A diagnostic cycle test for McArdle's disease

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.

Diagnosis and management of metabolic myopathies

Metabolic myopathies are a set of rare inborn errors of metabolism leading to disruption in energy production. Relevant to skeletal muscle, glycogen storage disease and fatty acid oxidation defects can lead to exercise intolerance, rhabdomyolysis, and weakness in children and adults, distinct from the severe forms that involve multiple-organ systems. These nonspecific, dynamic symptoms along with conditions that mimic metabolic myopathies can make diagnosis challenging. Clinicians can shorten the time to diagnosis by recognizing the typical clinical phenotypes and performing next generation sequencing. With improved access and affordability of molecular testing, clinicians need to be well-versed in resolving variants of uncertain significance relevant to metabolic myopathies. Once identified, patients can improve quality of life, safely engage in exercise, and reduce episodes of rhabdomyolysis by modifying diet and lifestyle habits.

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.

A first in literature: anesthesia management in kidney transplant surgery of a patient with McArdle disease

McArdle disease is an inherited myopathy that autosomal recessive inheritance and is also known as glycogen storage disease type 5. Myoglobinuria, increase in serum CK level and darkening of urine color secondary to myoglobinuria are typical. Patients may have symptoms associated with increased rhabdomyolysis secondary acute renal failure or hyperkalemia after long and strenuous exercise periods. Today, many studies in the literature have shown that transplantation is superior to dialysis in patients with end-stage renal disease. Our case is a 53-year-old male patient with the diagnosis of McArdle syndrome who was going to have a kidney transplant. The patient had essential hypertension and history of HBsAg+. Total intravenous anesthesia technique was chosen as the anesthesia technique because inhaled anesthetic agents may trigger malignant hyperthermia in the patient. We didn't experience any perioperative complications in our patient. In conclusion, renal transplantation performed with total intravenous in a McArdle syndrome patient may be a simple and effective technique.

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.

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-β-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.

The Second Wind in McArdle Patients: Fitness Matters

Background: The “second wind” (SW) phenomenon—commonly referring to both an initial period of marked intolerance to dynamic exercise (e.g., brisk walking) that is not followed by perceived improvement and disappearance of previous tachycardia (i.e., the actual “SW”) until 6–10 min has elapsed—is an almost unique feature of McArdle disease that limits adherence to an active lifestyle. In this regard, an increase in the workload eliciting the SW could potentially translate into an improved patients’ exercise tolerance in daily life. We aimed to determine whether aerobic fitness and physical activity (PA) levels are correlated with the minimum workload eliciting the SW in McArdle patients—as well as with the corresponding heart rate value. We also compared the SW variables and aerobic fitness indicators in inactive vs. active patients.

Methods: Fifty-four McArdle patients (24 women, mean ± SD age 33 ± 12 years) performed 12-min constant-load and maximum ramp-like cycle-ergometer tests for SW detection and aerobic fitness [peak oxygen uptake (VO_2peak) and workload and ventilatory threshold] determination, respectively. They were categorized as physically active/inactive during the prior 6 months (active = reporting ≥150 min/week or ≥75 min/week in moderate or vigorous-intensity aerobic PA, respectively) and were also asked on their self-report of the SW.

Results: Both peak and submaximal indicators of aerobic fitness obtained in the ramp tests were significantly correlated with the workload of the SW test, with a particularly strong correlation for the VO_2peak and peak workload attained by the patients (both Pearson’s coefficients > 0.70). Twenty (seven women) and 24 patients (18 women) were categorized as physically active and inactive, respectively. Not only the aerobic fitness level [∼18–19% higher values of VO_2peak (ml⋅kg^–1⋅min^–1)] but also the workload of the SW tests was significantly higher in active than in inactive patients. All the inactive patients reported that they experienced the SW during walking/brisk walking in daily life, whereas active patients only reported experiencing this phenomenon during more strenuous activities (very brisk walking/jogging and bicycling).

Conclusion: A higher aerobic fitness and an active lifestyle are associated with a higher workload eliciting the so-called SW phenomenon in patients with McArdle disease, which has a positive impact on their exercise tolerance during daily living.

International patient group harnesses social media to help inform rare disease research: use of a low carbohydrate ketogenic diet in McArdle disease

PURPOSE OF REVIEW

To highlight the benefits of a partnership between patient advocacy organizations (PAO) and clinical researchers in order to clinically study the prior anecdotal patient experience of utilizing a low-carbohydrate ketogenic diet (LCKD) to manage McArdle disease.

RECENT FINDINGS

The bedside-to-bench and back again method of translational research is well suited to explore anecdotal patient experiences, particularly for rare diseases. In McArdle disease, patients have explored the use of a LCKD to address the ubiquitous occurrence of physical activity intolerance and minimize associated adverse events. The International Association for Muscle Glycogen Storage Disease harnessed the power of social media to better understand this patient-centric finding and has subsequently partnered with clinical researchers to convey patient-centered priorities for research. From this partnership, a series of clinical and survey studies have been initiated.

SUMMARY

Collaboration between PAOs and clinical researchers has the potential to foster patient empowerment, advance anecdotal experiences into scientific hypotheses, and ultimately guide the development of management guidelines that reflect the patient perspective.

