What can metabolic myopathies teach us about exercise physiology?

The Application of Creatine Supplementation in Medical Rehabilitation

Numerous health conditions affecting the musculoskeletal, cardiopulmonary, and nervous systems can result in physical dysfunction, impaired performance, muscle weakness, and disuse-induced atrophy. Due to its well-documented anabolic potential, creatine monohydrate has been investigated as a supplemental agent to mitigate the loss of muscle mass and function in a variety of acute and chronic conditions. A review of the literature was conducted to assess the current state of knowledge regarding the effects of creatine supplementation on rehabilitation from immobilization and injury, neurodegenerative diseases, cardiopulmonary disease, and other muscular disorders. Several of the findings are encouraging, showcasing creatine's potential efficacy as a supplemental agent via preservation of muscle mass, strength, and physical function; however, the results are not consistent. For multiple diseases, only a few creatine studies with small sample sizes have been published, making it difficult to draw definitive conclusions. Rationale for discordant findings is further complicated by differences in disease pathologies, intervention protocols, creatine dosing and duration, and patient population. While creatine supplementation demonstrates promise as a therapeutic aid, more research is needed to fill gaps in knowledge within medical rehabilitation.

Targeted Therapies for Metabolic Myopathies Related to Glycogen Storage and Lipid Metabolism: a Systematic Review and Steps Towards a 'Treatabolome'

BACKGROUND

Metabolic myopathies are a heterogenous group of muscle diseases typically characterized by exercise intolerance, myalgia and progressive muscle weakness. Effective treatments for some of these diseases are available, but while our understanding of the pathogenesis of metabolic myopathies related to glycogen storage, lipid metabolism and β-oxidation is well established, evidence linking treatments with the precise causative genetic defect is lacking.

OBJECTIVE

The objective of this study was to collate all published evidence on pharmacological therapies for the aforementioned metabolic myopathies and link this to the genetic mutation in a format amenable to databasing for further computational use in line with the principles of the "treatabolome" project.

METHODS

A systematic literature review was conducted to retrieve all levels of evidence examining the therapeutic efficacy of pharmacological treatments on metabolic myopathies related to glycogen storage and lipid metabolism. A key inclusion criterion was the availability of the genetic variant of the treated patients in order to link treatment outcome with the genetic defect.

RESULTS

Of the 1,085 articles initially identified, 268 full-text articles were assessed for eligibility, of which 87 were carried over into the final data extraction. The most studied metabolic myopathies were Pompe disease (45 articles), multiple acyl-CoA dehydrogenase deficiency related to mutations in the ETFDH gene (15 articles) and systemic primary carnitine deficiency (8 articles). The most studied therapeutic management strategies for these diseases were enzyme replacement therapy, riboflavin, and carnitine supplementation, respectively.

CONCLUSIONS

This systematic review provides evidence for treatments of metabolic myopathies linked with the genetic defect in a computationally accessible format suitable for databasing in the treatabolome system, which will enable clinicians to acquire evidence on appropriate therapeutic options for their patient at the time of diagnosis.

The ratio of maximal handgrip force and maximal cycloergometry power as a diagnostic tool to screen for metabolic myopathies

