Myotonia fluctuans

Autosomal-dominantly inherited nondystrophic myotonic disorders are an interesting group of muscle diseases that provide considerable opportunity for future molecular genetic studies to identify the genes responsible for specific membrane functions. A family with such a myotonic disorder is described with features that are distinctly different from myotonia congenita and paramyotonia congenita. Five members were affected in three generations. The myotonia fluctuated to an unusual degree. It did not worsen with cold but increased markedly with potassium loading. Muscle weakness never occurred. Analysis of the contraction force of the flexor digitorum muscle showed a unique type of myotonia, namely, exercise-induced delayed-onset myotonia. Microelectrode studies done on one muscle biopsy specimen revealed a normal chloride conductance of the muscle fiber membrane.

Functional testing

Functional testing is useful to monitor the natural history of several neuromuscular disorders, and to measure the efficacy of therapeutic agents in clinical trials. A major limitation of functional testing is that a single test is often not appropriate throughout all stages of disease. The same limitation holds true for other measurements of disease progression. MMT and quantitative myometry become technically difficult to perform and lack sensitivity to disease progression at some stages in the course of DMD, ALS, and spinal muscular atrophy. Other limitations of functional testing are its lack of sensitivity to detect a subtle improvement or decline in muscle strength, and the difficulty of applying standard statistical methods to analyze disease progression or therapeutic efficacy. The advantages of functional testing outweigh the limitations. Function testing uses daily activities to monitor disease progression. Patient's appreciate improvements in function more readily than improvements in muscle strength. Functional testing is easily standardized and is reliable. It can be performed, with little or no expense, in almost any clinical setting. The primary challenge for investigators who wish to incorporate functional tests is to identify functional tests that best measure the natural history of the specific disease under investigation. Further, functional testing allows the clinician to provide an educated guess on the likely future course of disease.

Phenytoin-induced improvement in muscle cramping and insulin action in three patients with the syndrome of insulin resistance, acanthosis nigricans, and acral hypertrophy

Phenytoin sodium has been used to treat muscle cramps of diverse causes, and is known to increase insulin sensitivity during long-term use. We have previously described a syndrome of insulin resistance, acanthosis nigricans, and acral hypertrophy with continual muscle cramping. The effect of 300 mg/d of phenytoin (Dilantin) on muscle cramping and carbohydrate economy was studied in three affected patients and four control subjects. Oral glucose tolerance tests, euglycemic insulin infusion studies, and monocyte insulin binding tests were conducted before and after phenytoin administration. All three patients had notable improvement in muscle cramps. In response to phenytoin, metabolic improvements were variable, with improvement characteristically better in patients with less severe baseline metabolic abnormalities. Patient 1, with the mildest degree of glucose intolerance, had decreased fasting insulin and blood glucose levels, improved glucose tolerance, and insulin-mediated glucose disposal, associated with an increase in monocyte insulin receptors. Patient 2 had reduced fasting plasma glucose and insulin levels and improved oral glucose tolerance, suggesting a beneficial effect on carbohydrate metabolism. Patient 3, with the most severely impaired carbohydrate economy, showed no metabolic improvement despite marked lessening of muscle pain. These clinical characteristics were unaffected in control subjects. We conclude that phenytoin is of value in the therapy of muscle cramps and glucose intolerance in patients with this syndrome.

Potassium uptake in muscle during paramyotonic weakness

Previous studies have suggested that an abnormal release of potassium from muscle may accompany attacks of paramyotonic weakness. We investigated 3 patients with paramyotonia congenita before and after the induction of forearm muscle weakness by exercise in cold water. Two of these patients had paralysis periodica paramyotonica and the 3rd had paramyotonia congenita. At the time of paramyotonic weakness there was a marked increase in the arterialized-venous concentration difference of potassium across forearm muscle. This indicated a significant uptake of potassium by forearm muscle in all 3 patients. Normal controls showed a slight release of potassium both at rest and after exercise in cold water. These results suggest that (1) the sodium-potassium pump of the muscle fiber is operating efficiently during paramyotonic weakness; and (2) there is a different mechanism responsible for the generalized weakness that occurs in hyperkalemic periodic paralysis.

