Effect of vitamin B6 supplementation in McArdle's disease: a strategic case study

Mitochondrial Redox Status Regulates Glycogen Metabolism via Glycogen Phosphorylase Activity

Mitochondria and glycogen are co-distributed in skeletal muscles to regulate the metabolic status. Mitochondria are also redox centers that regulate the muscle function during exercise. However, the pathophysiological relationship between the mitochondrial redox status and glycogen metabolism in the muscle remains unclear. In the present study, we examined the pathological effects of mitochondrial dysfunction induced by mitochondrial superoxide dismutase (SOD2) depletion on glycogen metabolism. We found that muscle glycogen was significantly accumulated in association with motor dysfunction in mice with a muscle-specific SOD2 deficiency. Muscle glycogen phosphorylase (GP-M) activity, which is a key enzyme for glycogen degradation at times when energy is needed (e.g., during exercise), was significantly decreased in the mutant muscle. Moreover, the GP-M activity on normal muscle sections decreased after treatment with paraquat, a superoxide generator. In contrast, treatment with antioxidants reversed the GP-M activity and motor disturbance of the mutant mice, indicating that GP-M activity was reversibly regulated by the redox balance. These results demonstrate that the maintenance of the mitochondrial redox balance regulates glycogen metabolism via GP-M activity.

Preclinical Research in Glycogen Storage Diseases: A Comprehensive Review of Current Animal Models

GSD are a group of disorders characterized by a defect in gene expression of specific enzymes involved in glycogen breakdown or synthesis, commonly resulting in the accumulation of glycogen in various tissues (primarily the liver and skeletal muscle). Several different GSD animal models have been found to naturally present spontaneous mutations and others have been developed and characterized in order to further understand the physiopathology of these diseases and as a useful tool to evaluate potential therapeutic strategies. In the present work we have reviewed a total of 42 different animal models of GSD, including 26 genetically modified mouse models, 15 naturally occurring models (encompassing quails, cats, dogs, sheep, cattle and horses), and one genetically modified zebrafish model. To our knowledge, this is the most complete list of GSD animal models ever reviewed. Importantly, when all these animal models are analyzed together, we can observe some common traits, as well as model specific differences, that would be overlooked if each model was only studied in the context of a given GSD.

Novel Asp511Thr mutation in McArdle disease with acute kidney injury caused by rhabdomyolysis

McArdle disease (glycogen storage disease type V) is a rare hereditary metabolic myopathy. It can be overlooked clinically because it often presents as chronic asymptomatic hypercreatine phosphokinasemia (hyperCKemia). However, vigorous exercise or infections can trigger severe rhabdomyolysis. We present the case of a patient with long-term idiopathic hyperCKemia who, after contracting an upper respiratory tract infection, developed severe rhabdomyolysis and acute kidney injury. Upon hemodialysis, his renal function recovered and CK levels fell to below baseline, and maintenance therapy with vitamin B6 was also started. A molecular diagnosis of McArdle disease was subsequently made. Whole-exome sequencing revealed homozygous c1538delG (p.Asp511Thr fs*28) mutations in the PYGM gene, which was a novel mutation. Therefore, when investigating idiopathic hyperCKemia, glycogen storage disorders should also be considered.

Genes and exercise intolerance: insights from McArdle disease

McArdle disease (glycogen storage disease type V) is caused by inherited deficiency of a key enzyme in muscle metabolism, the skeletal muscle-specific isoform of glycogen phosphorylase, “myophosphorylase,” which is encoded by the PYGM gene. Here we review the main pathophysiological, genotypic, and phenotypic features of McArdle disease and their interactions. To date, moderate-intensity exercise (together with pre-exercise carbohydrate ingestion) is the only treatment option that has proven useful for these patients. Furthermore, regular physical activity attenuates the clinical severity of McArdle disease. This is quite remarkable for a monogenic disorder that consistently leads to the same metabolic defect at the muscle tissue level, that is, complete inability to use muscle glycogen stores. Further knowledge of this disorder would help patients and enhance understanding of exercise metabolism as well as exercise genomics. Indeed, McArdle disease is a paradigm of human exercise intolerance and PYGM genotyping should be included in the genetic analyses that might be applied in the coming personalized exercise medicine as well as in future research on genetics and exercise-related phenotypes.