Metabolic myopathies comprise a diverse group of inborn errors of intermediary metabolism affecting skeletal muscle, and often present clinically as an inability to perform normal exercise. Our aim was to use the maximal mechanical performances achieved during two functional tests, isometric handgrip test and cycloergometer, to identify metabolic myopathies among patients consulting for exercise-induced myalgia. Eighty-three patients with exercise-induced myalgia and intolerance were evaluated, with twenty-three of them having a metabolic myopathy (McArdle, n = 9; complete myoadenylate deaminase deficiency, n = 10; respiratory chain deficiency, n = 4) and sixty patients with non-metabolic myalgia. In all patients, maximal power (MP) was determined during a progressive exercise test on a cycloergometer and maximal voluntary contraction force (MVC) was assessed using a handgrip dynamometer. The ratio between percent-predicted values for MVC and MP was calculated for each subject (MVC%pred:MP%pred ratio). In patients with metabolic myopathy, the MVC%pred:MP%pred ratio was significantly higher compared to non-metabolic myalgia (1.54 ± 0.62 vs. 0.92 ± 0.25; p < 0.0001). ROC analysis of MVC%pred:MP%pred ratio showed AUC of 0.843 (0.758–0.927, 95% CI) for differentiating metabolic myopathies against non-metabolic myalgia. The optimum cutoff was taken as 1.30 (se = 69.6%, sp = 96.7%), with a corresponding diagnostic odd ratio of 66.3 (12.5–350.7, 95% CI). For a pretest probability of 15% in our tertiary reference center, the posttest probability for metabolic myopathy is 78.6% when MVC%pred:MP%pred ratio is above 1.3. In conclusion, the MVC%pred:MP%pred ratio is appropriate as a screening test to distinguish metabolic myopathies from non-metabolic myalgia.

Exercise efficiency impairment in metabolic myopathies

Metabolic myopathies are muscle disorders caused by a biochemical defect of the skeletal muscle energy system resulting in exercise intolerance. The primary aim of this research was to evaluate the oxygen cost (∆V’O₂/∆Work-Rate) during incremental exercise in patients with metabolic myopathies as compared with patients with non-metabolic myalgia and healthy subjects. The study groups consisted of eight patients with muscle glycogenoses (one Tarui and seven McArdle diseases), seven patients with a complete and twenty-two patients with a partial myoadenylate deaminase (MAD) deficiency in muscle biopsy, five patients with a respiratory chain deficiency, seventy-three patients with exercise intolerance and normal muscle biopsy (non-metabolic myalgia), and twenty-eight healthy controls. The subjects underwent a cardiopulmonary exercise test (CPX Medgraphics) performed on a bicycle ergometer. Pulmonary V’O₂ was measured breath-by-breath throughout the incremental test. The ∆V’O₂/∆Work-Rate slope for exercise was determined by linear regression analysis. Lower oxygen consumption (peak percent of predicted, mean ± SD; p < 0.04, one-way ANOVA) was seen in patients with glycogenoses (62.8 ± 10.2%) and respiratory chain defects (70.8 ± 23.3%) compared to patients with non-metabolic myalgia (100.0 ± 15.9%) and control subjects (106.4 ± 23.5%). ∆V’O₂/∆Work-Rate slope (mLO₂.min⁻¹.W⁻¹) was increased in patients with MAD absent (12.6 ± 1.5), MAD decreased (11.3 ± 1.1), glycogenoses (14.0 ± 2.5), respiratory chain defects (13.1 ± 1.2), and patients with non-metabolic myalgia (11.3 ± 1.3) compared with control subjects (10.2 ± 0.7; p < 0.001, one-way ANOVA). In conclusion, patients with metabolic myopathies display an increased oxygen cost during exercise and therefore can perform less work for a given VO₂ consumption during daily life-submaximal exercises.

Effects of AMPD1 common mutation on the metabolic-chronotropic relationship: Insights from patients with myoadenylate deaminase deficiency

PURPOSE

Current evidence indicates that the common AMPD1 gene variant is associated with improved survival in patients with advanced heart failure. Whilst adenosine has been recognized to mediate the cardioprotective effect of C34T AMPD1, the precise pathophysiologic mechanism involved remains undefined to date. To address this issue, we used cardio-pulmonary exercise testing data (CPX) from subjects with myoadenylate deaminase (MAD) defects.

METHODS

From 2009 to 2013, all the patients referred in our laboratory to perform a metabolic exercise testing, i.e. a CPX with measurements of muscle metabolites in plasma during and after exercise testing, were prospectively enrolled. Subjects that also underwent an open muscle biopsy for diagnosis purpose were finally included. The metabolic-chronotropic response was assessed by calculating the slope of the linear relationship between the percent heart rate reserve and the percent metabolic reserve throughout exercise. MAD activity was measured using the Fishbein's technique in muscle biopsy sample. The common AMPD1 mutation was genotyped and the AMPD1 gene was sequenced to screen rare variants from blood DNA.