Randomized, double-blind six-month trial of prednisone in Duchenne's muscular dystrophy

We performed a randomized, double-blind, controlled six-month trial of prednisone in 103 boys with Duchenne's muscular dystrophy (age, 5 to 15 years). The patients were assigned to one of three regimens: prednisone, 0.75 mg per kilogram of body weight per day (n = 33); prednisone, 1.5 mg per kilogram per day (n = 34); or placebo (n = 36). The groups were initially comparable in all measures of muscle function. Both prednisone groups had significant improvement of similar degree in the summary scores of muscle strength and function. Improvement began as early as one month and peaked by three months. At six months the high-dose prednisone group, as compared with the placebo group, had improvement in the time needed to rise from a supine to a standing position (3.4 vs. 6.2 seconds), to walk 9 m (7.0 vs. 9.7 seconds), and to climb four stairs (4.0 vs. 7.1 seconds), in lifting a weight (2.1 vs. 1.2 kg), and in forced vital capacity (1.7 vs. 1.5 liters) (P less than 0.001 for all comparisons). There was an increase in urinary creatinine excretion (261 vs. 190 mg per 24 hours), which suggested an increase in total muscle mass. However, the prednisone-treated patients who had required long-leg braces (n = 5) or wheelchairs (n = 11) continued to require them. The most frequent side effects were weight gain, cushingoid appearance, and excessive hair growth. We conclude from this six-month study that prednisone improves the strength and function of patients with Duchenne's muscular dystrophy. However, further research is required to identify the mechanisms responsible for these improvements and to determine whether prolonged treatment with corticosteroids may be warranted despite their side effects.

Rippling muscle disease

Six patients from two families with an autosomal dominantly inherited disease, apparently a myopathy, are described. Their major complaint was muscle stiffness, primarily in the legs. The muscles displayed an unusual sensitivity to stretch, manifested by rippling waves of muscle contraction. These rippling contractions were not accompanied by muscle fiber action potentials. Nonspecific, mild abnormalities were seen on muscle biopsy. These findings raise the possibility that there is an intracellular derangement in the muscle fiber responsible for the muscle rippling; further studies are necessary to establish the underlying pathophysiologic condition.

Randomized controlled trial of testosterone in myotonic dystrophy

Because testosterone has an anabolic effect in myotonic dystrophy, we conducted a 12-month, randomized, double-blind therapeutic trial of testosterone enanthate (3 mg/kg/wk) in 40 men with myotonic dystrophy. We evaluated strength by manual muscle tests, quantitative myometry, pulmonary function, and quantitative functional assessment. A sustained, significant elevation of testosterone levels was produced but there was no effect on any measurement of muscle strength. Muscle mass as estimated by creatinine excretion and lean body mass (40K method) increased significantly. We conclude that testosterone does not improve strength in myotonic dystrophy despite increasing muscle mass.

Treatment of muscular dystrophies

1. Specific therapies to cure the muscular dystrophies are not yet available. Therapeutic trials designed on the basis of our understanding of the pathophysiology of these disorders have had only limited success. 2. However, recent investigations in Duchenne muscular dystrophy have identified the abnormal gene and the missing or defective gene product, dystrophin. 3. These discoveries provide information which will lead to more rational and specific therapeutic approaches. 4. The advances in genetic research have led to more effective preventive therapy. Gene mapping has been applied successfully in carrier detection and antenatal diagnosis, and specific gene probes will soon become available for carrier testing for the two most common forms of muscular dystrophy, Duchenne muscular dystrophy and myotonic dystrophy. 5. Supportive therapies for muscular dystrophy patients now include respiratory support for selected patients with chronic respiratory insufficiency. 6. This review will focus on the two most common muscular dystrophies, Duchenne muscular dystrophy and myotonic dystrophy.