McArdle disease: a unique study model in sports medicine

McArdle disease is arguably the paradigm of exercise intolerance in humans. This disorder is caused by inherited deficiency of myophosphorylase, the enzyme isoform that initiates glycogen breakdown in skeletal muscles. Because patients are unable to obtain energy from their muscle glycogen stores, this disease provides an interesting model of study for exercise physiologists, allowing insight to be gained into the understanding of glycogen-dependent muscle functions. Of special interest in the field of muscle physiology and sports medicine are also some specific (if not unique) characteristics of this disorder, such as the so-called 'second wind' phenomenon, the frequent exercise-induced rhabdomyolysis and myoglobinuria episodes suffered by patients (with muscle damage also occurring under basal conditions), or the early appearance of fatigue and contractures, among others. In this article we review the main pathophysiological features of this disorder leading to exercise intolerance as well as the currently available therapeutic possibilities. Patients have been traditionally advised by clinicians to refrain from exercise, yet sports medicine and careful exercise prescription are their best allies at present because no effective enzyme replacement therapy is expected to be available in the near future. As of today, although unable to restore myophosphorylase deficiency, the 'simple' use of exercise as therapy seems probably more promising and practical for patients than more 'complex' medical approaches.

McArdle Disease: Update of Reported Mutations and Polymorphisms in the PYGM Gene

McArdle disease is an autosomal-recessive disorder caused by inherited deficiency of the muscle isoform of glycogen phosphorylase (or "myophosphorylase"), which catalyzes the first step of glycogen catabolism, releasing glucose-1-phosphate from glycogen deposits. As a result, muscle metabolism is impaired, leading to different degrees of exercise intolerance. Patients range from asymptomatic to severely affected, including in some cases, limitations in activities of daily living. The PYGM gene codifies myophosphoylase and to date 147 pathogenic mutations and 39 polymorphisms have been reported. Exon 1 and 17 are mutational hot-spots in PYGM and 50% of the described mutations are missense. However, c.148C>T (commonly known as p.R50X) is the most frequent mutation in the majority of the studied populations. No genotype-phenotype correlation has been reported and no mutations have been described in the myophosphorylase domains affecting the phosphorylated Ser-15, the 280's loop, the pyridoxal 5'-phosphate, and the nucleoside inhibitor binding sites. A newly generated knock-in mouse model is now available, which renders the main clinical and molecular features of the disease. Well-established methods for diagnosing patients in laboratories around the world will shorten the frequent ∼20-year period stretching from first symptoms appearance to the genetic diagnosis.

Sodium valproate increases the brain isoform of glycogen phosphorylase: looking for a compensation mechanism in McArdle disease using a mouse primary skeletal-muscle culture in vitro

McArdle disease, also termed ‘glycogen storage disease type V’, is a disorder of skeletal muscle carbohydrate metabolism caused by inherited deficiency of the muscle-specific isoform of glycogen phosphorylase (GP-MM). It is an autosomic recessive disorder that is caused by mutations in the PYGM gene and typically presents with exercise intolerance, i.e. episodes of early exertional fatigue frequently accompanied by rhabdomyolysis and myoglobinuria. Muscle biopsies from affected individuals contain subsarcolemmal deposits of glycogen. Besides GP-MM, two other GP isoforms have been described: the liver (GP-LL) and brain (GP-BB) isoforms, which are encoded by the PYGL and PYGB genes, respectively; GP-BB is the main GP isoform found in human and rat foetal tissues, including the muscle, although its postnatal expression is dramatically reduced in the vast majority of differentiated tissues with the exception of brain and heart, where it remains as the major isoform. We developed a cell culture model from knock-in McArdle mice that mimics the glycogen accumulation and GP-MM deficiency observed in skeletal muscle from individuals with McArdle disease. We treated mouse primary skeletal muscle cultures in vitro with sodium valproate (VPA), a histone deacetylase inhibitor. After VPA treatment, myotubes expressed GP-BB and a dose-dependent decrease in glycogen accumulation was also observed. Thus, this in vitro model could be useful for high-throughput screening of new drugs to treat this disease. The immortalization of these primary skeletal muscle cultures could provide a never-ending source of cells for this experimental model. Furthermore, VPA could be considered as a gene-expression modulator, allowing compensatory expression of GP-BB and decreased glycogen accumulation in skeletal muscle of individuals with McArdle disease.

Summary: Use of this in vitro model showed that sodium valproate (VPA) can reverse the muscle phenotype from a McArdle-like to a normal histological and biochemical profile.

The pathogenomics of McArdle disease--genes, enzymes, models, and therapeutic implications

Numerous biomedical advances have been made since Carl and Gerty Cori discovered the enzyme phosphorylase in the 1940s and the Scottish physician Brian McArdle reported in 1951 a previously 'undescribed disorder characterized by a gross failure of the breakdown in muscle of glycogen'. Today we know that this disorder, commonly known as 'McArdle disease', is caused by inherited deficiency of the muscle isoform of glycogen phosphorylase (GP). Here we review the main aspects of the 'pathogenomics' of this disease including, among others: the spectrum of mutations in the gene (PYGM) encoding muscle GP; the interplay between the different tissue GP isoforms in cellular cultures and in patients; what can we learn from naturally occurring and recently laboratory-generated animal models of the disease; and potential therapies.