Exercise, Nutrition, and Supplements in the Muscle Carnitine Palmitoyl-Transferase II Deficiency: New Theoretical Bases for Potential Applications

Carnitine palmitoyltransferase II (CPTII) deficiency is the most frequent inherited disorder regarding muscle fatty acid metabolism, resulting in a reduced mitochondrial long-chain fatty acid oxidation during endurance exercise. This condition leads to a clinical syndrome characterized by muscle fatigue and/or muscle pain with a variable annual frequency of severe rhabdomyolytic episodes. While since the CPTII deficiency discovery remarkable scientific advancements have been reached in genetic analysis, pathophysiology and diagnoses, the same cannot be said for the methods of treatments. The current recommendations remain those of following a carbohydrates-rich diet with a limited fats intake and reducing, even excluding, physical activity, without, however, taking into account the long-term consequences of this approach. Suggestions to use carnitine and medium chain triglycerides remain controversial; conversely, other potential dietary supplements able to sustain muscle metabolism and recovery from exercise have never been taken into consideration. The aim of this review is to clarify biochemical mechanisms related to nutrition and physiological aspects of muscle metabolism related to exercise in order to propose new theoretical bases of treatment which, if properly tested and validated by future trials, could be applied to improve the quality of life of these patients.

A New Glycogen Storage Disease Caused by a Dominant PYGM Mutation

OBJECTIVE

Glycogen storage diseases (GSDs) are severe human disorders resulting from abnormal glucose metabolism, and all previously described GSDs segregate as autosomal recessive or X-linked traits. In this study, we aimed to molecularly characterize the first family with a dominant GSD.

METHODS

We describe a dominant GSD family with 13 affected members presenting with adult-onset muscle weakness, and we provide clinical, metabolic, histological, and ultrastructural data. We performed exome sequencing to uncover the causative gene, and functional experiments in the cell model and on recombinant proteins to investigate the pathogenic effect of the identified mutation.

RESULTS

We identified a heterozygous missense mutation in PYGM segregating with the disease in the family. PYGM codes for myophosphorylase, the enzyme catalyzing the initial step of glycogen breakdown. Enzymatic tests revealed that the PYGM mutation impairs the AMP-independent myophosphorylase activity, whereas the AMP-dependent activity was preserved. Further functional investigations demonstrated an altered conformation and aggregation of mutant myophosphorylase, and the concurrent accumulation of the intermediate filament desmin in the myofibers of the patients.

INTERPRETATION

Overall, this study describes the first example of a dominant glycogen storage disease in humans, and elucidates the underlying pathomechanisms by deciphering the sequence of events from the PYGM mutation to the accumulation of glycogen in the muscle fibers. ANN NEUROL 2020;88:274-282.

Mastication and Oral Motor Function in McArdle Disease: Patient Reported Complaints

BACKGROUND

Exertional myalgia and cramps of the limb and trunk muscles are typical in McArdle disease, but mastication and oral motor limitations have not been systematically investigated before.

OBJECTIVE

Determine the reported prevalence and characteristics of limitations on oral motor activities, mastication, swallowing, and other oral motor activities in patients with McArdle disease.

METHODS

An observational study was carried out in 28 patients using a standardised questionnaire on mastication and oral motor function.

RESULTS

57% of the participants reported difficulties with mastication. Muscle cramps during mastication occurred in 57% of the patients. A feeling that food remains in the throat occurred in 32%. Adaptations during mastication were needed in 42% of the patients. Mastication difficulties, muscle cramps during mastication and mastication adjustments were inversely correlated with age (r2 = - 0.445, p < 0.05; r2 = - 0.509, p < 0.01; r2 = - 0.456, p < 0.05). Feeling of food remaining in throat, cramps during mastication and during other oral motor muscle activities, were correlated with disease severity (r2 = 0.476, p < 0.01; r2 = 0.463, p < 0.01; r2 = 0.461, p < 0.01; r2 = 0.432, p < 0.05).

CONCLUSIONS

In short, reported mastication difficulties are prevalent in people with McArdle disease, most frequently in younger people. Therefore, awareness of mastication and oral motor problems is required.

Manifesting heterozygotes in McArdle disease: a myth or a reality-role of statins

McArdle disease is an autosomal recessive condition caused by deficiency of the PYGM gene-encoded muscle isoform of glycogen phosphorylase. Some cases of "manifesting" heterozygotes or carriers (i.e., patients who show some McArdle-like symptoms or signs despite being carriers of only one mutated PYGM allele) have been reported in the literature but there is controversy, with misdiagnosis being a possibility. The purpose of our study was to determine if there are actually "manifesting" heterozygotes of McArdle disease and, if existing, whether statin treatment can trigger such condition. Eighty-one relatives of McArdle patients (among a total of 16 different families) were studied. We determined whether they were carriers of PYGM mutations and also collected information on exercise tests (second wind and modified Wingate anaerobic test) and statin intake. We found 50 carriers and 31 non-carriers of PYGM mutations. Although we found existence of heterozygotes manifesting some exercise-related muscle problems such as exacerbated myalgia or weakness, they only accounted for 14% of the carriers and muscle symptoms were milder than those commonly reported in patients. Further, no carrier (whether reporting symptoms or not) showed the second wind phenomenon or a flat blood lactate response to maximal-intensity exercise, both of which are hallmarks of McArdle disease. On the other hand, statin myotoxicity was not associated with muscle symptom onset.

