onset acid maltase deficiency (Pompe disease).

In 2010, Genzyme began marketing the synthetic enzyme as Lumizyme, for those with the later-onset form of the disease. Since Myozyme and Lumizyme became commercially available, the outlook for people of all ages with Pompe disease is considerably brighter.

The enzyme replacement therapy, which requires intravenous infusions of the drug, has significantly improved survival in patients with infantile-onset Pompe disease, as well as respiratory function and walking endurance in older patients.

In August 2021, another therapeutic option became available to people with late-onset Pompe disease. The FDA granted accelerated marketing approval to Genzyme for a second-generation enzyme replacement therapy, Nexviazyme, that is able to enter target cells more efficiently. In clinical trials, Nexviazyme treatment showed comparable safety profile and efficacy to treatment with Lumizyme.

Respiratory care

Acid maltase deficiency and debrancher enzyme deficiency can weaken respiratory muscles. If you’re at risk for these problems, your breathing should be monitored regularly by a specialist. Also, be conscious of symptoms such as unusual shortness of breath on exertion or morning headaches that may indicate that your breathing is compromised. You may require ventilatory assistance at night.

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