Feasibility of resistance training in adult McArdle patients: clinical outcomes and muscle strength and mass benefits

We analyzed the effects of a 4-month resistance (weight lifting) training program followed by a 2-month detraining period in 7 adult McArdle patients (5 female) on: muscle mass (assessed by DXA), strength, serum creatine kinase (CK) activity and clinical severity. Adherence to training was ≥84% in all patients and no major contraindication or side effect was noted during the training or strength assessment sessions. The training program had a significant impact on total and lower extremities’ lean mass (P < 0.05 for the time effect), with mean values increasing with training by +855 g (95% confidence interval (CI): 30, 1679) and +547 g (95%CI: 116, 978), respectively, and significantly decreasing with detraining. Body fat showed no significant changes over the study period. Bench press and half-squat performance, expressed as the highest value of average muscle power (W) or force (N) in the concentric-repetition phase of both tests showed a consistent increase over the 4-month training period, and decreased with detraining. Yet muscle strength and power detraining values were significantly higher than pre-training values, indicating that a training effect was still present after detraining. Importantly, all the participants, with no exception, showed a clear gain in muscle strength after the 4-month training period, e.g., bench press: +52 W (95% CI: 13, 91); half-squat: +173 W (95% CI: 96, 251). No significant time effect (P > 0.05) was noted for baseline or post strength assessment values of serum CK activity, which remained essentially within the range reported in our laboratory for McArdle patients. All the patients changed to a lower severity class with training, such that none of them were in the highest disease severity class (3) after the intervention and, as such, they did not have fixed muscle weakness after training. Clinical improvements were retained, in all but one patient, after detraining, such that after detraining all patients were classed as class 1 for disease severity.

Pharmacological and nutritional treatment for McArdle disease (Glycogen Storage Disease type V)

Background McArdle disease (Glycogen Storage Disease type V) is caused by an absence of muscle phosphorylase leading to exercise intolerance,myoglobinuria rhabdomyolysis and acute renal failure. This is an update of a review first published in 2004.Objectives To review systematically the evidence from randomised controlled trials (RCTs) of pharmacological or nutritional treatments for improving exercise performance and quality of life in McArdle disease.Search methods We searched the Cochrane Neuromuscular Disease Group Specialized Register, CENTRAL, MEDLINE and EMBASE on 11 August 2014.Selection criteria We included RCTs (including cross-over studies) and quasi-RCTs. We included unblinded open trials and individual patient studies in the discussion. Interventions included any pharmacological agent or nutritional supplement. Primary outcome measures included any objective assessment of exercise endurance (for example aerobic capacity (VO2) max, walking speed, muscle force or power and fatigability). Secondary outcome measures included metabolic changes (such as reduced plasma creatine kinase and a reduction in the frequency of myoglobinuria), subjective measures (including quality of life scores and indices of disability) and serious adverse events.Data collection and analysis Three review authors checked the titles and abstracts identified by the search and reviewed the manuscripts. Two review authors independently assessed the risk of bias of relevant studies, with comments from a third author. Two authors extracted data onto a specially designed form.Main results We identified 31 studies, and 13 fulfilled the criteria for inclusion. We described trials that were not eligible for the review in the Discussion. The included studies involved a total of 85 participants, but the number in each individual trial was small; the largest treatment trial included 19 participants and the smallest study included only one participant. There was no benefit with: D-ribose,glucagon, verapamil, vitamin B6, branched chain amino acids, dantrolene sodium, and high-dose creatine. Minimal subjective benefit was found with low dose creatine and ramipril only for patients with a polymorphism known as the D/Dangiotens in converting enzyme(ACE) phenotype. A carbohydrate-rich diet resulted in better exercise performance compared with a protein-rich diet. Two studies of oral sucrose given at different times and in different amounts before exercise showed an improvement in exercise performance. Four studies reported adverse effects. Oral ribose caused diarrhoea and symptoms suggestive of hypoglycaemia including light-headedness and hunger. In one study, branched chain amino acids caused a deterioration of functional outcomes. Dantrolene was reported to cause a number of adverse effects including tiredness, somnolence, dizziness and muscle weakness. Low dose creatine (60 mg/kg/day) did not cause side-effects but high-dose creatine (150 mg/kg/day) worsened the symptoms of myalgia.Authors' conclusions Although there was low quality evidence of improvement in some parameters with creatine, oral sucrose, ramipril and a carbohydrate rich diet, none was sufficiently strong to indicate significant clinical benefit.