Missense mutations have unexpected consequences: The McArdle disease paradigm

McArdle disease is a disorder of muscle glycogen metabolism caused by mutations in the PYGM gene, encoding for the muscle-specific isoform of glycogen phosphorylase (M-GP). The activity of this enzyme is completely lost in patients' muscle biopsies, when measured with a standard biochemical test which, does not allow to determine M-GP protein levels. We aimed to determine M-GP protein levels in the muscle of McArdle patients, by studying biopsies of 40 patients harboring a broad spectrum of PYGM mutations and 22 controls. Lack of M-GP protein was found in muscle in the vast majority (95%) of patients, irrespective of the PYGM genotype, including those carrying missense mutations, with few exceptions. M-GP protein biosynthesis is not being produced by PYGM mutations inducing premature termination codons (PTC), neither by most PYGM missense mutations. These findings explain the lack of PYGM genotype-phenotype correlation and have important implications for the design of molecular-based therapeutic approaches.

Clinical utility gene card for McArdle disease

Name of the disease (synonyms) McArdle disease (glycogenosis type V; glycogen storage disease V (GSDV); PYGM deficiency; muscle glycogen phosphorylase deficiency; myophosphorylase deficiency). OMIM# of the disease #232600. Name of the analysed genes or DNA/chromosome segments Muscle glycogen phosphoryalse (PYGM). OMIM# of the gene(s) #608455.Review of the analytical and clinical validity as well as of the clinical utility of DNA-based testing for variants in the PYGM gene(s) in⊠ diagnostic,⊠ predictive and⊠ prenatal settings and for⊠ risk assessment in relatives.

The role of electrodiagnosis with long exercise test in mcardle disease

INTRODUCTION

In this study we evaluated the role of an electrodiagnostic provocative test (long exercise test) in McArdle disease.

METHODS

Twenty-five McArdle patients and 2 control groups underwent an electrodiagnostic protocol with long exercise test (LET), consisting of recording the compound muscle action potential (CMAP) before and after 5 minutes of isometric contraction.

RESULTS

The LET disclosed a postexercise decrease in CMAP amplitude in 23 of 25 McArdle patients. The immediate and long-lasting decrease differentiated McArdle patients from controls. Patients with a normal LET demonstrated milder symptoms and/or residual myophosphorylase activity.

DISCUSSION

The LET is a sensitive, safe, and noninvasive provocative test that may guide clinicians toward molecular analysis of the myophosphorylase gene. The abnormalities observed on LET point toward complex biochemical mechanisms determined by the absence of myophosphorylase, beyond simple glycolytic blockade (ionic pump dysfunction, sarcolemmal inexcitability). The normal LET in patients with milder symptoms indicates a relationship of the LET with clinical severity, thus identifying it as a potential outcome measure. Muscle Nerve, 2018.

Metabolic myopathies: a practical approach

Metabolic myopathies are a diverse group of rare genetic disorders and their associated muscle symptoms may be subtle. Patients may present with indolent myopathic features, exercise intolerance or recurrent rhabdomyolysis. Diagnostic delays are common and clinicians need a high index of suspicion to recognise and differentiate metabolic myopathies from other conditions that present in a similar fashion. Standard laboratory tests may be normal or non-specific, particularly between symptomatic episodes. Targeted enzyme activity measurement and next-generation genetic sequencing are increasingly used. There are now specific enzyme replacement therapies available, and other metabolic strategies and gene therapies are undergoing clinical trials. Here, we discuss our approach to the adult patient with suspected metabolic myopathy. We outline key features in the history and examination and discuss some mimics of metabolic myopathies. We highlight some disorders of glycogen and fatty acid utilisation that present in adulthood and outline current recommendations on management.

Genotypic and phenotypic features of all Spanish patients with McArdle disease: a 2016 update

BACKGROUND

We recently described the genotype/phenotype features of all Spanish patients diagnosed with McArdle disease as of January 2011 (n = 239, prevalence of ~1/167,000) (J Neurol Neurosurg Psychiatry 2012;83:322-8). Several caveats were however identified suggesting that the prevalence of the disease is actually higher.

METHODS

We have now updated main genotype/phenotype data, as well as potential associations within/between them, of all Spanish individuals currently diagnosed with McArdle disease (December 2016).

RESULTS

Ninety-four new patients (all Caucasian) have been diagnosed, yielding a prevalence of ~1/139,543 individuals. Around 55% of the mutated alleles have the commonest PYGM pathogenic mutation p.R50X, whereas p.W798R and p.G205S account for 10 and 9% of the allelic variants, respectively. Seven new mutations were identified: p.H35R, p.R70C, p.R94Q, p.L132WfsX163, p.Q176P, p.R576Q, and c.244-3_244-2CA. Almost all patients show exercise intolerance, the second wind phenomenon and high serum creatine kinase activity. There is, however, heterogeneity in clinical severity, with 8% of patients being asymptomatic during normal daily life, and 21% showing limitations during daily activities and fixed muscle weakness. A major remaining challenge is one of diagnosis, which is often delayed until the third decade of life in 72% of new patients despite the vast majority (86%) reporting symptoms before 20 years. An important development is the growing proportion of those reporting a 4-year improvement in disease severity (now 34%) and following an active lifestyle (50%). Physically active patients are more likely to report an improvement after a 4-year period in the clinical course of the disease than their inactive peers (odds ratio: 13.98; 95% confidence interval: 5.6, 34.9; p < 0.001). Peak oxygen uptake is also higher in the former (20.7 ± 6.0 vs. 16.8 ± 5.3 mL/kg/min, p = 0.0013). Finally, there is no association between PYGM genotype and phenotype manifestation of the disease.