Distribution of insulin receptor-like immunoreactivity in the rat forebrain

Previous studies have suggested that insulin may play a role in the hormonal regulation of neurotransmitter metabolisms within the central nervous system. In order to provide additional information to support this hypothesis, we examined the distribution of insulin receptors within the forebrain of adult male rats. Insulin receptors were localized by immunocytochemistry, using an antibody directed against the carboxy-terminus of the beta-subunit of the insulin receptor. The antibody specificity was tested by immunoprecipitation of brain insulin receptors with antiserum and the purity of the receptor-antibody preparation was determined using hormone binding-assays with radiolabeled insulin and insulin-like growth factor-l. Insulin receptor-like immunoreactivity was found in a widespread, but selective, distribution on neurons throughout the rat forebrain. Double-labeling with glial fibrillary acidic protein did not demonstrate any detectable insulin receptor-like immunoreactivity on glial cells. Areas with the highest density of insulin receptor-like immunoreactivity were found in the olfactory bulbs, hypothalamus and median eminence, medial habenula, subthalamic nucleus, subfornical organ, CA 1/2 pyramidal cell layer of the hippocampus and piriform cortex. Double-staining of hypothalamic sections with somatostatin and vasopressin antisera revealed insulin receptor-like immunoreactivity on a subpopulation of somatostatin neurons in the periventricular region and on vasopressin neurons in the supraoptic nucleus. A moderately dense insulin receptor-like immunoreactivity was observed in layers II-IV of cerebral cortex, medial amygdala, reticular thalamic nucleus, zona incerta, and preoptic and septal regions, whereas a low density of insulin receptor-like immunoreactive neurons was found in basolateral amygdala and most thalamic regions. The basal ganglia and most parts of the thalamus were almost devoid of insulin receptor-like immunoreactivity. Our findings provide morphological support for a direct action of insulin on selected regions of the rat forebrain and suggest that the insulin receptor may modulate synaptic transmission or the release of neurotransmitters and peptide hormones in the CNS.

Clinical investigation in Duchenne muscular dystrophy: penicillamine and vitamin E

A double-blind controlled trial of penicillamine and vitamin E against placebo was conducted in 106 boys with Duchenne muscular dystrophy. Nine dropped out of the study after being randomized. Patients were evaluated using an established protocol that measures muscle strength, joint contractures, functional grade, and pulmonary function. The trial lasted for 18 months. The power of the study to detect a 75% slowing of the disease was 0.95 at a P less than 0.05. No therapeutic effect was observed in the patients taking penicillamine and vitamin E as compared with those taking placebo.

Inflammatory myopathies

The inflammatory myopathies are a heterogeneous group of disorders with recent evidence demonstrating differences in clinical features, pathologic changes, pathogenesis, and response to therapy. The inflammatory myopathies generally produce predominantly proximal, symmetric muscle weakness and wasting. Additional criteria for diagnosis include elevated serum muscle enzymes, myopathic features on EMG, and muscle biopsy abnormalities, including muscle fiber necrosis, degeneration, and inflammatory infiltrates. Inclusion body myositis is distinctive in that distal weakness is most commonly equal to or greater than proximal weakness and muscle biopsy reveals rimmed, cytoplasmic vacuoles, eosinophilic inclusions in the cytoplasm, and nucleus and abnormal filamentous structures. Autoimmune mechanisms seem likely to be involved in the pathogenesis of these disorders and viral infection may be etiologically involved in some of these diseases. The differences in the site of immune-mediated damages suggest an angiography in dermatomyositis while direct muscle fiber involvement is more likely in polymyositis and inclusion body myositis. Therapy of these disorders is similar although some, particularly inclusion body myositis, may be particularly resistant to therapy. Prednisone is currently recommended as the first treatment with azathioprine or methotrexate added after 3 months if steroids are ineffective.

Hypertrophy of the calf with S-1 radiculopathy

Occasionally, chronic denervation is associated with enlargement rather than atrophy of muscle tissue. We studied seven patients with S-1 radiculopathy who developed ipsilateral enlargement of the calf. On the basis of results of calf muscle biopsies, it seems likely that the enlargement was due to muscle fiber hypertrophy and atrophy combined with an increase in connective tissue. Computed tomograms of the legs revealed no evidence of tumor. Surgical decompression of the involved S-1 nerve root had no obviously beneficial effect in reducing the calf enlargement. More information is needed to define the natural course of the calf enlargement and to clarify the pathophysiologic processes involved.