Adeno-associated virus-mediated gene therapy for metabolic myopathy

Metabolic myopathies are a diverse group of rare diseases in which impaired breakdown of stored energy leads to profound muscle dysfunction ranging from exercise intolerance to severe muscle wasting. Metabolic myopathies are largely caused by functional deficiency of a single gene and are generally subcategorized into three major types of metabolic disease: mitochondrial, lipid, or glycogen. Treatment varies greatly depending on the biochemical nature of the disease, and unfortunately no definitive treatments exist for metabolic myopathy. Since this group of diseases is inherited, gene therapy is being explored as an approach to personalized medical treatment. Adeno-associated virus-based vectors in particular have shown to be promising in the treatment of several forms of metabolic myopathy. This review will discuss the most recent advances in gene therapy efforts for the treatment of metabolic myopathies.

[Metabolic and mitochondrial myopathies]

Metabolic myopathies include a broad group of diseases involving inherited enzyme defects in the various metabolic pathways and skeletal musculature. They show an extensive phenotypic variability of symptoms and different ages of manifestation. Symptoms often included intolerance to duress or permanent paresis. Some forms of metabolic myopathy, in particular mitochondriopathy, are associated with multsystemic organ participation. The diagnostics must be adjusted to individual cases and carried out in stages. Primary investigations should include blood parameters (e.g. creatine kinase measurement, muscle load tests and determination of the acylcarnitine spectrum) and a second step includes muscle biopsy for histological and enzyme investigations and special molecular genetic tests although the causative enzyme defect cannot be clarified in every case. On the other hand by means of a thorough investigation it is particularly important in patients with load intolerance to differentiate between other causes, in particular psychosomatic diseases. If this is not done there is a danger of classifying the symptoms of a metabolic myopathy as a somatoform disorder. Therapy is mostly symptom-oriented as Pompe disease is the only one which can be treated with enzyme replacement therapy.

Confirmation of the efficacy of vitamin B6 supplementation for McArdle disease by follow-up muscle biopsy

No effective treatment for McArdle disease exists.We report a Japanese patient with McArdle disease who was treated with vitamin B(6) supplementation (60-90 mg/day). After treatment, increased muscle phosphorylase activity was confirmed by follow-up muscle biopsy (3.8 times higher than pretreatment levels). Increased lactate levels were seen on the forearm exercise test, and regular work activities could be resumed. Vitamin B(6) supplementation can enhance residual phosphorylase activity and improve insufficient anaerobic glycolysis of skeletal muscle.

Pharmacological and nutritional treatment for McArdle disease (Glycogen Storage Disease type V)

BACKGROUND

McArdle disease (Glycogen Storage Disease type V) is caused by an absence of muscle phosphorylase leading to exercise intolerance, myoglobinuria rhabdomyolysis and acute renal failure.

OBJECTIVES

To review systematically the evidence from randomized controlled trials of pharmacological or nutritional treatments for improving exercise performance and quality of life in McArdle disease.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group Specialised Register (17 May 2010), the Cochrane Central Register of Controlled Trials (Issue 2, 2010 in The Cochrane Library), MEDLINE (January 1966 to May 2010) and EMBASE (January 1980 to May 2010) using the search terms 'McArdle disease', 'Glycogen Storage Disease type V' and 'muscle phosphorylase deficiency'.

SELECTION CRITERIA

We included randomized controlled trials (including cross-over studies) and quasi-randomised trials. Unblinded open trials and individual patient studies were included in the discussion. Interventions included any pharmacological agent or nutritional supplement. Primary outcome measures included any objective assessment of exercise endurance (for example aerobic capacity (VO(2)) max, walking speed, muscle force or power and fatigability). Secondary outcome measures included metabolic changes (such as reduced plasma creatine kinase and a reduction in the frequency of myoglobinuria), subjective measures (including quality of life scores and indices of disability) and serious adverse events.

DATA COLLECTION AND ANALYSIS

Three review authors checked the titles and abstracts identified by the search and reviewed the manuscripts. In the first review two authors (RQ and RB) independently assessed methodological quality of relevant studies and extracted data onto a specially designed form. In this update methodological quality of data was assessed by RQ and AM with comments from BS.

MAIN RESULTS

We identified 31 studies,13 fulfilled the criteria for inclusion. Excluded trials are included in the Discussion. The largest treatment trial included 19 subjects. There was no benefit with: D-ribose, glucagon, verapamil, vitamin B(6), branched chain amino acids, dantrolene sodium, and high dose creatine. Minimal benefit was found with low dose creatine and ramipril only for patients with a polymorphism known as the D/D angiotensin converting enzyme (ACE) phenotype. A carbohydrate-rich diet resulted in better exercise performance compared with a protein-rich diet. Two studies of oral sucrose given at different times and in different amounts before exercise showed an improvement in exercise performance.