CONCLUSIONS

The reported prevalence of McArdle disease grows exponentially despite frequent, long delays in genetic diagnosis, suggesting that many patients remain undiagnosed. Until a genetic cure is available (which is not predicted in the near future), current epidemiologic data support that adoption of an active lifestyle is the best medicine for these patients.

Impaired glycogen breakdown and synthesis in phosphoglucomutase 1 deficiency

OBJECTIVE

We investigated metabolism and physiological responses to exercise in an 18-year-old woman with multiple congenital abnormalities and exertional muscle fatigue, tightness, and rhabdomyolysis.

METHODS

We studied biochemistry in muscle and fibroblasts, performed mutation analysis, assessed physiological responses to forearm and cycle-ergometer exercise combined with stable-isotope techniques and indirect calorimetry, and evaluated the effect of IV glucose infusion and oral sucrose ingestion on the exercise response.

RESULTS

Phosphoglucomutase type 1 (PGM1) activity in muscle and fibroblasts was severely deficient and PGM1 in muscle was undetectable by Western blot. The patient was compound heterozygous for missense (R422W) and nonsense (Q530X) mutations in PGM1. Forearm exercise elicited no increase in lactate, but an exaggerated increase in ammonia, and provoked a forearm contracture. Comparable to patients with McArdle disease, the patient developed a 'second wind' with a spontaneous fall in exercise heart rate and perceived exertion. Like in McArdle disease, this was attributable to an increase in muscle oxidative capacity. Carbohydrate oxidation was blocked during exercise, and the patient had exaggerated oxidation of fat to fuel exercise. Exercise heart rate and perceived exertion were lower after IV glucose and oral sucrose. Muscle glycogen level was low normal.

CONCLUSIONS

The second wind phenomenon has been considered to be pathognomonic for McArdle disease, but we demonstrate that it can also be present in PGM1 deficiency. We show that severe loss of PGM1 activity causes blocked muscle glycogenolysis that mimics McArdle disease, but may also limit glycogen synthesis, which broadens the phenotypic spectrum of this disorder.

Home-based aerobic exercise training improves skeletal muscle oxidative metabolism in patients with metabolic myopathies

Aerobic training can be effective in patients with mitochondrial myopathies (MM) and McArdle's disease (McA). The aim of the study was to use noninvasive functional evaluation methods, specifically aimed at skeletal muscle oxidative metabolism, to evaluate the effects of an aerobic exercise training (cycle ergometer, 12 wk, 4 days/wk, ∼65-70% of maximal heart rate) in 6 MM and 7 McA. Oxygen uptake and skeletal muscle vastus lateralis fractional O2 extraction by near-infrared spectroscopy were assessed during incremental and low-intensity constant work rate (CWR) exercises before (BEFORE) and at the end (AFTER) of training. Peak O2 uptake increased significantly with training both in MM [14.7 ± 1.2 vs. 17.6 ± 1.4 ml·kg(-1)·min(-1) (mean ± SD)] and in McA (18.5 ± 1.8 ml·kg(-1)·min(-1) vs. 21.6 ± 1.9). Peak skeletal muscle fractional O2 extraction increased with training both in MM (22.0 ± 6.7 vs. 32.6 ± 5.9%) and in McA (18.5 ± 6.2 vs. 37.2 ± 7.2%). During low-intensity CWR in both MM and McA: V̇o2 kinetics became faster in AFTER, but only in the patients with slow V̇o2 kinetics in BEFORE; the transient overshoot in fractional O2 extraction kinetics disappeared. The level of habitual physical activity was not higher 3 mo after training (FOLLOW-UP vs. PRE). In MM and McA patients a home-based aerobic training program significantly attenuated the impairment of skeletal muscle oxidative metabolism and improved variables associated with exercise tolerance. Our findings indicate that in MM and McA patients near-infrared spectroscopy and V̇o2 kinetics can effectively detect the functional improvements obtained by training.

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.

Phenotype consequences of myophosphorylase dysfunction: insights from the McArdle mouse model

KEY POINTS

This is the first study to analyse the effect of muscle glycogen phosphorylase depletion in metabolically different muscle types. In McArdle mice, muscle glycogen phosphorylase is absent in both oxidative and glycolytic muscles. In McArdle mice, the glycogen debranching enzyme (catabolic) is increased in oxidative muscles, whereas the glycogen branching enzyme (anabolic) is increased in glycolytic muscles. In McArdle mice, total glycogen synthase is decreased in both oxidative and glycolytic muscles, whereas the phosphorylated inactive form of the enzyme is increased in both oxidative and glycolytic enzymes. In McArdle mice, glycogen content is higher in glycolytic muscles than in oxidative muscles. Additionally, in all muscles analysed, the glycogen content is higher in males than in females. The maximal endurance capacity of the McArdle mice is significantly lower compared to heterozygous and wild-type mice.