RESULTS

Sixty-seven patients were included in the study; 5 had complete MAD deficiency, 11 had partial MAD deficiency, and 51 had normal MAD activity. Compared with normal MAD activity subjects, MAD deficient subjects appeared to have a lower-than-expected metabolic-chronotopic response during exercise. The metabolic-chronotropic relationship is more closely correlated with MAD activity in skeletal muscle (Rs = 0.57, p = 5.93E-7, Spearman correlation) than the presence of the common AMPD1 gene variant (Rs = 0.34, p = 0.005). Age-predicted O2 pulse ratio is significantly increased in MAD deficient subjects, indicating a greater efficiency of the cardiovascular system to deliver O2 (p < 0.01, Scheffé's post hoc test).

CONCLUSION

The metabolic-chronotropic response is decreased in skeletal muscle MAD deficiency, suggesting a biological mechanism by which AMPD1 gene exerts cardiac effect.

Higher oxidative stress in skeletal muscle of McArdle disease patients

McArdle disease (MCD) is an autosomal recessive condition resulting from skeletal muscle glycogen phosphorylase deficiency. The resultant block in glycogenolysis leads to an increased flux through the xanthine oxidase pathway (myogenic hyperuricemia) and could lead to an increase in oxidative stress. We examined markers of oxidative stress (8-isoprostane and protein carbonyls), NAD(P)H-oxidase, xanthine oxidase and antioxidant enzyme (superoxide dismutase, catalase and glutathione peroxidase) activity in skeletal muscle of MCD patients (N = 12) and controls (N = 12). Eight-isoprostanes and protein carbonyls were higher in MCD patients as compared to controls (p < 0.05). There was a compensatory up-regulation of catalase protein content and activity (p < 0.05), mitochondrial superoxide dismutase (MnSOD) protein content (p < 0.01) and activity (p < 0.05) in MCD patients, yet this increase was not sufficient to protect the muscle against elevated oxidative damage. These results suggest that oxidative stress in McArdle patients occurs and future studies should evaluate a potential role for oxidative stress contributing to acute pathology (rhabdomyolysis) and possibly later onset fixed myopathy.

Diagnostic Algorithm for Glycogenoses and Myoadenylate Deaminase Deficiency Based on Exercise Testing Parameters: A Prospective Study

Aim

Our aim was to evaluate the accuracy of aerobic exercise testing to diagnose metabolic myopathies.

Methods

From December 2008 to September 2012, all the consecutive patients that underwent both metabolic exercise testing and a muscle biopsy were prospectively enrolled. Subjects performed an incremental and maximal exercise testing on a cycle ergometer. Lactate, pyruvate, and ammonia concentrations were determined from venous blood samples drawn at rest, during exercise (50% predicted maximal power, peak exercise), and recovery (2, 5, 10, and 15 min). Biopsies from vastus lateralis or deltoid muscles were analysed using standard techniques (reference test). Myoadenylate deaminase (MAD) activity was determined using p-nitro blue tetrazolium staining in muscle cryostat sections. Glycogen storage was assessed using periodic acid-Schiff staining. The diagnostic accuracy of plasma metabolite levels to identify absent and decreased MAD activity was assessed using Receiver Operating Characteristic (ROC) curve analysis.

Results

The study involved 51 patients. Omitting patients with glycogenoses (n = 3), MAD staining was absent in 5, decreased in 6, and normal in 37 subjects. Lactate/pyruvate at the 10th minute of recovery provided the greatest area under the ROC curves (AUC, 0.893 ± 0.067) to differentiate Abnormal from Normal MAD activity. The lactate/rest ratio at the 10th minute of recovery from exercise displayed the best AUC (1.0) for discriminating between Decreased and Absent MAD activities. The resulting decision tree achieved a diagnostic accuracy of 86.3%.

