The Second Wind in McArdle Patients: Fitness Matters

Exercise Intolerance in McArdle Disease: A Role for Cardiac Impairment? A Preliminary Study in Humans and Mice

INTRODUCTION

Whether cardiac impairment can be fully discarded in McArdle disease-the paradigm of "exercise intolerance," caused by inherited deficiency of the skeletal muscle-specific glycogen phosphorylase isoform ("myophosphorylase")-remains to be determined.

METHODS

Eight patients with McArdle disease and seven age/sex-matched controls performed a 15-min moderate, constant-load cycle-ergometer exercise bout followed by a maximal ramp test. Electrocardiographic and two-dimensional transthoracic (for cardiac dimension's assessment) and speckle tracking (for left ventricular global longitudinal strain (GLS) assessments) echocardiographic evaluations were performed at baseline. Electrocardiographic and GLS assessments were also performed during constant-load exercise and immediately upon maximal exertion. Four human heart biopsies were obtained in individuals without McArdle disease, and in-depth histological/molecular analyses were performed in McArdle and wild-type mouse hearts.

RESULTS

Exercise intolerance was confirmed in patients ("second wind" during constant-load exercise, -55% peak power output vs controls). As opposed to controls, patients showed a decrease in GLS during constant-load exercise, especially upon second wind occurrence, but with no other between-group difference in cardiac structure/function. Human cardiac biopsies showed that all three glycogen phosphorylase-myophosphorylase, but also liver and especially brain-isoforms are expressed in the normal adult heart, thereby theoretically compensating for eventual myophosphorylase deficiency. No overall histological (including glycogen depots), cytoskeleton, metabolic, or mitochondrial (morphology/network/distribution) differences were found between McArdle and wild-type mouse hearts, except for lower levels of pyruvate kinase M2 and translocase of outer-membrane 20-kDa subunit in the former.

CONCLUSIONS

This study provides preliminary evidence that cardiac structure and function seem to be preserved in patients with McArdle disease. However, the role for an impaired cardiac contractility associated with the second wind phenomenon should be further explored.

Experiences of living with GSD5 (McArdle) disease: challenges and strategies. A qualitative study in the Netherlands

PURPOSE

Glycogen Storage Disease type 5 (GSD5 or McArdle disease) is caused by deficient glycogen phosphorylase enzyme activity in skeletal muscles. Individuals with GSD5 experience symptoms like muscle pain, fatigue, and tachycardia during exertion. Our study aimed to explore the lived experiences of individuals with GSD5, focusing on their daily challenges, the process of being diagnosed, and management strategies.

METHODS

Participants were invited to share their life experiences through in-depth, semi-structured interviews, and the collected data was analyzed using thematic analysis.

RESULTS

Using purposeful sampling, 13 individuals with GSD5 were recruited for the study. The analysis identified four key themes: "experiencing incomprehensible difficulties," "diagnosis as an explanation," "finding ways to manage daily challenges," and "listening to your body." These themes reflect diverse experiences of daily functioning, physical challenges faced, the pivotal role of diagnosis in understanding symptoms, and the adoption of management strategies like using the 'second wind' phenomenon. Participants emphasized the importance of support networks and adaptive lifestyle changes in effectively managing their condition.

CONCLUSIONS

Early diagnosis and tailored management strategies are critical for improving outcomes and quality of life in individuals with GSD5. Timely diagnosis and comprehensive multidisciplinary care are essential for effectively managing the complexities of this rare metabolic disorder.

Dose-response effect of pre-exercise carbohydrates under muscle glycogen unavailability: Insights from McArdle disease

BACKGROUND

This study aimed to determine the effect of different carbohydrate (CHO) doses on exercise capacity in patients with McArdle disease-the paradigm of "exercise intolerance", characterized by complete muscle glycogen unavailability-and to determine whether higher exogenous glucose levels affect metabolic responses at the McArdle muscle cell (in vitro) level.

METHODS

Patients with McArdle disease (n = 8) and healthy controls (n = 9) underwent a 12-min submaximal cycling constant-load bout followed by a maximal ramp test 15 min after ingesting a non-caloric placebo. In a randomized, double-blinded, cross-over design, patients repeated the tests after consuming either 75 g or 150 g of CHO (glucose:fructose = 2:1). Cardiorespiratory, biochemical, perceptual, and electromyographic (EMG) variables were assessed. Additionally, glucose uptake and lactate appearance were studied in vitro in wild-type and McArdle mouse myotubes cultured with increasing glucose concentrations (0.35, 1.00, 4.50, and 10.00 g/L).

