McArdle's Disease: A Differential Diagnosis of Metabolic Myopathies.

McArdle's disease, also known as glycogen storage disease type V or McArdle syndrome, is a pure muscle myopathy with an autosomal recessive inheritance pattern. It is caused by mutations in the gene that encodes muscle phosphorylase. Symptoms typically begin in late adolescence or early adulthood, presenting as exercise intolerance. This review focuses on the diagnosis of McArdle's disease, initially manifesting as a clinical picture of rhabdomyolysis in an 18-year-old male patient with a history of minor thalassemia who had been followed in pediatric consultation since age three for failure to thrive. After excluding common causes such as alcohol consumption, drug use, traumatic muscle compression, and other conditions, the diagnosis of McArdle's disease was considered. The diagnosis was supported by laboratory tests showing myoglobinuria and elevated creatine kinase levels, as well as the absence of increased serum lactate following ischemic exercise. Genetic testing confirmed the presence of mutations in the PYGM gene, corroborating the diagnosis. Treatment includes administering a diet rich in slow-absorbing carbohydrates, regular low-intensity physical exercise, and, in some cases, supplementation with vitamin B6 and creatine. The prognosis is generally favorable with proper disease management, although vigorous exercise should be avoided to prevent complications such as severe muscle injury and rhabdomyolysis. Although McArdle's disease is a rare condition, it is likely underdiagnosed. Ideally, it should be considered in the differential diagnosis of rhabdomyolysis in all patients with symptoms of exercise intolerance and/or recurrent myoglobinuria.

New perspectives for the treatment and follow-up of glycogen storage disease type V: DL-3-hydroxybutyric acid with modified Atkins diet and quadriceps femoris shear wave elastography.

OBJECTIVES: Glycogen storage disease type V is caused by the mutations in muscle glycogen phosphorylase gene. This is the first report which DL-3-hydroxybutyric acid was used in combination with modified Atkins diet for the treatment of a patient with glycogen storage disease type V and quadriceps femoris shear wave elastography was performed to evaluate the treatment efficacy. CASE PRESENTATION: A 13-year-old girl was referred with fatigue and muscle cramps with exercise and there were no pathological findings in physical examination. Creatine kinase levels with 442 U/L. No phosphorylase enzyme activity was detected in muscle biopsy, a homozygous c.1A>G (p.M1V) pathogenic mutation was found in PYGM gene. She was started on DL-3-hydroxybutyric acid and modified Atkins diet at age 16. Her walking and stair climbing capacity increased, the need for rest during exercise decreased. The stiffness of the quadriceps femoris exhibited a reduction. CONCLUSIONS: DL-3-hydroxybutyric acid and modified Atkins diet may provide an alternative fuel and shear wave elastography may be useful in demonstrating treatment efficacy. More clinical and pre-clinical studies are obviously needed to reach more definite conclusions.

An autosomal recessive variant in PYGM causes myophosphorylase deficiency in Red Angus composite cattle.

BACKGROUND: Between 2020 and 2022, eight calves in a Nebraska herd (composite Simmental, Red Angus, Gelbvieh) displayed exercise intolerance during forced activity. In some cases, the calves collapsed and did not recover. Available sire pedigrees contained a paternal ancestor within 2-4 generations in all affected calves. Pedigrees of the calves' dams were unavailable, however, the cows were ranch-raised and retained from prior breeding seasons, where bulls used for breeding occasionally had a common ancestor. Therefore, it was hypothesized that a de novo autosomal recessive variant was causative of exercise intolerance in these calves. RESULTS: A genome-wide association analysis utilizing SNP data from 6 affected calves and 715 herd mates, followed by whole-genome sequencing of 2 affected calves led to the identification of a variant in the gene PYGM (BTA29:g.42989581G > A). The variant, confirmed to be present in the skeletal muscle transcriptome, was predicted to produce a premature stop codon (p.Arg650*). The protein product of PYGM, myophosphorylase, breaks down glycogen in skeletal muscle. Glycogen concentrations were fluorometrically assayed as glucose residues demonstrating significantly elevated glycogen concentrations in affected calves compared to cattle carrying the variant and to wild-type controls. The absence of the PYGM protein product in skeletal muscle was confirmed by immunohistochemistry and label-free quantitative proteomics analysis; muscle degeneration was confirmed in biopsy and necropsy samples. Elevated skeletal muscle glycogen persisted after harvest, resulting in a high pH and dark-cutting beef, which is negatively perceived by consumers and results in an economic loss to the industry. Carriers of the variant did not exhibit differences in meat quality or any measures of animal well-being. CONCLUSIONS: Myophosphorylase deficiency poses welfare concerns for affected animals and negatively impacts the final product. The association of the recessive genotype with dark-cutting beef further demonstrates the importance of genetics to not only animal health but to the quality of their product. Although cattle heterozygous for the variant may not immediately affect the beef industry, identifying carriers will enable selection and breeding strategies to prevent the production of affected calves.

