THE METABOLIC AND TOXIC MYOPATHIES in Microsoft Office

Encoder QR Code JIS X 0510 in Microsoft Office THE METABOLIC AND TOXIC MYOPATHIES

THE METABOLIC AND TOXIC MYOPATHIES
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has been mapped to the distal portion of the long arm of chromosome 17 (17q23) In the second (childhood) form, onset is during the second year, with delay in walking and slowly progressive weakness of shoulder, pelvic girdle, and trunk muscles The toe walking, waddling gait, enlargement of calf muscles, and lumbar lordosis resemble those of Duchenne dystrophy Cardiomyopathy is exceptional, hepatomegaly is less frequent than in the infantile form, and mental retardation was present in only 2 of 18 cases reported by DiMauro et al Death occurs between 3 and 24 years of age, usually from ventilatory insuf ciency In the third, or adult, form there is a more benign truncal and proximal limb myopathy The weakness is slowly progressive over many years, and death is usually the result of weakness of respiratory muscles At times the only severe weakness is of the diaphragm, as in the case reported by Sivak and colleagues, making adult acid maltase de ciency part of a select group of neuromuscular disorders that may present in this way (along with motor neuron disease, nemaline rod myopathy, and myasthenia gravis) The liver and heart are not enlarged Creatine kinase (CK) values are increased, as they are in all forms of the disease The EMG discloses a number of abnormalities brief motor unit potentials, brillation potentials, positive waves, bizarre high-frequency discharges, and occasional myotonic discharges (without clinical evidence of myotonia) The disease must be differentiated from other chronic adult polymyopathies, including polymyositis and the endocrine myopathies, and from motor neuron disease Aside from the elevation of the muscle-derived enzymes CK and aldolase, blood studies are normal The diagnosis in all forms of acid maltase de ciency is readily con rmed by muscle biopsy The sarcoplasm is vacuolated and most vacuoles contain periodic acid Schiff (PAS)-positive diastase-digestible material; they stain intensely for acid phosphatase The glycogen particles lie in aggregates; electron microscopy shows some of them to occupy lysosomal vesicles (ie, this is a lysosomal storage disease) and others to lie free The myo brils are disrupted, and some muscle bers degenerate Glycogen accumulation is more pronounced in type 1 bers As indicated earlier, in the more severe infantile form of acid maltase de ciency, heart muscle and the large neurons of the spinal cord and brainstem may also accumulate glycogen and degenerate The difference in severity between infant and adult forms may relate to the completeness of enzyme de ciency, but possibly other factors are also at work since more than one of the three types may occur in the same family Treatment The adult who is threatened by respiratory failure should be observed frequently with measurements of vital capacity and blood gases Umpleby and coworkers reported that a low-carbohydrate, high-protein diet may be bene cial A few of our patients died unexpectedly during sleep Respiratory support (rocking bed, nasal positive pressure, and negative-pressure cuirass) may prolong life Enzyme replacement therapy is being investigated in the treatment of Pompe disease Recombinant acid alpha-glucosidase is injected intravenously and the early results are promising Myophosphorylase De ciency (Type V Glycogenosis; McArdle Disease) and Phosphofructokinase De ciency (Type VII Glycogenosis; Tarui Disease) These disorders are considered together since they are clinically virtually identical and both express themselves by the development of muscle cramps (actually true physiologic contractures, page 1194) in response to exercise a
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feature that distinguishes them from other glycogenoses In both these diseases an otherwise normal child, adolescent, or adult begins to complain of weakness and stiffness and sometimes pain on using the limbs Muscle contraction and relaxation are normal when the patient is in repose, but strenuous exercise, either isometric (carrying heavy weights) or dynamic (climbing stairs or walking uphill), causes the muscles to shorten (contracture), since they are unable to relax After vigorous exercise, episodes of myoglobinuria are common; these may result in renal failure With mild sustained activity, the patient experiences progressive fatigue and weakness, which diminish following a brief pause The patient can then resume his activities at the original pace ( second-wind phenomenon) During the second-wind phase, the patient copes with his symptoms by increasing cardiac output and substituting free fatty acids and blood-borne glucose for muscle glycogen (Braakhekke et al) The primary abnormality in McArdle disease is a de ciency of myophosphorylase, which prevents the conversion of glycogen to glucose-6-phosphate Phosphofructokinase de ciency (Tarui disease) interferes with the conversion of glucose-6-phosphate to glucose-1-phosphate; the defect in the latter condition is also present in red blood cells (Layzer et al) The gene for myophosphorylase has been localized to the long arm of chromosome 11 (11q13), and analysis of DNA from the patient s leukocytes can be used for diagnosis The muscle (M) subunit of the phosphofructokinase protein in Tarui disease localizes to a gene defect on chromosome 1 This defect predominates in Ashkenazi Jewish men Clinical variants of these disorders are well known Some patients, with no previous symptoms of cramps or myoglobinuria, develop progressive weakness of limb muscles in the sixth or seventh decade One of these older patients came to our attention because of chronically elevated levels of CK and mild muscle cramping after climbing stairs In others, rapidly progressive weakness became evident in infancy, with early death from respiratory failure These unusual forms are not directly related to severity of the enzyme de ciencies The contracted muscles in these disorders, unlike muscles in other involuntary spasms, no longer use energy, and they are more or less electrically silent (ie, no electrical activity is recorded from maximally contracted muscle during the cramps induced by ischemic exercise); moreover, the muscle does not produce lactic acid This shortened state is spoken of as physiologic contracture Ischemia contributes to this condition by denying glucose to the muscle, which cannot function adequately on fatty acids and nonglucose substrates These features are the basis of the forearm ischemic exercise test, which, while controversial in its use, may be helpful if performed carefully in the diagnosis of both McArdle and Tarui disease An indwelling catheter is placed in the antecubital vein and a basal blood sample is obtained Above the elbow, a sphygmomanometer cuff is in ated to exceed arterial pressure After 1 min of vigorous hand exercise (30 hand closures against an ergometer), blood samples are obtained at 1 and 3 min Normal individuals show a three- to vefold increase in blood lactate In patients with either McArdle or Tarui disease, the lactate fails to rise This procedure has reportedly caused a localized rhabdomyolysis (Meinck et al), for which reason Griggs and associates recommend that the test be carried out without a blood pressure cuff Problems with consistency in conducting the test and processing blood samples for lactate limit its validity unless it is performed by experienced individuals and laboratories
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