ABSTRACT

McArdle disease, caused by inherited deficiency of the enzyme muscle glycogen phosphorylase (GP-MM), is arguably the paradigm of exercise intolerance. The recent knock-in (p.R50X/p.R50X) mouse disease model allows an investigation of the phenotypic consequences of muscle glycogen unavailability and the physiopathology of exercise intolerance. We analysed, in 2-month-old mice [wild-type (wt/wt), heterozygous (p.R50X/wt) and p.R50X/p.R50X)], maximal endurance exercise capacity and the molecular consequences of an absence of GP-MM in the main glycogen metabolism regulatory enzymes: glycogen synthase, glycogen branching enzyme and glycogen debranching enzyme, as well as glycogen content in slow-twitch (soleus), intermediate (gastrocnemius) and glycolytic/fast-twitch (extensor digitorum longus; EDL) muscles. Compared with wt/wt, exercise capacity (measured in a treadmill test) was impaired in p.R50X/p.R50X (∼48%) and p.R50X/wt mice (∼18%). p.R50X/p.R50X mice showed an absence of GP-MM in the three muscles. GP-MM was reduced in p.R50X/wt mice, especially in the soleus, suggesting that the function of 'slow-twitch' muscles is less dependent on glycogen catabolism. p.R50X/p.R50X mice showed increased glycogen debranching enzyme in the soleus, increased glycogen branching enzyme in the gastrocnemius and EDL, as well as reduced levels of mucle glycogen synthase protein in the three muscles (mean ∼70%), reflecting a protective mechanism for preventing deleterious glycogen accumulation. Additionally, glycogen content was highest in the EDL of p.R50X/p.R50X mice. Amongst other findings, the present study shows that the expression of the main muscle glycogen regulatory enzymes differs depending on the muscle phenotype (slow- vs. fast-twitch) and that even partial GP-MM deficiency affects maximal endurance capacity. Our knock-in model might help to provide insights into the importance of glycogen on muscle function.

Rhabdomyolysis: a genetic perspective

Rhabdomyolysis (RM) is a clinical emergency characterized by fulminant skeletal muscle damage and release of intracellular muscle components into the blood stream leading to myoglobinuria and, in severe cases, acute renal failure. Apart from trauma, a wide range of causes have been reported including drug abuse and infections. Underlying genetic disorders are also a cause of RM and can often pose a diagnostic challenge, considering their marked heterogeneity and comparative rarity.

In this paper we review the range of rare genetic defects known to be associated with RM. Each gene has been reviewed for the following: clinical phenotype, typical triggers for RM and recommended diagnostic approach. The purpose of this review is to highlight the most important features associated with specific genetic defects in order to aid the diagnosis of patients presenting with hereditary causes of recurrent RM.

Diagnostic power of the non-ischaemic forearm exercise test in detecting glycogenosis type V

BACKGROUND AND PURPOSE

This was a retrospective study to assess the diagnostic value of the non-ischaemic forearm exercise test in detecting McArdle's disease.

METHODS

The study is a retrospective diagnostic study over 15 years (1999-2013) on a referred sample of patients suffering from exercise intolerance and various muscle complaints, generally with elevated creatine kinase (CK). In all, 1226 patients underwent the non-ischaemic forearm exercise test. Blood lactate, ammonia and CK levels were analyzed. DNA analyses and/or muscle biopsies were assessed to confirm the diagnosis of McArdle's disease. The results of 60 volunteers were used to compare with the results of study subjects.

RESULTS

In this cohort, 40 patients were finally diagnosed with McArdle's disease. Absolute values of lactate and ammonia rise were used to discriminate all McArdle patients from healthy patients. A sensitivity and specificity of respectively 100% and 99.7% were calculated. The 24-h CK level showed no significant difference from the CK level at the day of the test and confirms the safety of the test.

CONCLUSIONS

This study has formally assessed the diagnostic value of the non-ischaemic forearm exercise test in the detection of McArdle's disease. Very high sensitivity and specificity were observed. Furthermore, the test is easy to set up and to perform, it is non-traumatic and cost effective. It may circumvent a muscle biopsy in McArdle patients presenting the most common mutations. Hence, it is a perfect and safe screening instrument to detect patients with McArdle's disease. Glycogen storage disease type III patients, however, may show similar patterns to McArdle patients.

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.

From exercise intolerance to functional improvement: the second wind phenomenon in the identification of McArdle disease

McArdle disease is the most common of the glycogen storage diseases. Onset of symptoms is usually in childhood with muscle pain and restricted exercise capacity. Signs and symptoms are often ignored in children or put down to ‘growing pains’ and thus diagnosis is often delayed. Misdiagnosis is not uncommon because several other conditions such as muscular dystrophy and muscle channelopathies can manifest with similar symptoms. A simple exercise test performed in the clinic can however help to identify patients by revealing the second wind phenomenon which is pathognomonic of the condition. Here a patient is reported illustrating the value of using a simple 12 minute walk test.