Fatigue and associated factors in 172 patients with McArdle disease: An international web-based survey.

McArdle disease is an autosomal recessive inherited disease caused by pathogenic variants in the PYGM gene, resulting in virtual absence of the myophosphorylase enzyme in skeletal muscle. Patients experience physical activity intolerance, muscle pain, and muscle fatigue. This study aimed to investigate other fatigue domains with the Multidimensional Fatigue Inventory (MFI-20) along with an investigation of potential contributing factors, including relevant disease and lifestyle-related factors. We conducted a survey in an international cohort of patients with McArdle disease. The survey included questions on demographics and McArdle disease-related symptoms, and the questionnaires: MFI-20, Insomnia Severity Index (ISI), and International Physical Activity Questionnaire Short-Form (IPAQ-SF). One hundred seventy-four responses were included in the data analyses. We found relatively high fatigue scores in all five domains (general fatigue (12.9 ± 2.2), mental fatigue (10.1 ± 4.1), physical fatigue (13.7 ± 4.1), reduced activity (12.1 ± 4.1), and reduced motivation (10.4 ± 3.4)). Fatigue associated with McArdle symptom severity (p < 0.005), lower levels of physical activity (assessed by IPAQ-SF) (p < 0.05), and poor sleep (assessed by ISI) (p < 0.05). These findings call for clinical focus and future research into fatigue, sleep and mental health in patients with McArdle disease.

Exercise induced muscle weakness in a young adult: McArdle's disease unusual presentation.

McArdle's disease (Glycogen storage disease type V) is a rare inherited autosomal recessive disease involving defect in enzyme, glycogen phosphorylase (PYGM) which results in accumulation of glycogen mainly affecting skeletal muscles. It commonly presents in childhood and rarely in adults with symptoms like exercise intolerance, muscle weakness, cramps and fatigue. Herein, we report an unusual case of a 22 years old male in Pakistan with probable McArdle's Disease presenting with repeated episodes of generalized cramping muscle pain, exercise intolerance and haematuria. The diagnostic approach to identifying this disease as well as the differentials of other rare types of skeletal muscle disorders that should be kept in mind while dealing with a similar clinical picture, irrespective of the age of presentation, have been discussed.

Can a modified ketogenic diet be a nutritional strategy for patients with McArdle disease? Results from a randomized, single-blind, placebo-controlled, cross-over study.

BACKGROUND: McArdle disease is caused by myophosphorylase deficiency leading to blocked glycogenolysis in skeletal muscle. Consequently, individuals with McArdle disease have intolerance to physical activity, muscle fatigue, and pain. These symptoms vary according to the availability of alternative fuels for muscle contraction. In theory, a modified ketogenic diet (mKD) can provide alternative fuels in the form of ketone bodies and potentially boost fat oxidation. METHODS: This randomized, single-blind, placebo-controlled, cross-over study aimed to investigate if a mKD improves exercise capacity in individuals with McArdle disease. Participants were randomized to follow a mKD (75-80% fat, 15% protein, 5-10% carbohydrates) or placebo diet (PD) first for three weeks, followed by a wash-out period, and then the opposite diet. The primary outcome was change in heart rate during constant-load cycling. Secondary outcomes included change in plasma metabolites, perceived exertion, indirect calorimetry measures, maximal exercise capacity, and patient-reported outcomes. RESULTS: Fifteen out of 20 patients with genetically verified McArdle disease completed all study visits, and 14 were included in the data analyses. We found that the mKD induced a metabolic shift towards increased fat oxidation (∼60% increase), and a 19-fold increase in plasma ß-hydroxybutyrate (p < 0.05). The mKD did not improve heart rate responses during constant-load cycling but did improve patient-reported outcomes and maximal exercise capacity (∼20% increase) compared to the PD. CONCLUSION: The mKD did not alleviate all McArdle disease-related symptoms but did induce some positive changes. To date, no satisfactory treatment options exist other than exercise training. To that end, a mKD can be a possible nutritional strategy for some individuals with McArdle disease who are motivated to undertake a restrictive diet. CLINICAL TRIAL REGISTRATION: clinical trials.gov: NCT04044508.