RESULTS

Compared with controls, patients showed the "classical" second-wind phenomenon (after prior disproportionate tachycardia, myalgia, and excess electromyographic activity during submaximal exercise, all p < 0.05) and an impaired endurance exercise capacity (-51% ventilatory threshold and -55% peak power output, both p < 0.001). Regardless of the CHO dose (p < 0.05 for both doses compared with the placebo), CHO intake increased blood glucose and lactate levels, decreased fat oxidation rates, and attenuated the second wind in the patients. However, only the higher dose increased ventilatory threshold (+27%, p = 0.010) and peak power output (+18%, p = 0.007). In vitro analyses revealed no differences in lactate levels across glucose concentrations in wild-type myotubes, whereas a dose-response effect was observed in McArdle myotubes.

CONCLUSION

CHO intake exerts beneficial effects on exercise capacity in McArdle disease, a condition associated with total muscle glycogen unavailability. Some of these benefits are dose dependent.

Acute ketone supplementation in the absence of muscle glycogen utilization: Insights from McArdle disease

BACKGROUND & AIMS

Ketone supplementation is gaining popularity. Yet, its effects on exercise performance when muscle glycogen cannot be used remain to be determined. McArdle disease can provide insight into this question, as these patients are unable to obtain energy from muscle glycogen, presenting a severely impaired physical capacity. We therefore aimed to assess the effects of acute ketone supplementation in the absence of muscle glycogen utilization (McArdle disease).

METHODS

In a randomized cross-over design, patients with an inherited block in muscle glycogen breakdown (i.e., McArdle disease, n = 8) and healthy controls (n = 7) underwent a submaximal (constant-load) test that was followed by a maximal ramp test, after the ingestion of a placebo or an exogenous ketone ester supplement (30 g of D-beta hydroxybutyrate/D 1,3 butanediol monoester). Patients were also assessed after carbohydrate (75 g) ingestion, which is currently considered best clinical practice in McArdle disease.

RESULTS

Ketone supplementation induced ketosis in all participants (blood [ketones] = 3.7 ± 0.9 mM) and modified some gas-exchange responses (notably increasing respiratory exchange ratio, especially in patients). Patients showed an impaired exercise capacity (-65 % peak power output (PPO) compared to controls, p < 0.001) and ketone supplementation resulted in a further impairment (-11.6 % vs. placebo, p = 0.001), with no effects in controls (p = 0.268). In patients, carbohydrate supplementation resulted in a higher PPO compared to ketones (+21.5 %, p = 0.001) and a similar response was observed vs. placebo (+12.6 %, p = 0.057).

CONCLUSIONS

In individuals who cannot utilize muscle glycogen but have a preserved ability to oxidize blood-borne glucose and fat (McArdle disease), acute ketone supplementation impairs exercise capacity, whereas carbohydrate ingestion exerts the opposite, beneficial effect.

Toward an Understanding of GSD5 (McArdle disease): How Do Individuals Learn to Live with the Metabolic Defect in Daily Life

BACKGROUND

Glycogen storage disease type 5 (GSD) is an autosomal recessive inherited metabolic myopathy caused by a deficiency of the enzyme muscle glycogen phosphorylase. Individuals with GSD5 experience physical activity intolerance.

OBJECTIVE

This patient-led study aimed to capture the daily life experiences of GSD5, with a focus on adapting to and coping with their physical activity intolerance.

METHODS

An online survey was composed in close collaboration with patient organizations. It consisted of customized and validated questionnaires on demographics, general health and comorbidities, physical activity, psychosocial well-being and functioning, pain, fatigue and adapting to and coping with GSD5.

RESULTS

One hundred sixty-two participants (16 countries) participated. The majority, n = 86 (69%) were from the Netherlands, USA or UK. We observed a high rate of misdiagnosis prior to GSD5 diagnosis (49%), surprisingly a relatively high proportion had not been diagnosed by DNA testing which is the gold standard. Being diagnosed had a strong impact on emotional status, daily life activities and important life choices. A large proportion had not received any rehabilitation (41%) nor medical treatment (57%) before diagnosis. Engagement in vigorous and moderate physical activity was reduced. Health related quality of life was low, most likely related to low physical health. The median Fatigue Severity Score was 4.3, indicating moderate to severe fatigue. Participants themselves had found various ways to adapt to and cope with their disability. The adaptations concerned all aspect of their life, including household chores, social and physical activities, and work. In addition to lack of support, participants reported limited availability of information sources.

CONCLUSION

Participants have provided guidance for newly diagnosed people, including the advice to accept one's limited abilities and maintain an active lifestyle. We conclude that adequate counseling on ways of adapting and coping is expected to increase both health-related quality of life and physical activity.