The significance of clinical and laboratory features in the diagnosis of glycogen storage disease type v: a case report

Glycogen storage disease type V (GSD-V) is the most common disorder of muscle glycogenosis with characteristic clinical and laboratory findings. A 32-yr-old woman complained of exercise intolerance and myoglobulinuria since early adolescence. She reported several episodes of second-wind phenomenon. Physical examination did not show any neurological abnormality, including fixed muscle weakness or atrophy. Serum creatine kinase level was 1,161 IU/L at rest. The result of the non-ischemic forearm exercise test was compatible with GSD-V. Mutation analysis identified the compound heterozygous mutations of the PYGM, p.D510fs and p.F710del, which has not yet been reported in Korea. The present case recognizes that detail clinical and laboratory analysis is the first step in the diagnosis of GSD-V.

Rhabdomyolysis: review of the literature

Rhabdomyolysis is a serious and potentially life threatening condition. Although consensus criteria for rhabdomyolysis is lacking, a reasonable definition is elevation of serum creatine kinase activity of at least 10 times the upper limit of normal followed by a rapid decrease of the sCK level to (near) normal values. The clinical presentation can vary widely, classical features are myalgia, weakness and pigmenturia. However, this classic triad is seen in less than 10% of patients. Acute renal failure due to acute tubular necrosis as a result of mechanical obstruction by myoglobin is the most common complication, in particular if sCK is >16.000 IU/l, which may be as high as 100,000 IU/l. Mortality rate is approximately 10% and significantly higher in patients with acute renal failure. Timely recognition of rhabdomyolysis is key for treatment. In the acute phase, treatment should be aimed at preserving renal function, resolving compartment syndrome, restoring metabolic derangements, and volume replacement. Most patients experience only one episode of rhabdomyolysis, mostly by substance abuse, medication, trauma or epileptic seizures. In case of recurrent rhabdomyolysis, a history of exercise intolerance or a positive family history for neuromuscular disorders, further investigations are needed to identify the underlying, often genetic, disorder. We propose a diagnostic algorithm for use in clinical practice.

The "second wind" in McArdle's disease patients during a second bout of constant work rate submaximal exercise

Patients with McArdle's disease (McA) typically show the "second-wind" phenomenon, a sudden decrease in heart rate (HR) and an improved exercise tolerance occurring after a few minutes of exercise. In the present study, we investigated whether in McA a first bout of exercise determines a second wind during a second bout, separated by the first by a few minutes of recovery. Eight McA (44 ± 4 yr) and a control group of six mitochondrial myopathy patients (51 ± 6 yr) performed two repetitions (CWR1 and CWR2) of 6-min constant work rate exercise (∼50% of peak work rate) separated by 6-min (SHORT) or 18-min (LONG) recovery. Pulmonary O2 uptake (Vo2), HR, cardiac output, rates of perceived exertion, vastus lateralis oxygenation {changes in deoxygenated Hb and myoglobin Mb concentrations, Δ[deoxy(Hb+Mb)], by near-infrared spectroscopy} were determined. In McA, Vo2 (0.86 ± 0.2 vs. 0.95 ± 0.1 l/min), HR (113 ± 10 vs. 150 ± 13 beats/min), cardiac output (11.6 ± 0.6 vs. 15.0 ± 0.8 l/min), and rates of perceived exertion (11 ± 2 vs. 14 ± 3) were lower, whereas Δ[deoxy(Hb+Mb)] was higher (14.7 ± 2.3 vs. -0.1 ± 4.6%) in CWR2-SHORT vs. CWR1; the "overshoot" of Δ[deoxy(Hb+Mb)] and the "slow component" of Vo2 kinetics disappeared in CWR2-SHORT. No differences (vs. CWR1) were observed in McA during CWR2-LONG, or in mitochondrial myopathy patients during both CWR2-SHORT and -LONG. A second-wind phenomenon was observed in McA during the second of two consecutive 6-min constant-work rate submaximal exercises. The second wind was associated with changes of physiological variables, suggesting an enhanced skeletal muscle oxidative metabolism. The second wind was not described after a longer (18-min) recovery period.

McArdle's disease (glycogen storage disease type V) and anesthesia--a case report and review of the literature

McArdles disease (glycogen storage disease type v) is a rare condition in which energy-metabolism in the muscle is hampered. A case report is presented and the possible risk for perioperative complications including malignant hyperthermia is discussed. A checklist for the anesthesiological management of patients with McArdles disease is provided. A short overview of anesthesiological challenges and perioperative complications of other glycogen storage diseases is given.

Exercise testing in metabolic myopathies

Metabolic myopathies are a group of genetic disorders specifically affecting glucose/glycogen, lipid, and mitochondrial metabolism. The main metabolic myopathies that are evaluated in this article are the mitochondrial myopathies, fatty acid oxidation defects, and glycogen storage disease. This article focuses on the usefulness of exercise in the evaluation of genetic metabolic myopathies.