Clinical and Laboratory Findings on Glycogen Storage Disease Type V: Results from a Retrospective Observational Study in a Tertiary Hospital.

Introduction - Glycogen storage disease type V (GSDV, MIM #232600) is an autosomal recessive metabolic myopathy caused by pathogenic variants in the PYGM gene. The characteristic symptoms of exercise intolerance, myalgia, and cramps, which improve after a few minutes of rest, are frequently unrecognized in affected children. When there is clinical suspicion, the initial approach with a forearm exercise test has diagnostic value by detecting low post-exercise plasma lactate-to-ammonia ratio values. The diagnostic algorithm is followed by genetic testing if the results suggest myophosphorylase deficiency. Methods - This was a retrospective observational study conducted based on reviewing medical records of patients with GSDV in a tertiary hospital. We assessed demographic variables, including the timing of onset and diagnosis, relevant clinical characteristics, and whether genetic testing was performed, including its results. Results/Case Report - Our goal was to review the GSDV cases in our center to assess our cohort's diagnostic timing and clinical and genetic characteristics. We identified 28 patients from 24 families, three with consanguinity. The mean age at the time of the study was 43 years. While most (26/28; 93%) recalled their first symptoms in childhood/adolescence, only 25% (7/28) were diagnosed then. All patients had exercise intolerance and CK elevation, while about half reported the second wind phenomenon. Genetic testing was performed in 22 patients, revealing biallelic PYGM variants (9 homozygous, 13 compound heterozygous) as the most common (p.R50*). Conclusion - GSDV is rare and presents in the pediatric age, with subtle manifestations often underestimated for decades. A late diagnosis may negatively impact the psychosocial development of affected children. It is essential to recognize some unique features that facilitate diagnosis: history of exercise intolerance, the second wind sign, and high resting serum CK levels. Identifying the disease-causing variants in PYGM is currently the gold standard for diagnosis as it is less invasive than performing a muscle biopsy, and may promptly diagnose the condition and avoid wrongful labelling of patients.

Repeated oral sucrose dosing after the second wind is unnecessary in patients with McArdle disease: Results from a randomized, placebo-controlled, double-blind, cross-over study.

It is well-established that oral sucrose ingested shortly before exercise improves early exercise tolerance in individuals with McArdle disease. This is by supplying blood-borne glucose for muscle metabolism to compensate for the blocked glycogenolysis. The present study investigated if individuals with McArdle disease could benefit further from repeated sucrose ingestion during prolonged exercise. In this double-blind, placebo-controlled, cross-over study, the participants were randomized to ingest either sucrose or placebo first and subsequently the opposite on two separate days. The participants ingested the drink 10 min before and thrice (after 10, 25, and 40 min) during a 60-min submaximal exercise test on a cycle ergometer. The primary outcome was exercise capacity as indicated by heart rate (HR) and perceived exertion (PE) responses to exercise. Secondary outcomes included changes in blood metabolites, insulin and carbohydrate, and fatty acid oxidation rates during exercise. Nine participants with McArdle disease were included in the study. We confirmed improvement of exercise capacity with oral sucrose vs. placebo during early exercise (pre-second wind) indicated by lower peak HR and PE (p < 0.02). We found no further beneficial effect with repeated sucrose versus placebo ingestion during prolonged exercise, as indicated by no difference in HR or PE post-second wind (p > 0.05). Glucose, lactate, insulin, and carbohydrate oxidation rates increased, and fatty acid oxidation decreased with sucrose versus placebo (p ≤ 0.0002). We can conclude that repeated sucrose ingestion is not recommended during prolonged exercise. This finding can prevent excessive caloric intake and reduce the risk of obesity and insulin resistance.