Conclusion

The present algorithm provides a non-invasive test to accurately predict absent and decreased MAD activity, facilitating the selection of patients for muscle biopsy and target appropriate histochemical analysis.

Preliminary observations on high energy phosphates and metabolic pathway and transporter potentials in extensor carpi radialis brevis and trapezius muscles of women with work-related myalgia

This study compared both the extensor carpi radialis brevis (ECRB) and the trapezius (TRAP) muscles of women with work-related myalgia (WRM) with healthy controls (CON) to determine whether abnormalities existed in cellular energy status and the potentials of the various metabolic pathways and segments involved in energy production and substrate transport. For both the ECRB (CON, n = 6-9; WRM, n = 13) and the TRAP (CON, n = 6-7; WRM, n = 10), no differences (P > 0.05) were found for the concentrations (in millimoles per kilogram of dry mass) of ATP, PCr, lactate, and glycogen. Similarly, with one exception, the maximal activities (in moles per milligram of protein per hour) of mitochondrial enzymes representative of the citric acid cycle (CAC), the electron transport chain (ETC), and β-oxidation, as well as the cytosolic enzymes involved in high energy phosphate transfer, glycogenolysis, glycolysis, lactate oxidation, and glucose phosphorylation were not different (P > 0.05). The glucose transporters GLUT1 and GLUT4, and the monocarboxylate transporters MCT1 and MCT4, were also normal in WRM. It is concluded that, in general, abnormalities in the resting energy and substrate state, the potential of the different metabolic pathways and segments, as well as the glucose and monocarboxylate transporters do not appear to be involved in the cellular pathophysiology of WRM.

Skeletal muscle and beyond: the role of exercise as a mediator of systemic mitochondrial biogenesis

It has been known for more than 4 decades that exercise causes increases in skeletal muscle mitochondrial enzyme content and activity (i.e., mitochondrial biogenesis). Increasing evidence now suggests that exercise can induce mitochondrial biogenesis in a wide range of tissues not normally associated with the metabolic demands of exercise. Perturbations in mitochondrial content and (or) function have been linked to a wide variety of diseases, in multiple tissues, and exercise may serve as a potent approach by which to prevent and (or) treat these pathologies. In this context, the purpose of this review is to highlight the effects of exercise, and the underlying mechanisms therein, on the induction of mitochondrial biogenesis in skeletal muscle, adipose tissue, liver, brain, and kidney.

Metabolic myopathies: update 2009

Metabolic myopathies are inborn errors of metabolism that result in impaired energy production due to defects in glycogen, lipid, mitochondrial, and possibly adenine nucleotide metabolism. Fatty acid oxidation defects (FAOD), glycogen storage disease, and mitochondrial myopathies represent the 3 main groups of disorders, and some consider myoadenylate deaminase (AMPD1 deficiency) to be a metabolic myopathy. Clinically, a variety of neuromuscular presentations are seen at different ages of life. Newborns and infants commonly present with hypotonia and multisystem involvement (liver and brain), whereas onset later in life usually presents with exercise intolerance with or without progressive muscle weakness and myoglobinuria. In general, the glycogen storage diseases result in high-intensity exercise intolerance, whereas the FAODs and the mitochondrial myopathies manifest predominately during endurance-type activity or under fasted or other metabolically stressful conditions. The clinical examination is often normal, and testing requires various combinations of exercise stress testing, serum creatine kinase activity and lactate concentration determination, urine organic acids, muscle biopsy, neuroimaging, and specific genetic testing for the diagnosis of a specific metabolic myopathy. Prenatal screening is available in many countries for several of the FAODs through liquid chromatography-tandem mass spectrometry. Early identification of these conditions with lifestyle measures, nutritional intervention, and cofactor treatment is important to prevent or delay the onset of muscle weakness and to avoid potential life-threatening complications such as rhabdomyolysis with resultant renal failure or hepatic failure. This article will review the key clinical features, diagnostic tests, and treatment recommendations for the more common metabolic myopathies, with an emphasis on mitochondrial myopathies.