AUTHORS' CONCLUSIONS

Although there was low quality evidence of improvement in some parameters with creatine, oral sucrose, ramipril and a carbohydrate rich diet, none was sufficiently strong to indicate significant clinical benefit.

A case of McArdle disease: efficacy of vitamin B6 on fatigability and impaired glycogenolysis

McArdle disease is a glycogenetic myopathy caused by a deficit of myophosphorylase inherited in an autosomal recessive pattern. Here, we report a case of McArdle disease in which fatigability was the only subjective complaint. Objective neurological findings were normal except for very mild muscle weakness in limbs and an elevated serum creatine kinase level. Ischemic forearm exercise test showed deficient glycogenolysis. In the muscle biopsy specimen, periodic acid Schiff (PAS) stained subsarcolemmal glycogen was increased and the muscle phosphorylase A activity was decreased. After administration of vitamin B6, fatigability was diminished and ischemic forearm exercise test showed improved glycogenolysis. Vitamin B6 may be beneficial for McArdle disease, especially for its easy fatigability.

Effect of energy compound on skeletal muscle strain injury and regeneration in rats

This study was designed to determine whether the supplement of energy compound could attenuate strain-induced damage to skeletal muscle in rats. Energy compound is a saline mixture of the following ingredients: ATP (10mg), Coenzyme-A (50 units), Coenzyme-Q(10) (50mg), Cytochrome C (30 mg) and Vitamin B(6) (50mg). Experimental animals were injured in right gastrocnemius muscles by a strain injury model. Energy compound groups were given energy compound 10 ml/kg body weight per day since injured, while saline groups were given saline at the same dose. And a sham operation was performed on the right hindlimb of control group. Plasma was centrifuged to measure lactate dehydrogenase (LDH), lactic acid (La) and creatine kinase (CK) on 3, 7 and 14 d post injury. Muscles were removed and fixed for histology observation and immunohistochemistry assay of desmin and vimentin. The results showed a similar tendency of plasma CK, La and LDH in saline and energy compound groups, while the lower level was found in the energy-compound group. The histological examination of muscle sections revealed a lower degree of damage in the energy compound group in which the expression levels of desmin and vimentin were higher than in the saline group. It is suggested that energy compound supplement may attenuate strain-induced muscle damage and facilitate its regeneration.

Pharmacological and nutritional treatment for McArdle disease (Glycogen Storage Disease type V)

BACKGROUND

McArdle disease (Glycogen Storage Disease type V) is caused by the absence of the glycolytic enzyme, muscle phosphorylase. People present with exercise-induced pain, cramps, fatigue, and myoglobinuria, which can result in acute renal failure if it is severe.

OBJECTIVES

To systematically review the evidence from randomised controlled trials of pharmacological or nutritional treatments in improving exercise performance and quality of life in McArdle disease.

SEARCH STRATEGY

We updated the review by searching the Cochrane Neuromuscular Disease Group Trials Register (November 2007), MEDLINE (January 1966 to November 2007) and EMBASE (January 1980 to November 2007) using the search terms 'McArdle disease' and its synonym 'Glycogen Storage Disease type V'.

SELECTION CRITERIA

We included randomised controlled trials (including crossover studies) and quasi-randomised trials. Open trials and individual patient studies with no participant or observer blinding were included in the discussion. Types of interventions included any pharmacological agent or micronutrient or macronutrient supplementation. Primary outcome measures included any objective assessment of exercise endurance (for example aerobic capacity (VO(2)) max, walking speed, muscle force or power and improvement in fatiguability). Secondary outcome measures included metabolic changes (such as reduced plasma creatine kinase activity and a reduction in the frequency of myoglobinuria), subjective measures (including quality of life scores and indices of disability) and serious adverse events.

DATA COLLECTION AND ANALYSIS

Three review authors checked the titles and abstracts identified by the search and reviewed the manuscripts. Two review authors (RQ and RB) independently assessed methodological quality of the full text of potentially relevant studies and extracted data onto a specially designed form.

MAIN RESULTS

We reviewed 24 studies. Twelve trials fulfilled the criteria for inclusion, with two being first identified in this update. The 12 excluded trials are included in the discussion. The largest treatment trial included 19 cases. The other trials included fewer than 12 cases. As there were only single trials for a given intervention we were unable to undertake a meta-analysis.