Patient-Reported Experiences with a Low-Carbohydrate Ketogenic Diet: An International Survey in Patients with McArdle Disease.

The low-carbohydrate ketogenic diet (LCKD) has attracted increased attention in recent years as a potential treatment option for individuals with McArdle disease (glycogen storage disease type V), and despite the absence of strong scientific evidence of the LCKD's benefits, increased numbers of individuals with McArdle disease have tried a LCKD. The objective of this study was to collect patient-reported experiences with a LCKD. We aimed to estimate the immediate prevalence of individuals that had tried a LCKD in an international McArdle disease cohort, and we aimed to report on the patient-reported experiences with the diet, both positive and negative. A total of 183 responses were collected from individuals with McArdle disease from 18 countries. We found that one-third of the cohort had tried a LCKD, and almost 90% experienced some degree of positive effect, with the most prominent effects on McArdle disease-related core symptoms (e.g., activity intolerance, muscle pain, and muscle fatigue). Adverse effects were rare and generally rated as mild to moderate. These patient-reported findings underline the need for randomized clinical trials to decisively determine if a LCKD is a suitable nutritional strategy for patients with McArdle disease. The results from this study can prompt and contribute to the design of such a clinical trial.

Diagnosis and genetic analysis of a case with glycogen storage disease type V caused by compound heterozygous mutations in the PYGM gene.

Glycogen storage disease type V is an autosomal recessive genetic disorder caused by muscle glycogen phosphorylase (PYGM) deficiency, which is characterized by exercise intolerance, second wind phenomena and high level of serum creatine kinase. In this study, we reported a Chinese young man with glycogen storage disease type V, with lower extremity weakness after exercise, increased creatine kinase, and slight fat infiltration in the posterior group of thigh muscle by magnetic resonance imaging (MRI). The proband had complex heterozygous PYGM disease-causing mutations, including c.308T>C (p.L103P) variant transmitted from the mother and c.260_261delCT (p.S87Ffs*23) from the father, of which the former was a novel PYGM mutation. This study enriched the PYGM pathogenic gene mutation spectrum, contributed to improve clinicians' understanding of glycogen storage disease type V and provided a reference for further genetic study of the disease.

Enfermedad de McArdle en cuatro pacientes pediátricos. Algoritmo diagnóstico ante una intolerancia al ejercicio / McArdle’s disease in four pediatric patients. Diagnostic algorithm for exercise intolerance

Introducción: La enfermedad de McArdle está causada por una mutación en el gen PYGM y déficit de miofosforilasa muscular, resultando alterada la liberación de glucosa-1-P a partir del glucógeno. Suele manifestarse en la infancia con cansancio precoz y excesivo, mialgias, calambres y contracturas o rabdomiólisis, aunque no suele diagnosticarse hasta la etapa adulta. La creatincinasa se incrementa durante el ejercicio. Se presentan cuatro pacientes pediátricos, se resume la fisiopatología y se propone un algoritmo diagnóstico. Pacientes y métodos: Pacientes con edades entre 6 y 14 años. Se describe la anamnesis, la exploración física, la bioquímica, el electromiograma, el test de isquemia y el estudio genético, con biopsia muscular a un solo paciente. Se elabora un algoritmo a partir del test de isquemia. Resultados: En los tres varones, las mialgias aparecieron tras finalizar cada sesión deportiva, con un fenómeno second wind en un caso. Se apreció un test de isquemia sin elevación del lactato y marcada elevación del amonio en todos, una biopsia muscular con depósitos de glucógeno y ausencia de miofosforilasa, y gen PYGM con mutaciones homocigotas en todos. El tratamiento dietético les atenuó la sintomatología durante los ejercicios aeróbicos. Conclusiones: El test de isquemia resultó muy útil para demostrar una disfunción en la glucólisis anaeróbica. Se destaca que el suplemento oral de glucosa es muy útil para la enfermedad de McArdle, pero está contraindicado en los seis defectos de la glucólisis anaeróbica. El algoritmo también permite orientar el defecto de 20 miopatías metabólicas o estructurales, que se resumen.(AU)