Clinical investigation of Duchenne muscular dystrophy. Interesting results in a trial of prednisone

We investigated the effect of high-dose prednisone therapy in 33 boys with Duchenne muscular dystrophy. The drug was given daily in doses of 1.5 mg/kg of body weight (to a maximum of 80 mg) for six months. Muscle strength, joint contractures, timed functional tests, functional ability, and pulmonary function were measured at the beginning and end of the treatment period. The trial was designed using natural history controls, and the power of the study was 0.80 to detect a slowing of 50% in the rate of progression. During the period of the trial, muscle strength, functional grades, timed functional tests, and pulmonary function improved. Contractures followed the expected natural history of the illness.

Clinical investigation of Duchenne muscular dystrophy. A methodology for therapeutic trials based on natural history controls

Between 1979 and 1987 we documented the natural history of Duchenne muscular dystrophy in 170 patients, aged from 3 to 23 years, by making serial measurements in over 5000 individual evaluations. This database makes it possible to design and conduct therapeutic trials using natural history controls. Such trials do not replace the need for randomized placebo-controlled trials of promising agents but they do require fewer patients, are cost-effective, and permit the use of high-risk therapy where toxicity monitoring may be important. Natural history-controlled trials, therefore, may serve as a screening method for new therapeutic agents. Drugs showing a significant benefit can then be evaluated in a randomized controlled trial.

Lack of rapid enhancement of insulin action after oral glucose challenge in myotonic dystrophy

Oral glucose administration to normal humans stimulates insulin release and simultaneously enhances the action of insulin by producing a rapid increase in tissue insulin sensitivity by a mechanism separate from the amount of hormone released. We determined whether insulin-resistant patients with myotonic dystrophy lose the ability to produce the normal rapid increase in tissue insulin action after oral glucose. Nine ambulatory, nonobese men with myotonic dystrophy were studied with 120-min euglycemic insulin infusions (20 mU X m-2 X min-1) given before and after glucose ingestion (4 and 5 patients received 15- and 25-g loads, respectively). Identical studies were performed in nonobese normal volunteers (16 and 13 patients received 15- and 25-g oral glucose loads, respectively). Glucose infusion rates at 20-120 min (GIR20-120) during euglycemic insulin infusions without prior glucose were 2.87 +/- 0.6 mg X kg-1 X min-1 in patients with myotonic dystrophy compared to 4.70 +/- 0.3 mg X kg-1 X min-1 in normal subjects. Euglycemic insulin infusions after glucose ingestion were begun after arterialized blood glucose values had returned to baseline. After glucose ingestion by normal subjects, GIR20-120 increased by 44.4 +/- 7.1% (P less than .0001) and by 46.8 +/- 8.6% (P less than .0002) with 15- and 25-g glucose loads, respectively. GIR20-120 in the nine patients with myotonic dystrophy showed no significant increase after glucose ingestion. These results confirmed the existence of a decrease in whole-body insulin sensitivity in myotonic dystrophy and indicated that the patients lack the normal mechanism that enhances insulin action after oral glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical investigation in Duchenne dystrophy. VI. Double-blind controlled trial of nifedipine

Excessive accumulation of intracellular calcium in Duchenne muscular dystrophy (DMD) may be a necessary step in the process that causes muscle damage in this disease. Because of this possibility, a controlled trial of the calcium channel blocking agent nifedipine was undertaken. One hundred and five patients were randomized and treated in a double-blind manner for 18 months. Muscle strength, contractures, functional ability, cardiopulmonary changes, and laboratory data were monitored. The dose of nifedipine was 0.75-1 mg/kg/day in the first 6 months and 1.5-2 mg/kg/day for the next 12 months. Satisfactory blood levels of nifedipine were attained. The study had a power greater than 0.99 to detect a slowing of the illness to 25% of its original rate of progression. No significant improvement was demonstrated in the treated group. One or more of the frequent mild side effects of flushing, dizziness, and leg edema, often associated with the use of nifedipine in adults, occurred transiently in approximately one-half of the patients in the nifedipine group and in 21% of the placebo group. Four patients died, two on nifedipine and two on placebo. This study demonstrates that nifedipine is safe to administer in children, but that it is without beneficial effect on the course of DMD.