Diagnosis and management of children with McArdle Syndrome (GSD V) in New South Wales

Glycogen storage type V (GSD V-McArdle Syndrome) is a rare neuromuscular disorder characterised by severe pain early after the onset of physical activity. A recent series indicated a diagnostic delay of 29 years; hence reports of children affected by the disorder are uncommon (Lucia et al., 2021, Neuromuscul Disord, 31, 1296-1310). This paper presents eight patients with a median onset age of 5.5 years and diagnosis of 9.5 years. Six patients had episodes of rhabdomyolysis with creatine kinase elevations >50 000 IU/L. Most episodes occurred in relation to eccentric non-predicted activities rather than regular exercise. One of the patients performed a non-ischaemic forearm test. One patient was diagnosed subsequent to a skeletal muscle biopsy, and all had confirmatory molecular genetic diagnosis. Three were homozygous for the common PYGM:c.148C > T (p.Arg50*) variant. All but one patient had truncating variants. All patients were managed with structured exercise testing to help them identify 'second-wind', and plan an exercise regimen. In addition all also had an exercise test with 25 g maltodextrin which had statistically significant effect on ameliorating ratings of perceived exertion. GSD V is under-recognised in paediatric practice. Genetic testing can readily diagnose the condition. Careful identification of second-wind symptomatology during exercise with the assistance of a multi-disciplinary team, allows children to manage activities and tolerate exercise. Maltodextrin can be used for structured exercise, but excessive utilisation may lead to weight gain. Early intervention and education may improve outcomes into adult life.

Exercise testing and prescription in patients with inborn errors of muscle energy metabolism

Skeletal muscle is a dynamic organ requiring tight regulation of energy metabolism in order to provide bursts of energy for effective function. Several inborn errors of muscle energy metabolism (IEMEM) affect skeletal muscle function and therefore the ability to initiate and sustain physical activity. Exercise testing can be valuable in supporting diagnosis, however its use remains limited due to the inconsistency in data to inform its application in IEMEM populations. While exercise testing is often used in adults with IEMEM, its use in children is far more limited. Once a physiological limitation has been identified and the aetiology defined, habitual exercise can assist with improving functional capacity, with reports supporting favourable adaptations in adult patients with IEMEM. Despite the potential benefits of structured exercise programs, data in paediatric populations remain limited. This review will focus on the utilisation and limitations of exercise testing and prescription for both adults and children, in the management of McArdle Disease, long chain fatty acid oxidation disorders, and primary mitochondrial myopathies.

Data from the European registry for patients with McArdle disease (EUROMAC): functional status and social participation

BACKGROUND

The European registry for individuals with GSD5 and other muscle glycogenosis (EUROMAC) was launched to register rare muscle glycogenosis in Europe, to facilitate recruitment for research trials and to learn about the phenotypes and disseminate knowledge about the diseases. A network of twenty collaborating partners from eight European countries and the US contributed data on rare muscle glycogenosis in the EUROMAC registry.

METHODS

Following the initial report on demographics, neuromuscular features and comorbidity (2020), we here present the data on social participation, previous and current treatments (medication, supplements, diet and rehabilitation) and limitations. Furthermore, the following questionnaires were used: Fatigue severity scale (FSS), WHO Disability Assessment Scale (DAS 2.0), health related quality of life (SF36) and International Physical Activity Questionnaire (IPAQ).

RESULTS

Of 282 participants with confirmed diagnoses of muscle glycogenosis, 269 had GSD5. Of them 196 (73%) completed all questionnaires; for the others, the data were incomplete. The majority, 180 (67%) were currently working. Previous medical treatments included pain medication (23%) and rehabilitation treatment (60%). The carbohydrate-rich diet was reported to be beneficial for 68%, the low sucrose diet for 76% and the ketogenic diet for 88%. Almost all participants (93%) reported difficulties climbing stairs. The median FSS score was 5.22, indicating severe fatigue. The data from the WHODAS and IPAQ was not of sufficient quality to be interpreted.

CONCLUSIONS

The EUROMAC registry have provided insight into the functional and social status of participants with GSD5: most participants are socially active despite limitations in physical and daily life activities. Regular physical activity and different dietary approaches may alleviate fatigue and pain.

Development of Continuum of Care for McArdle disease: A practical tool for clinicians and patients

McArdle disease (glycogen storage disease type V; GSDV) is a rare genetic disease caused by the inability to break down glycogen in skeletal muscle due to a deficiency in myophosphorylase. Glycolysis is only partially blocked in GSDV, as muscle fibres can take up circulating glucose and convert it to glucose-6-phosphate downstream of the metabolic block. Because skeletal muscle predominantly relies on anaerobic energy during the first few minutes of transition from rest to activity, and throughout more intense activities, individuals with GSDV experience muscle fatigue/pain, tachypnea, and tachycardia during these activities. If warning signs are not heeded, a muscle contracture may rapidly occur, and if significant, may lead to acute rhabdomyolysis. Without a cure or treatment, individuals with GSDV must be consistent in employing proper management techniques; however, this can be challenging due to the nuances inherent in this metabolic myopathy. The International Association for Muscle Glycogen Storage Disease collaborated with an international team of five expert clinicians to identify areas of learning to achieve an optimal state. A Continuum of Care model was developed that outlines five pivotal steps (diagnosis; understanding; acceptance; learning and exercise) to streamline assessments and more succinctly assist clinicians in determining patient-specific learning needs. This model serves as a translational tool to help optimize care for this patient population.