INTRODUCTION: McArdle’s disease is caused by a mutation in the PYGM gene, causing a muscle myophosphorylase deficiency, altering the release of glucose-1-P from glycogen. It usually manifests itself in childhood with early and excessive tiredness, myalgias, cramps and contractures or rhabdomyolysis, although it is not usually diagnosed until adulthood. Creatine kinase increases sharply during exercise. Four pediatric patients are presented, the pathophysiology is summarized, and a diagnostic algorithm is proposed. PATIENTS AND METHODS: Ages between 6 and 14 years, the anamnesis, physical examination, biochemistry, elec­tro­myogram, ischemia test and genetic study are described. Muscle biopsy in a single patient. The algorithm was developed from the ischemia test. RESULTS: In the three men, myalgias appeared after finishing each sports session. Phenomenon ‘second wind’ in one case. Ischemia test without lactate elevation and marked ammonia elevation in all. Only one muscle biopsy with glycogen deposits and absence of myophosphorylase. PYGM gene with homozygous mutations in all. Dietary treatment attenuated their symptoms during aerobic exercises. CONCLUSIONS:The ischemia test was very useful to demonstrate a dysfunction in anaerobic glycolysis. It is worth noting that oral glucose supplementation is very useful in McArdle disease, but is contraindicated in all six defects of anaerobic glycolysis. The algorithm also allows targeting the defect of 20 metabolic or structural myopathies, which are summarized.(AU)

McArdle disease in a patient with anorexia nervosa: a case report.

BACKGROUND: McArdle disease is an autosomal recessive genetic disorder caused by a deficiency of the glycogen phosphorylase (myophosphorylase) enzyme, which muscles need to break down glycogen into glucose for energy. Symptoms include exercise intolerance, with fatigue, muscle pain, and cramps being manifested during the first few minutes of exercise, which may be accompanied by rhabdomyolysis. CASE PRESENTATION: This case report describes for the first time the clinical features, diagnosis and management of a 20 year-old patient with anorexia nervosa and McArdle disease, documented by means of muscle biopsy. CONCLUSION: Anorexia nervosa and McArdle disease interact in a detrimental bidirectional way. In addition, some laboratory parameter alterations (e.g., elevated values of creatine kinase) commonly attributed to the specific features of eating disorders (e.g., excessive exercising) may delay the diagnosis of metabolic muscle diseases. On the other hand, the coexistence of a chronic disease, such as McArdle disease, whose management requires the adoption of a healthy lifestyle, can help to engage patients in actively addressing their eating disorder.

McArdle disease and pregnancy: A case report and scoping review of pregnancy outcomes.

McArdle disease is an autosomal recessive disorder affecting skeletal muscle glycogen metabolism. Limited data are available regarding pregnancy outcomes with this genetic condition. We present a recent case of a woman with McArdle disease, along with a scoping review of all published literature regarding pregnancy and delivery outcomes for women with McArdle disease. A total of 35 cases are summarised. Overall, pregnancy does not worsen or increase the risk for disease flare. Women can successfully deliver vaginally, with consideration of an assisted second stage recommended to reduce the risk of postpartum rhabdomyolysis.

No effect of oral ketone ester supplementation on exercise capacity in patients with McArdle disease and healthy controls: A randomized placebo-controlled cross-over study.