AUTHORS' CONCLUSIONS

There is no evidence of significant benefit from any specific nutritional or pharmacological treatment in McArdle disease. In one small trial low dose creatine produced slight benefit but high dose creatine caused myalgia. Ingestion of oral sucrose immediately before exercise reduced perceived ratings of exertion and heart rate and improved exercise tolerance. This treatment will not influence sustained or unexpected exercise and may cause significant weight gain. A carbohydrate rich diet did benefit patients. Because of the rarity of McArdle disease, there is a need to develop international multicentre collaboration and standardised assessment protocols for future treatment trials.

Molecular genetics of McArdle's disease

This review highlights recent advances in our understanding of McArdle's disease, including the mechanisms involved in the regulation of the clinical phenotype. The latest molecular genetic studies have demonstrated the genetic heterogeneity of the disorder, with more than 65 mutations identified to date. There is not a specific treatment for McArdle's disease, but some nutritional treatments in combination with aerobic conditioning could improve the quality of life in most patients.

Aerobic conditioning: An effective therapy in McArdle's disease

OBJECTIVE

Susceptibility to exertional cramps and rhabdomyolysis in myophosphorylase deficiency (McArdle's disease [MD]) may lead patients to shun exercise. However, physical inactivity may worsen exercise intolerance by further reducing the limited oxidative capacity caused by blocked glycogenolysis. We investigated whether aerobic conditioning can safely improve exercise capacity in MD.

METHODS

Eight MD patients (4 men and 4 women; age range, 33-61 years) pedalled a cycle ergometer for 30 to 40 minutes a day, 4 days a week, for 14 weeks, at an intensity corresponding to 60 to 70% of maximal heart rate. We monitored serum creatine kinase levels; changes in peak cycle work, oxygen uptake, and cardiac output; presence and magnitude of a spontaneous and glucose-induced second wind; and citrate synthase and beta-hydroxyacyl coenzyme A dehydrogenase enzyme activities in quadriceps muscle.

RESULTS

The prescribed exercise program increased average work capacity (36%), oxygen uptake (14%), cardiac output (15%), and citrate synthase and beta-hydroxyacyl coenzyme A dehydrogenase enzyme levels (80 and 62%, respectively) without causing pain or cramping or increasing serum creatine kinase. A spontaneous and glucose-induced second wind was present and was of similar magnitude in each patient before and after training.

INTERPRETATION

Moderate aerobic exercise is an effective means of improving exercise capacity in MD by increasing circulatory delivery and mitochondrial metabolism of bloodborne fuels.

Recurrent Rhabdomyolysis in a Collegiate Athlete

PURPOSE

Hereditary metabolic disorders can cause rhabdomyolysis in athletes. Team physicians should be aware of the presentation, workup, and management of the most common of these disorders, carnitine palmitoyltransferase (CPT) II deficiency and muscle phosphorylase deficiency.

METHODS

The case of a collegiate athlete with recurrent bouts of rhabdomyolysis is presented, and the diagnostic workup is discussed.

RESULTS

The patient described in this case has CPT II deficiency. The diagnosis and management of CPT II deficiency and muscle phosphorylase deficiency (McArdle's disease) are discussed.

CONCLUSION

Athletes with rhabdomyolysis, in the absence of an obvious cause such as drug toxicity, severe trauma, or excessive exercise, should be evaluated for the presence of a metabolic myopathy.

What can metabolic myopathies teach us about exercise physiology?

Exercise physiologists are interested in metabolic myopathies because they demonstrate how knocking out a component of a specific biochemical pathway can alter cellular metabolism. McArdle's disease (myophosphorylase deficiency) has often been studied in exercise physiology to demonstrate the influence of removing the major anaerobic energy supply to skeletal muscle. Studies of patients with McArdle's disease have shown the increased reliance on blood-borne fuels, the importance of glycogen to maximal aerobic capacity, and the use of nutritional strategies to bypass metabolic defects. Myoadenylate deaminase deficiency is the most common metabolic enzyme deficiency in human skeletal muscle. It is usually compensated for endogenously and does not have a major influence on high-energy power output. Nutritional interventions such as carbohydrate loading and carbohydrate supplementation during exercise are essential components of therapy for patients with fatty acid oxidation defects. Cases of mitochondrial myopathies illustrate the importance of peripheral oxygen extraction for maximal aerobic capacity and show how both exercise and nutritional interventions can partially compensate for these mutations. In summary, metabolic myopathies provide important insights into regulatory and nutritional aspects of the major biochemical pathways of intermediary metabolism in human skeletal muscle. Key words: myoadenylate deaminase deficiency, MELAS syndrome, McArdle's disease, mitochondrial disease, inborn errors of metabolism.