Physical training for McArdle disease

BACKGROUND

McArdle disease is a rare metabolic myopathy caused by a complete absence of the enzyme muscle glycogen phosphorylase. Affected people experience symptoms of fatigue and cramping within minutes of exercise and are at risk for acute muscle injury (rhabdomyolysis) and acute renal failure. If the first few minutes of exercise are paced, a 'second wind' will occur enabling exercise to continue. This is due to mobilisation and utilisation of alternative fuel substrates. Aerobic training appears to improve work capacity by increasing cardiovascular fitness.

OBJECTIVES

To assess the effects of aerobic training in people with McArdle disease.

SEARCH METHODS

We searched the Cochrane Neuromuscular Disease Group Specialized Register (11 January 2011), CENTRAL (2010, Issue 4), MEDLINE (January 1966 to January 2011) and EMBASE (January 1980 to January 2011).

SELECTION CRITERIA

All randomised and quasi-randomised controlled studies of aerobic exercise training in people of all ages with McArdle disease.

DATA COLLECTION AND ANALYSIS

Two authors identified possible studies for inclusion and assessed their methodological quality. Had more than one study of sufficient methodological quality been identified we would have undertaken a meta-analysis.

MAIN RESULTS

There were no randomised or quasi-randomised controlled trials of aerobic training in people with McArdle disease. However, three open studies using small numbers of participants provided some evidence that aerobic training improves fitness without adverse events in people with McArdle disease.

AUTHORS' CONCLUSIONS

Evidence from non-randomised studies using small numbers of patients suggest that it would be safe and worthwhile for larger controlled trials of aerobic training to be undertaken in people with McArdle disease.

Clinical and laboratory features of patients with myophosphorylase deficiency (McArdle disease)

Mutations of PYGM, the gene encoding human myophosphorylase, produce a metabolic myopathy characterised by exercise intolerance and, in some patients, myoglobinuria. To illustrate the clinical and laboratory features of myophosphorylase deficiency, we describe 10 patients diagnosed in Auckland, New Zealand, between 1989 and 2009. We review the clinical, biochemical, and histologic features and the results of mutation analysis. All patients reported exercise intolerance since childhood or the teenage years, starting within minutes of moderate or intense exertion. The "second wind" phenomenon, or myoglobinuria, were each reported in about half the patients. The serum creatine kinase concentration was elevated in all patients where this had been measured. Muscle biopsies revealed subsarcolemmal vacuolation and histochemical absence of myophosphorylase. Analysis of PYGM showed mutations in all alleles, most commonly Arg49Ter or Gly204Ser. One patient harbored a novel mutation, Pro488Arg, predicted to seriously disrupt the tertiary structure of the enzyme. Myophosphorylase deficiency produces a fairly uniform set of symptoms, and consistent elevation of the serum creatine kinase concentration. The diagnosis can be confirmed in most patients by mutation analysis using a blood sample.

Exercise stress testing in children with metabolic or neuromuscular disorders

The role of exercise as a diagnostic or therapeutic tool in patients with a metabolic disease (MD) or neuromuscular disorder (NMD) is relatively underresearched. In this paper we describe the metabolic profiles during exercise in 13 children (9 boys, 4 girls, age 5–15 yrs) with a diagnosed MD or NMD. Graded cardiopulmonary exercise tests and/or a 90-min prolonged submaximal exercise test were performed. During exercise, respiratory gas-exchange and heart rate were monitored; blood and urine samples were collected for biochemical analysis at set time points. Several characteristics in our patient group were observed, which reflected the differences in pathophysiology of the various disorders. Metabolic profiles during exercises CPET and PXT seem helpful in the evaluation of patients with a MD or NMD.

Excessive skeletal muscle recruitment during strenuous exercise in McArdle patients

We compared the cardiorespiratory response and muscle recruitment [as determined by electromyography (EMG)] of 37 McArdle patients [19 males, 37.4 ± 2.8 years, body mass index (BMI): 25.1 ± 4.7 kg m(-2)] and 33 healthy controls (18 males, 36.4 ± 10.0 years, BMI: 25.7 ± 3.8 kg m(-2)) during cycle-ergometer exercise (an incremental test to exhaustion and a 12-min submaximal constant workload test). We obtained cardiorespiratory [oxygen uptake and heart rate (HR)] and EMG data (rectus femoris and vastus lateralis muscles). During the incremental test, the patients exhibited the expected hyperkinetic cardiovascular response shown by a marked increase in the slope of the HR:Power relationship (p < 0.001). Throughout the incremental test and at the point of fatigue, the patients produced significantly less power than the controls (peak power output: 67 ± 21 vs. 214 ± 56 watts respectively, p < 0.001), yet they demonstrated significantly higher levels of muscle activity for a given absolute power. During the constant workload test, patients displayed higher levels of EMG activity than the controls during the second half of the test, despite a lower power production (34 ± 13 vs. 94 ± 29 watts respectively, p < 0.001). In conclusion, since the McArdle patients required more motor unit recruitment for a given power output, our data suggest that the state of contractility of their muscles is reduced compared with healthy people. Excessive muscle recruitment for a given load could be one of the mechanisms explaining the exercise intolerance of these patients.

Metabolic myopathies: functional evaluation by analysis of oxygen uptake kinetics

PURPOSE

The aim was to identify additional noninvasive tools allowing to detect and to quantify the metabolic impairment in patients with mitochondrial myopathies (MM) or McArdle's disease (McA).