Muscle glycogen unavailability and fat oxidation rate during exercise: Insights from McArdle disease

Carbohydrate availability affects fat metabolism during exercise; however, the effects of complete muscle glycogen unavailability on maximal fat oxidation (MFO) rate remain unknown. Our purpose was to examine the MFO rate in patients with McArdle disease, comprising an inherited condition caused by complete blockade of muscle glycogen metabolism, compared to healthy controls. Nine patients (three women, aged 36 ± 12 years) and 12 healthy controls (four women, aged 40 ± 13 years) were studied. Several molecular markers of lipid transport/metabolism were also determined in skeletal muscle (gastrocnemius) and white adipose tissue of McArdle (Pygm p.50R*/p.50R*) and wild-type male mice. Peak oxygen uptake ( V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ ), MFO rate, the exercise intensity eliciting MFO rate (FATmax) and the MFO rate-associated workload were determined by indirect calorimetry during an incremental cycle-ergometer test. Despite having a much lower V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ (24.7 ± 4 vs. 42.5 ± 11.4 mL kg^-1  min^-1 , respectively; P < 0.0001), patients showed considerably higher values for the MFO rate (0.53 ± 0.12 vs. 0.33 ± 0.10 g min^-1 , P = 0.001), and for the FATmax (94.4 ± 7.2 vs. 41.3 ± 9.1 % of V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ , P < 0.0001) and MFO rate-associated workload (1.33 ± 0.35 vs. 0.81 ± 0.54 W kg^-1 , P = 0.020) than controls. No between-group differences were found overall in molecular markers of lipid transport/metabolism in mice. In summary, patients with McArdle disease show an exceptionally high MFO rate, which they attained at near-maximal exercise capacity. Pending more mechanistic explanations, these findings support the influence of glycogen availability on MFO rate and suggest that these patients develop a unique fat oxidation capacity, possibly as an adaptation to compensate for the inherited blockade in glycogen metabolism, and point to MFO rate as a potential limiting factor of exercise tolerance in this disease. KEY POINTS: Physically active McArdle patients show an exceptional fat oxidation capacity. Maximal fat oxidation rate occurs near-maximal exercise capacity in these patients. McArdle patients' exercise tolerance might rely on maximal fat oxidation rate capacity. Hyperpnoea might cloud substrate oxidation measurements in some patients. An animal model revealed overall no higher molecular markers of lipid transport/metabolism.

Long-Term Exercise Intervention in Patients with McArdle Disease: Clinical and Aerobic Fitness Benefits

INTRODUCTION

The long-term effects of exercise in patients with McArdle disease-the paradigm of "exercise intolerance"-are unknown. This is an important question because the severity of the disease frequently increases with time.

PURPOSE

This study aimed to study the effects of a long-term exercise intervention on clinical and fitness-related outcomes in McArdle patients.

METHODS

Seventeen patients (exercise group: n = 10, 6 male, 38 ± 18 yr; control: n = 7, 4 male, 38 ± 18 yr) participated in a 2-yr unsupervised intervention including moderate-intensity aerobic (cycle-ergometer exercise for 1 h) and resistance (high load-low repetition circuit) training on 5 and 2-3 d·wk -1 , respectively. Patients were assessed at baseline and postintervention. Besides safety, outcomes included clinical severity (e.g., exercise intolerance features) on a 0-3 scale (primary outcome), and aerobic fitness, gross muscle efficiency, and body composition (total/regional fat, muscle, and bone mass; secondary outcomes).

RESULTS

The exercise program was safe and resulted in a reduction of 1 point (-1.0; 95% confidence interval, -1.6 to -0.5; P = 0.025) in clinical severity versus the control group, with 60% of participants in the exercise group becoming virtually asymptomatic and with no functional limitation in daily life activities. Compared with controls, the intervention induced significant and large benefits (all P < 0.05) in the workload eliciting the ventilatory threshold (both in absolute (watts, +37%) and relative units (watts per kilogram of total body mass or of lower-limb muscle mass, +44%)), peak oxygen uptake (in milliliters per kilogram per minute, +28%), and peak workload (in absolute (+27%) and relative units (+33%)). However, no significant changes were found for muscle efficiency or for any measure of body composition.

CONCLUSIONS

A 2-yr unsupervised intervention including aerobic and resistance exercise is safe and induces major benefits in the clinical course and aerobic fitness of patients with McArdle disease.