The thickness of the adductor pollicis muscle reflects the muscle compartment and may be used as a new anthropometric parameter for nutritional assessment

PURPOSE OF REVIEW

Contraction of the adductor pollicis muscle after electrical stimulation (electromyogram) or dynamometry (hand-grip tests) has been evaluated in a variety of clinical conditions as a parameter to assess nutritional status. However, adductor pollicis muscle thickness has not been investigated as an anthropometric parameter.

RECENT FINDINGS

Prolonged immobilization and non-use of lower and upper limb muscles causes atrophy. Adductor pollicis muscle function is normal in patients with stable chronic obstructive pulmonary disease and multiple sclerosis, whereas the musculature of lower limbs suffers more pronounced functional alterations. Structure and function are relatively preserved in upper limb muscles, probably because of the maintenance of some daily activities involving the arms. Inactivity as a result of a reduction in daily activities is probably the driving factor for these changes. Forearm immobilization for 21 days caused no significant change in muscle morphology, but caused a deterioration in muscle function. Virtually all routinely developed activities requiring opposition of the thumb muscle and repetitive exercise of one muscle group for a given period of time maintain muscle size and function. Apathy is often observed as malnutrition progresses, reducing daily working activity and aggravating adductor pollicis muscle loss besides the muscular catabolism caused by disease.

SUMMARY

This study provides the first estimates of adductor pollicis muscle thickness in normal healthy individuals. The adductor pollicis muscle has a positive correlation with anthropometric variables that estimate muscle mass, but fails to correlate with parameters that estimate fat mass. This measurement is now being evaluated as an anthropometric parameter in clinical studies.

Pharmacological and nutritional treatment for McArdle's disease (Glycogen Storage Disease type V)

BACKGROUND

McArdle's disease (Glycogen Storage Disease type V) is caused by the absence of the glycolytic enzyme, muscle phosphorylase. Patients present with exercise-induced pain, cramps, fatigue, myoglobinuria and acute renal failure, which can ensue if the myoglobinuria is severe.

OBJECTIVES

To systematically review the evidence from randomised controlled trials of pharmacological or nutritional treatments in improving exercise performance and quality of life in McArdle's disease.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group register (searched December 2001 and updated in December 2003), MEDLINE (January 1966 to December 2003) and EMBASE (January 1980 to December 2003) using the search term 'McArdle's disease and it's synonym 'Glycogen Storage Disease type V'.

SELECTION CRITERIA

We included randomised controlled trials (including crossover studies) and quasi-randomised trials. Open trials and individual patient studies with no patient or observer blinding were included in the discussion but not the review. Types of interventions included any pharmacological agent or micronutrient or macronutrient supplementation. Primary outcome measures included any objective assessment of exercise endurance (for example VO2 max, walking speed, muscle force/power and improvement in fatiguability). Secondary outcome measures included metabolic changes (such as reduced plasma creatine kinase activity and a reduction in the frequency of myoglobinuria); subjective measures (including quality of life scores and indices of disability); and serious adverse events.

DATA COLLECTION AND ANALYSIS

Two reviewers checked the titles and abstracts identified by the search, independently assessed methodological quality of the full text of potentially relevant studies and extracted data onto a specially designed form.

MAIN RESULTS

We reviewed 20 trials. Ten trials fulfilled the criteria for inclusion and ten trials were included in the discussion. The largest treatment trial included 19 cases, the other trials included fewer than 12 cases. As there were only single trials for a given intervention we were unable to undertake a meta-analysis.

REVIEWERS' CONCLUSIONS

It is not yet possible to recommend any specific treatment for McArdle's disease. Low dose creatine supplementation was shown to demonstrate a statistically significant benefit, albeit modest, in ischaemic exercise in a small number of patients. Ingestion of oral sucrose immediately prior to exercise reduces perceived ratings of exertion and heart rate and improves exercise tolerance. This treatment will not influence sustained or unexpected exercise and may cause significant weight gain. Because of the rarity of McArdle's disease, there is a need to develop multicentre collaboration and standardised assessment protocols for future treatment trials.

The effect of oral sucrose on exercise tolerance in patients with McArdle's disease

BACKGROUND

Energy metabolism in muscles relies predominantly on the breakdown of glycogen early in exercise. In patients with McArdle's disease, blocked glycogenolysis in muscles results in low exercise tolerance and can lead to muscle injury, particularly in the first minutes of exercise. We hypothesized that ingesting sucrose before exercise would increase the availability of glucose and would therefore improve exercise tolerance in patients with McArdle's disease.