Patients with glycogen storage disease type V (GSDV), also known as McArdle disease, have blocked glycogen breakdown due to myophosphorylase deficiency, leading to exercise intolerance, muscle pain, and risk of muscle damage. Blood-derived ketone bodies (KBs) constitute an alternative energy source that could fuel the muscle independent of glycogenolysis. However, except for long-time fasting or ketogenic dieting, KBs are present in low quantities. This led us to explore the effects of a drink containing exogenously produced KBs in the form of D-ß-hydroxybutyrate esters (KE) on exercise capacity and metabolism in patients with GSDV. Eight GSDV patients and four healthy controls (HC) were included in this placebo-controlled, cross-over study where subjects were randomized to receive a KE drink with 395 mgKE/kg or placebo drink on two separate days 25 min before a submaximal cycle exercise test. The primary outcome was exercise capacity as indicated by heart rate response (HR) to exercise. Secondary outcomes included perceived exertion (PE) and measures of KB, carbohydrate, and fat metabolism during exercise. In GSDV, the KE drink vs. placebo increased plasma KBs and KB oxidation (p ≤ 0.0001) but did not improve exercise capacity as judged from HR (p = 0.120) and PE (p = 0.109). In addition, the KE drink lowered plasma glucose, free fatty acids, and lowered lipolytic rate and glucose rate of appearance compared with placebo. Similar results were found in the HC group. The present study indicates that an increase in KB oxidation by oral KE supplementation does not improve exercise capacity in GSDV possibly because of KB-induced inhibition of lipolysis and liver glucose output. Thus, oral KE supplementation alone cannot be recommended as a treatment option for patients with GSDV.

High prevalence of paraspinal muscle involvement in adults with McArdle disease.

INTRODUCTION/AIMS: Very few studies analyzing the pattern of muscle involvement in magnetic resonance imaging (MRI) of patients with McArdle disease have been reported to date. We aimed to examine the pattern of muscle fat replacement in patients with McArdle disease. METHODS: We performed a retrospective study including all patients with genetically confirmed McArdle disease followed in our center from January 2010 to March 2021. Clinical data were collected from the medical record. Whole-body MRI was performed as part of the diagnostic evaluation. The distribution of muscle fat replacement and its severity were analyzed. RESULTS: Nine patients were included. Median age at onset was 7 y (range, 5-58) and median age at the time when MRI was performed was 57.3 y (range, 37.2-72.8). At physical examination, four patients had permanent weakness: in three the weakness was limited to paraspinal muscles, whereas in one the weakness involved the paraspinal and proximal upper limb muscles. Muscle MRI showed abnormalities in six of the seven studied patients. In all of them, fat replacement of paravertebral muscles was found. Other muscles frequently affected were the tongue in three, subscapularis in three, and long head of biceps femoris and semimembranosus in two. DISCUSSION: Our findings suggest that paraspinal muscle involvement is common in McArdle disease and support the need to include this disease in the differential diagnosis of the causes of paraspinal muscle weakness. Involvement of the tongue and subscapularis are also frequent in McArdle disease.

An elderly diabetic patient with McArdle disease and recurrent rhabdomyolysis: a potential association with late hypoinsulinemia?

BACKGROUND: McArdle disease is a myopathy caused by mutations in PYGM gene that is characterized by reduced or absent activity of myophosphorylase. Reports of patients with concomitant McArdle disease and diabetes are scarce. We report a case of a patient with a late diagnosis of McArdle disease and we postulate that symptoms may be related to hypoinsulinemia. CASE PRESENTATION: This report describes the evolution of an elderly diabetic patient with confirmed diagnosis of McArdle's disease based on the absence of myophosphorylase activity in the analysis of muscle biopsy, and a homozygous mutation in the PYGM gene. The variant - Chr11: 64.525 (p. Asn168*fs) has not been previously described. The diagnosis of McArdle disease was confirmed after two episodes of rhabdomyolysis, at 77 and 81 years of age, as the symptoms were, until then, discrete. The "second-wind phenomenon" was not spontaneously reported, but it was confirmed when directly questioned. We postulate that the later episodes of rhabdomyolysis occurred because of a progressive decrease in insulin production with a consequent reduction in the uptake of blood glucose by muscle cells, thus compromising the cellular energy balance. To our knowledge, this is the first report of recurrent rhabdomyolysis in an elderly diabetic patient with genetically proven McArdle disease. Our initial attempt to reduce insulin resistance with metformin and pioglitazone was not effective, possibly because of inadequate insulinemia. However, an improvement was evident after the administration of low doses of intermediate-acting insulin. CONCLUSIONS: In view of the patient's clinical evolution, we suggest the use of medication that reduces insulin resistance for patients with McArdle disease and type 2 diabetes, pre-diabetes or even normoglycemic metabolic syndrome.