Effect of testosterone on muscle protein synthesis in myotonic dystrophy

Muscle wasting in myotonic dystrophy may result from decreased muscle anabolic processes rather than from increased catabolism. Male patients with myotonic dystrophy often have low levels of circulating androgens, and androgen administration has been shown to increase their muscle mass. We have studied the effect of testosterone enanthate administration (3 mg/kg weekly for 3 months) on muscle and whole body protein synthesis in 6 male patients with myotonic dystrophy. Muscle protein synthesis was estimated from the rate of isotope incorporation into muscle protein obtained by quadriceps muscle biopsy during a primed continuous infusion of L-[1-13C]leucine. Testosterone administration resulted in a significant increase in muscle protein synthesis in all patients. Whole body protein synthesis did not increase, indicating that protein synthesis in other tissues may have declined. Muscle ribonucleic acid content rose significantly in response to testosterone administration, suggesting that testosterone initiated its effect by hormone receptor interaction with muscle nuclei.

Effect of testosterone on whole body amino acid utilization in myotonic dystrophy

Patients with myotonic dystrophy are markedly insulin resistant and have an associated abnormality in the regulation of arterialized amino acid concentrations during euglycemic insulin infusions. We studied the effect of testosterone treatment on whole body amino acid balance in myotonic dystrophy, since it increases muscle mass and muscle protein synthesis rate. Six patients with myotonic dystrophy underwent studies of glucose disposal and amino acid regulation during low dose insulin infusions with maintenance of euglycemia, prior to and after 10 to 13 weeks of testosterone (3 mg/kg/wk). Testosterone increased the insulin-stimulated decline of certain amino acids, but did not improve whole body glucose uptake. The anabolic effect of testosterone is separate from the anabolic effect of insulin.

Insulin resistance and regulation of serum amino acid levels in myotonic dystrophy

To quantify the degree of whole body insulin resistance in patients with myotonic dystrophy and to determine if these same patients display signs of a whole body decrease in the action of insulin on amino acid uptake and glucose disposal, three separate 120 min studies employing the euglycaemic insulin clamp technique (20, 80 and 200 m-units min-1 m-2) were performed on five ambulatory patients with myotonic dystrophy. The results were compared with findings obtained in identical studies in 21 normal volunteers. Myotonic dystrophy patients showed a slower, less marked decline in the serum concentration of insulin sensitive amino acids (threonine, valine, leucine, isoleucine, tyrosine, phenylalanine) during all three insulin infusions compared with normals. The greatest difference occurred at the low physiological elevations of insulin produced by the 20 m-units min-1 m-2 infusion. Alanine levels fell significantly below baseline in patients with myotonic dystrophy after 60 and 120 min of insulin infusion with all three rates of insulin infusion. Normal subjects had only a minimal, insignificant decline in arterialized alanine concentrations during the three different insulin infusions. Creatinine adjusted rates of whole body glucose disposal were 30-40% lower in the myotonic dystrophy group at all three doses of insulin compared with the normals. This demonstrates that their insulin resistance was not due simply to a reduction in muscle mass. The overall pattern of findings in these studies of patients with myotonic dystrophy indicates that there is a whole body derangement in the regulation of circulating amino acid levels by insulin as well as a marked decrease in the action of this hormone in stimulating glucose uptake by target tissues.