METHODS

In a single-blind, randomized, placebo-controlled crossover study, 12 patients with McArdle's disease drank 660 ml of a beverage that had been sweetened with artificial sweeteners (placebo) or with 75 g of sucrose after an overnight fast. Thirty to 40 minutes later, the patients rode a stationary bicycle at a constant workload for 15 minutes while the heart rate, level of perceived exertion, and venous blood glucose levels were monitored.

RESULTS

Supplemental sucrose increased the mean plasma glucose level by more than 36 mg per deciliter (2.0 mmol per liter) and resulted in a marked improvement in exercise tolerance in all patients. The mean (+/-SE) heart rate dropped by a maximum of 34+/-3 beats per minute (P<0.001), and the level of perceived exertion fell dramatically when the patients ingested glucose as compared with when they received the placebo.

CONCLUSIONS

This study suggests that the ingestion of sucrose before exercise can markedly improve exercise tolerance in patients with McArdle's disease. The treatment takes effect during the time when muscle injury commonly develops in these patients. In addition to increasing the patients' exercise capacity and sense of well-being, the treatment may protect against exercise-induced rhabdomyolysis.

Carbohydrate oxidation disorders of skeletal muscle

PURPOSE OF REVIEW

The major energy sources for muscle contraction are glycogen, glucose and fatty acids, and defects in their oxidative pathways cause metabolic myopathies. Eleven specific enzyme deficiencies of carbohydrate oxidation affect skeletal muscle alone or in combination with other tissues, such as liver, heart or red blood cells. These hereditary glycogen storage diseases cause two major clinical presentations: one characterized by fixed, often progressive muscle weakness, and the other by acute, intermittent, and reversible muscle dysfunction manifesting as exercise intolerance (myalgia on exertion, muscle contractures, myoglobinuria).

RECENT FINDINGS

The focus of this review is on recent developments in: clinical features, including a brief description of the newest identified glycogen storage disease type XIII; molecular genetic studies discussing genotype-phenotype correlations in some carbohydrate oxidation disorders; pathophysiological mechanisms, especially those assessed by non-invasive P magnetic resonance spectroscopy; and therapeutic approaches such as nutritional supplementation and gene therapy, including recombinant enzyme replacement.

SUMMARY

Although major progress has been made in an understanding of the molecular genetic bases of carbohydrate oxidation defects, the pathophysiology of exercise intolerance and muscle weakness remains to be further clarified. Gene therapy and dietary therapeutic regimes appear promising, but need to be actively investigated in the future.

Muscle glycogenoses

There are 11 hereditary disorders of glycogen metabolism affecting muscle alone or together with other tissues, and they cause two main clinical syndromes: episodic, recurrent exercise intolerance with cramps, myalgia, and myoglobinuria; or fixed, often progressive weakness. Great strides have been made in our understanding of the molecular bases of these disorders, all of which show remarkable genetic heterogeneity. In contrast, the pathophysiological mechanisms underlying acute muscle breakdown and chronic weakness remain unclear. Although glycogen storage diseases have been studied for decades, new biochemical defects are still being discovered, especially in the glycolytic pathway. In addition, the pathogenesis of polyglucosan deposition is being clarified both in traditional glycogenoses and in disorders such as Lafora's disease. In some conditions, combined dietary and exercise regimens may be of help, and gene therapy, including recombinant enzyme replacement, is being actively pursued.

Glycogen storage myopathies

The glycogen storage myopathies are caused by enzyme defects in the glycogenolytic or in the glycolytic pathway affecting skeletal muscle alone or in conjunction with other tissues. The authors review recent findings in this area, including a new entity, aldolase deficiency, and the wealth of molecular genetic data that are rapidly accumulating. Despite this progress, genotype-phenotyp3 correlations are still murky in most glycogen storage myopathies.

Glycogen storage diseases of muscle

Ten specific enzyme defects of glycogen metabolism affect skeletal muscle alone or in combination with other tissues. The newest addition to this group of disorders is the defect of aldolase A (glycogenosis type XII), a block in terminal glycolysis associated with myopathy and a hemolytic trait. The muscle glycogenoses cause two major syndromes, one characterized by exercise intolerance, cramps, and myoglobinuria, and the other dominated by fixed, often progressive weakness. This review considers sequentially recent advances in the following: clinical features or clinical variants, including a brief description of glycogenosis type XII; animal models, both spontaneous and genetically engineered; physiopathologic mechanisms, especially of the exercise intolerance and myoglobinuria; biochemical and molecular features--molecular defects are just beginning to be discovered for some glycogenoses (e.g. phosphorylase-b-kinase deficiency or branching enzyme deficiency), whereas they form long lists for others, such as acid maltase deficiency and myophosphorylase deficiency; and therapeutic approaches, including enzyme replacement and gene therapy.