| Important Medical Information Emergencies: The metabolic myopathies are such rare disorders that emergency room staffs are frequently unfamiliar with them. For this reason, some people carry a "protocol," composed by their treating neurologists, containing pertinent phone numbers and addresses of the patient's physicians, current medications, dietary considerations and guidelines for handling emergency situations. Another solution is to wear a MedicAlert bracelet. Emergency physicians can call the number on the bracelet and obtain information about the patient. For information, call 800-IDALERT or visit www.medicalert.org/home.asp. Anesthesia: People with metabolic myopathies are at high risk for a potentially fatal reaction to common general anesthetics (typically combinations of halothane and succinylcholine). This reaction, called malignant hyperthermia, can be avoided by using lower-risk anesthetics. However, it's a good idea to wear a MedicAlert bracelet stating this susceptibility in case you require emergency surgery. Metabolic myopathies covered in Muscular DystrophyAssociation's program include muscle phosphorylase deficiency (McArdle's disease), debrancher enzyme deficiency (Cori's or Forbes' disease), phosphofructokinase deficiency (Tarui's disease), phosphoglycerate kinase deficiency, phosphoglycerate mutase deficiency, lactate dehydrogenase deficiency, acid maltase deficiency (Pompe's disease), myoadenylate deaminase deficiency, carnitine deficiency and carnitine palmityl transferase deficiency. Each is caused by a specific genetic defect in the various pathways involved in food metabolism in the cell. Although each myopathy listed above has its own set of symptoms depending on the particular metabolic pathway that's blocked, two major problems can occur in all of them. First, tissues can suffer from a lack of energy, and, second, toxic metabolites may build up and destroy the tissue. With most metabolic myopathies, the biggest problem seems to be the inability to use specific fuel sources for energy. The major sources of fuel used by our muscles are carbohydrates (in the form of glucose and glycogen) and fats. Carnitine deficiency and carnitine palmityl transferase deficiency (CPT) interfere with fat metabolism by preventing the passage of fatty acids through the inner membrane of the mitochondria, energy-processing centers of cells. The remaining disorders, except for myoadenylate deficiency, are all defects in the carbohydrate pathway. The adult-onset forms of the metabolic myopathies can be severe, and encompass a wide range of symptoms and rates of progression. Exercise intolerance When metabolic myopathies interfere with the extraction of energy from food, the main result is the inability to perform some types of ordinary exercise, a situation called exercise intolerance. People with exercise intolerance become fatigued easily, either at the onset of exercise or after sustained exercise. Adults with these types of myopathies push themselves in physical activities, overexertion may be accompanied by acute episodes of muscle breakdown (rhabdomyolysis), producing skyrocketing creatine kinase levels, myoglobinuria (myoglobin in the urine that turns it a cola color) and intense muscle pain. Although many people describe this muscle pain as a "cramp," in reality, it's not a true cramp (spasmodic muscle contraction) and can, in fact, be more painful. Because these acute attacks can be so painful and may produce significant muscle injury and myoglobinuria-induced kidney damage, a major goal in living with this form of metabolic myopathy is to prevent attacks. In response, many people have learned to avoid activities that they know will trigger an attack, and have made dietary modifications designed to circumvent the blocked metabolic pathway. Different definitons of 'strenuous' Some think the standard warnings against "strenuous" exercise in metabolic myopathies understate the problem. Last June, Stout squatted down for a second and triggered an attack of rhabdomyolysis that cost him 10 percent of the muscle mass in both his thighs. He cites a list of other activities that could trigger problems in people with phosphorylase deficiency, including squatting, standing on tiptoes, and lifting, pushing or pulling heavy objects. Each of these activities involves "isometric" exercise, or strength exercise. Muscular Dystrophy Association research grantee Ronald Haller, director of the Neuromuscular Center of the Institute for Exercise and Environmental Medicine at Presbyterian Hospital of Dallas, points out that intense isometric muscle contractions constrict blood vessels and cut off blood flow to the muscles. Thus, during isometric exercise, the muscles must depend on anaerobic metabolism for energy (an anaerobic pathway is one that doesn't require oxygen to make ATP). Because the main fuel source for anaerobic metabolism is glycogen, those with defects in glycogen metabolism can develop significant muscle injury during this type of exercise. "That warning needs to be there up front," Stout says. "I didn't have a clue that squatting would be a high-risk activity. An important observation about the metabolic myopathies is that they can vary in severity — the degree of exercise intolerance may be different from one person to the next, as may be specific situations that trigger rhabdomyolysis. Also, although many people with adult-onset metabolic myopathy may be told that their conditions aren't progressive, repeated episodes of rhabdomyolysis, combined with the effects of aging, may eventually lead to some degree of chronic muscle weakness, in addition to exercise intolerance. Special Considerations in Metabolic Myopathies Myoglobinuria: Rust-colored urine caused by the presence of myoglobin. When overexertion triggers acute muscle breakdown (rhabdomyolysis), muscle proteins like creatine kinase and myoglobin are released into the blood. Myoglobin, which is reddish-brown in color, ultimately finds its way into the urine. In addition to being an indicator of acute muscle damage, myoglobinuria can cause permanent damage to the kidneys. Severe cases of rhabdomyolysis should be treated as emergencies and may require the introduction of intravenous fluids to avoid renal failure. Cardiac Care Debranchor enzyme deficiency, carnitine deficiency and acid maltase deficiency can lead to heart failure. In the case of primary carnitine deficiency, the only symptom may be heart failure; however, this disorder responds very well to carnitine supplementation. In acid maltase deficiency and CPT2 deficiency, a cardiologist who's familiar with your disorder should monitor heart function regularly. |
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| Some of the information in this article was taken from the MDA publication: Running on Empty written by Ruth Speary, Keith Stout & Chris Tapley |