Enhancement of insulin action after oral glucose ingestion

Previous investigations in normal humans and rats have shown an increase in insulin sensitivity and binding affinity of adipocytes isolated 1-3 h after glucose ingestion. To determine whether a rapid enhancement of the action of insulin follows glucose ingestion in vivo, the present studies have utilized 120-min 20 mU/m2 X min euglycemic insulin infusions before and after 7.5-, 15-, 25-, and 100-g oral glucose loads. Euglycemic insulin infusions after the carbohydrate challenge were begun after arterialized blood glucose and insulin values had returned to baseline. After 15- and 25-g oral glucose loads during the 20-120-min interval of insulin infusion, glucose infusion rates increased by 44 +/- 6% (P less than 0.0001) and 47 +/- 9% (P less than 0.0002), respectively. No significant differences in arterialized glucose or insulin values existed between basal and post-glucose insulin infusions. In addition, no significant differences in hepatic glucose production or counter-regulatory hormone levels were found between basal and post-glucose insulin infusions. Control infusion studies including subjects who ingested saline or mannitol failed to show an increase in insulin action. Studies were carried out to mimic the insulin curve seen after 15- and 25-g oral glucose loads. Euglycemic insulin infusions after these insulin simulation studies show a 34 +/- 7% enhancement compared to baseline euglycemic insulin infusions. These results demonstrate a rapid enhancement of insulin action after oral glucose challenge in normal humans. The insulin simulation studies suggest that insulin itself either directly or through release of another factor acts on muscle to increase insulin sensitivity. The increase in insulin action demonstrated in these investigations may represent an important regulatory mechanism to modulate tissue insulin sensitivity.

Lack of relationship of hypogonadism to muscle wasting in myotonic dystrophy

Myotonic dystrophy is frequently associated with testicular atrophy. Since androgens may play a role in the maintenance of muscle mass, we have studied the levels of plasma testosterone and gonadotropins and of urinary 17-ketosteroids in 22 men with myotonic dystrophy, 36 normal men, and 16 men (control group) with muscle wasting. Results were correlated with muscle mass as estimated by creatinine excretion and total body potassium. Patients with myotonic dystrophy had significantly lower testosterone and higher gonadotropin levels than normal, and these changes were progressive in longitudinal studies. Testosterone levels were also lower than normal in disease control subjects. There was no correlation between low testosterone levels and diminished muscle mass in either myotonic dystrophy or disease control patients. The low plasma concentration of testosterone in men with myotonic dystrophy and other neuromuscular diseases does not appear to be directly related to their muscle wasting. This study does not exclude the possibility that an alteration in testosterone receptor or tissue effects may contribute to a loss of muscle tissue.

Intravenous glucose infusion fails to alter monocyte insulin-binding affinity in normal subjects

Previous investigations have demonstrated an increase in monocyte insulin receptor affinity two and five hours following oral carbohydrate loading. The present studies were undertaken to see if intravenous (IV) glucose challenge provokes a similar increase in monocyte insulin binding affinity. 25 grams of glucose were given to 10 lean normals and monocytes were isolated for 125I-insulin tracer binding studies (8.4 X 10(-10) M) at 0, 1 and 5 hours after glucose loading. The mean data show that monocytes develop a small, statistically insignificant increase in insulin-binding affinity one hour after intravenous glucose (mean +/- SEM, 7.28 +/- 1.06 ng/ml compared to basal 50% insulin displacement value, B50, of 9.25 +/- 1.62 ng/ml). B50 values demonstrated no increase in binding affinity at five hours (10.77 +/- 2.22 ng/ml). Prior studies have shown a 50 to 70% decrease in B50 following oral glucose, indicating a rapid increase in receptor binding affinity after carbohydrate ingestion. In contrast the present studies have shown that after IV glucose six normals had no decrease in B50 at one or five hours, while the remaining four normals had a 35% decrease at one hour but no decrease at five hours. Intravenous glucose loading, unlike an oral carbohydrate challenge, fails to provoke an acute, consistent increase in monocyte insulin binding affinity at these time points. Elevations in plasma glucose and insulin do not by themselves induce the acute increase in receptor affinity.

Myotonic dystrophy: effect of testosterone on total body potassium and on creatinine excretion

Muscle wasting in myotonic dystrophy appears to reflect impaired anabolism rather than accelerated catabolism. We therefore investigated the effects of testosterone, an anabolic hormone, on muscle mass as estimated by creatinine excretion and total body potassium in nine patients with myotonic dystrophy. Weekly injections of testosterone for 10 to 13 weeks increased both creatinine excretion (19%) and total body potassium (16%) in all patients. Metabolic balance data showed a confirmatory accretion of nitrogen, potassium, and phosphorus. Because testosterone increases indirect measures of muscle mass, it may deserve a therapeutic trial in myotonic dystrophy.