Results of an open label feasibility study of sodium valproate in people with McArdle disease.

McArdle disease results from a lack of muscle glycogen phosphorylase in skeletal muscle tissue. Regenerating skeletal muscle fibres can express the brain glycogen phosphorylase isoenzyme. Stimulating expression of this enzyme could be a therapeutic strategy. Animal model studies indicate that sodium valproate (VPA) can increase expression of phosphorylase in skeletal muscle affected with McArdle disease. This study was designed to assess whether VPA can modify expression of brain phosphorylase isoenzyme in people with McArdle disease. This phase II, open label, feasibility pilot study to assess efficacy of six months treatment with VPA (20 mg/kg/day) included 16 people with McArdle disease. Primary outcome assessed changes in VO2peak during an incremental cycle test. Secondary outcomes included: phosphorylase enzyme expression in post-treatment muscle biopsy, total distance walked in 12 min, plasma lactate change (forearm exercise test) and quality of life (SF36). Safety parameters. 14 participants completed the trial, VPA treatment was well tolerated; weight gain was the most frequently reported drug-related adverse event. There was no clinically meaningful change in any of the primary or secondary outcome measures including: VO2peak, 12 min walk test and muscle biopsy to look for a change in the number of phosphorylase positive fibres between baseline and 6 months of treatment. Although this was a small open label feasibility study, it suggests that a larger randomised controlled study of VPA, may not be worthwhile.

Titrating a modified ketogenic diet for patients with McArdle disease: A pilot study.

Glycogen storage disease type V (GSDV) is a rare inborn error of carbohydrate metabolism. Patients present with exercise intolerance due to blocked glycogen breakdown in skeletal muscle. Introducing alternative fuel substrates, such as ketone bodies (KBs), could potentially alleviate muscle symptoms. This pilot study investigates which of three different modified ketogenic diet regimes is optimal for GSDV-patients to follow in a future large-scale study. Participants were randomised to follow one of three diet regimes for 3 weeks (#1: 65%/15%/20%; #2: 75%/15%/10%, or #3: 80%/15%/5%, fat/protein/carbohydrate). The primary outcome was exercise tolerance assessed by heart rate (HR) changes during constant load cycling. Secondary outcomes included levels of ketosis, and changes in perceived exertion and indirect calorimetry measures during exercise. Ten GSDV-patients were included. Eight completed the study. The other two were excluded. Diet #3 showed the highest average KB level (1.1 mmol/L) vs #2 (0.5 mmol/L) and #1 (0.3 mmol/L). Five patients reported subjective symptom relief, all of whom were on diets #2 and #3. All diet regimes seemed to improve fatty acid oxidation rates and exercise capacity as indicated by a small decrease in HR and perceived exertion. The results of this open-label pilot study show that diets #2 and #3 induce ketosis and improve symptoms and exercise capacity in GSDV-patients. Diet #2 had the highest acceptability score and was superior or equal to diet #3 in all other parameters, except level of ketosis. Based on this, we suggest testing diet #2 in a large-scale, placebo-controlled study in GSDV.

McArdle Disease: New Insights into Its Underlying Molecular Mechanisms.

McArdle disease, also known as glycogen storage disease type V (GSDV), is characterized by exercise intolerance, the second wind phenomenon, and high serum creatine kinase activity. Here, we recapitulate PYGM mutations in the population responsible for this disease. Traditionally, McArdle disease has been considered a metabolic myopathy caused by the lack of expression of the muscle isoform of the glycogen phosphorylase (PYGM). However, recent findings challenge this view, since it has been shown that PYGM is present in other tissues than the skeletal muscle. We review the latest studies about the molecular mechanism involved in glycogen phosphorylase activity regulation. Further, we summarize the expression and functional significance of PYGM in other tissues than skeletal muscle both in health and McArdle disease. Furthermore, we examine the different animal models that have served as the knowledge base for better understanding of McArdle disease. Finally, we give an overview of the latest state-of-the-art clinical trials currently being carried out and present an updated view of the current therapies.