METHODS

Kinetics of adjustment of pulmonary oxygen uptake (VO2 kinetics) during transitions to constant-load moderate-intensity cycle ergometer exercise were determined on 15 MM, 8 McA, 21 patients with signs and/or symptoms of metabolic myopathy but a negative biopsy ("patient controls"; P-CTRL), and 22 healthy untrained controls (CTRL).

RESULTS

VO2 kinetics were slower in MM and in McA versus P-CTRL and CTRL, slower in McA versus MM, and not significantly different between P-CTRL and CTRL. The time constants (tau) of the monoexponential function describing the VO2 kinetics were (X +/- SE) 59.2 +/- 8.5 s in MM, 87.6 +/- 16.4 s in McA, 36.9 +/- 3.1 s in P-CTRL, and 35.4 +/- 1.9 s in CTRL. In a subgroup of the patients (eight MM and seven McA), tau of VO2 kinetics were negatively correlated with two variables determined in a previous study (Grassi B, Marzorati M, Lanfranconi F, et al. Impaired oxygen extraction in metabolic myopathies: detection and quantification by near-infrared spectroscopy. Muscle Nerve. 2007;35:510-20): a) a muscle oxygenation index, obtained by near-infrared spectroscopy, estimating the peak capacity of skeletal muscle fractional O2 extraction; and b) VO2 peak.

CONCLUSIONS

In MM and McA patients, analysis of pulmonary VO2 kinetics during moderate-intensity exercise allows to identify and to quantify, noninvasively, the impairment of skeletal muscle oxidative metabolism. In these patients, the slower VO2 kinetics can be considered a marker of the impaired exercise tolerance. The present data could be useful for clinicians who need an objective, quantitative, and longitudinal evaluation of the impairment to be used in the follow-up of these patients as well as in the assessment of therapeutic interventions.

The second wind phenomenon in very young McArdle's patients

We investigated the phenomenon of second wind in four patients with McArdle's disease: a brother and sister (aged 4 and 12 years respectively) and two unrelated patients, a boy of 14 and a 17-year-old girl. We also studied the siblings' healthy 6-year-old sister. Each patient performed a 15-min exercise test at a constant workload and a subsequent graded exercise test until exhaustion. Overall the healthy girl and the youngest McArdle's patient, the 4-year-old boy, did not show a second wind phenomenon. Further, the peak cardio-respiratory capacity of the young McArdle's boy was normal for his age (32.3 mL 02/kg/min) and he did not report any function limitations during physical education classes.

Splice mutations preserve myophosphorylase activity that ameliorates the phenotype in McArdle disease

Over 100 mutations in the myophosphorylase gene, which cause McArdle disease, are known. All these mutations have resulted in a complete block of muscle glycogenolysis, and accordingly, no genotype-phenotype correlation has been identified in this condition. We evaluated physiologic and genetic features of two patients with a variant form of McArdle disease, associated with unusually high exercise capacity. Physiologic findings were compared to those in 47 patients with typical McArdle disease, and 17 healthy subjects. Subjects performed an ischaemic forearm exercise test to assess lactate and ammonia production. Peak oxidative capacity (VO2max) and cardiac output were determined, using cycle ergometry as the exercise modality. The two patients with atypical McArdle disease carried common mutations on one allele (R50X and G205S), and novel splice mutations in introns 3 [IVS3-26A>G (c.425-26A>G)] and 5 [IVS5-601G>A (c.856-601G>A)] on the other allele. Plasma lactate after ischaemic exercise decreased in all typical McArdle patients, but increased in the two atypical McArdle patients (10% of that in healthy subjects). Peak workload and oxidative capacity were 2-fold higher in patients with atypical McArdle disease compared to typical McArdle patients. Oxygen uptake, relative to cardiac output, was severely impaired in the 47 patients with typical McArdle disease, and partially normalized in the milder affected McArdle patients. These findings identify the first distinct genotype-phenotype relationship in McArdle disease, and indicate that minimal myophosphorylase activity ameliorates the typical McArdle disease phenotype by augmenting muscle oxidative capacity. The milder form of McArdle disease provides important clues to the level of functional myophosphorylase needed to support muscle oxidative metabolism.

McArdle disease: what do neurologists need to know?

McArdle disease (also known as glycogen storage disease type V) is a pure myopathy caused by an inherited deficit of myophosphorylase, the skeletal muscle isoform of the enzyme glycogen phosphorylase. The disease exhibits clinical heterogeneity, but patients typically experience exercise intolerance, that is, reversible, acute crises (early fatigue and contractures, sometimes with rhabdomyolysis and myoglobinuria) triggered by static muscle contractions (e.g. lifting weights) or dynamic exercise (e.g. climbing stairs or running). In this Review, we discuss the main features of McArdle disease, with the aim of providing neurologists with up-to-date, useful information to assist their patients. The topics covered include diagnostic tools-for example, molecular genetic diagnosis, the classic ischemic forearm test and the so-called 'second wind' phenomenon-and current therapeutic options-for example, a carbohydrate-rich diet and carbohydrate ingestion shortly before strenuous exercise, in combination with medically supervised aerobic training of low to moderate intensity.