Functional evaluation of muscle oxidative metabolism in metabolic myophaties. A cross-talk between exercise physiology and clinical medicine

Metabolic myopathies are a heterogeneous group of diseases characterized by genetically determined defects which impair skeletal muscle energy production and/or function. These diseases include errors of glycogen metabolism, lipid metabolism, and mitochondrial respiratory chain. The most common clinical features are muscle weakness, pain, easy fatigability, cramping and, sometimes myoglobinuria due to muscle fiber necrosis. Exercise intolerance is another hallmark of clinical features of metabolic myopathies. As a result, patients note undue fatigue and dyspnea during low levels of exertion, including moderate activities of daily living. Symptoms are usually first experienced in childhood or early adulthood, however, late-onset is well recognized. The diagnostic process of these diseases usually begins with a careful medical history, a physical and neurological examination to assess reflexes, strength and the distribution of weakness. Creatine kinase is an extremely useful laboratory test for the evaluation of patients with a suspected myopathy and electromyography may be used to rule out a number of other neuromuscular disorders that cause similar patterns of weakness. As for exercise testing, the six minute walking test and the forearm ischaemic lactate test have traditionally been employed to screen for suspected disorders of glycogen metabolism; however, they have been superseded by improved biochemical and genetic techniques. Confirmation of the diagnosis usually requires muscle biopsy and/or molecular genetic testing. Functional evaluation of oxidative metabolism during exercise provides information regarding the physiological responses required by the cardiovascular and respiratory systems to meet the metabolic demands of the skeletal muscle. Moreover, the study of the physiological adjustments to exercise in patients is of extreme interest also from a “basic science” point of view, allowing to investigate aspects related to the regulation and integration of physiological and bioenergetic responses. Although exercise testing is widely utilized by cardiology, pulmonary, and sports medicine clinicians as a means to assess heart failure, respiratory disease, or athletic capacity, only few neurologists utilize a comprehensive assessment of oxidative metabolism to clarify the etiology of exercise intolerance and unexplained dyspnea among patients with metabolic myopathies. In previous studies our group applied on mitochondrial myopathies (MM) and McArdle’s disease (McA) patients two non−invasive methods of functional evaluation specifically aimed at oxidative metabolism at the skeletal muscle level. The variables of functional evaluation that we investigated were: A) Skeletal muscle oxygenation indices during exercise, obtained by near−infrared spectroscopy (NIRS) and taken as estimates of the capacity of O2 extraction; B) Kinetics of adjustment of pulmonary O2 uptake (V’O2 kinetics) during the transition from rest to exercise. We demonstrated that these methods allow to identify and quantify, in MM and in McA, the metabolic impairment. Moreover, these studies represent a typical example of “translational medicine”, in which methods and tools developed over the years in the exercise physiology laboratory are taken to the bed of the patient. In this thesis will be reported data of four studies in which the above−mentioned tools of functional evaluation of muscle oxidative metabolism were utilized, with specific purposes, on patients with metabolic myopathies. In the first study we evaluated, during a 24-month follow-up, cardiovascular and metabolic responses to exercise of a 50-yr-old patient with glycogen storage disease type II (Pompe disease)undergoing enzyme replacement therapy (ERT). At the same constant-workload submaximal exercise, rate of perceived exertion, pulmonary ventilation, and heart rate were lower during ERT versus pre-treatment, suggesting an increased exercise tolerance. Peak oxygen uptake increased by approximately 35% after 1 month of treatment and did not significantly change thereafter. Also, peak cardiac output significantly increased during ERT, whereas peak skeletal muscle fractional O2 extraction was unchanged compared with pre-treatment. Thus, this case report suggest that ERT may increase peak exercise capacity and exercise tolerance at submaximal workloads in patients with glycogen storage disease type II after 1 month of therapy, without no further changes occurring up to 24 months. In the second study, we followed the same approach of the case study previously mentioned and we evaluated the effects of 12-month of ERT on physiological variables related to exercise tolerance of four patients with Pompe disease. Patients performed an incremental exercise on a cycle ergometer, up to voluntary exhaustion, before and after 12 months of ERT. Peak workload and oxygen uptake values significantly increased after ERT whereas the observed increases of both peak cardiac output and the NIRS-determined peak skeletal muscle fractional O2 extraction were not statistically significant. These findings suggest that in glycogen storage disease type II patients enzyme replacement therapy is associated with a mild improvement of exercise tolerance. Since exercise training could improve exercise tolerance, motor function and muscle strength, counteracting the general deconditioning typical of chronic diseases, in the future may be interesting to evaluate if exercise training could be helpful in increasing ERT clinical efficacy, improving patients' muscle function and ameliorating their quality of life. A new study based upon a collaboration between neurologists and exercise physiologists has now started and it should give the opportunity to better investigate crucial issues related to patients’ follow-up and treatment. In the third study we evaluated in McArdle’s (McA) patients whether a first bout of exercise determines a sudden decrease in heart rate (HR) and an improved exercise tolerance (the so-called “second-wind” phenomenon) during a second bout, separated by the first by a few minutes of recovery. A second-wind phenomenon (marked decrease in heart rate and in the rating of perceived exertion) was indeed observed in McA patients 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: enhanced O2 extraction; signs of better matching between intramuscular O2 delivery and O2 utilization; disappearance of the “slow component” of pulmonary VO2 kinetics. The second wind was not described in McA patients after a longer (18-min) recovery period or in patients affected by a mitochondrial myopathy who have similar exercise intolerance. Besides being of interest from a basic science point of view (elucidating the mechanisms responsible for the second wind in McA patients), results of the present study are of interest also from a clinical point of view, since they identify a method (a warm up moderate-intensity exercise, carried out a few minutes before performing a task) capable of significantly increasing exercise tolerance in these patients. Finally, still unpublished data of another study are presented in this thesis, demonstrating the utility of non−invasive functional evaluation methods utilized by physiologists in the follow−up of patients as well as in the evaluation of the effects of therapies and/or rehabilitation intervention (i.e. exercise training). Since at present the therapeutic interventions available for metabolic myopathies patients are very limited and evidence from the literature suggests that moderate−intensity aerobic exercise training represents a safe intervention, the variables of functional evaluation determined by the above−mentioned tools were utilized to evaluate, in MM and McA patients, the effects of a program of moderate−intensity aerobic exercise training carried out by the patients at their home. Peak O2 uptake, variable evaluating maximal aerobic power, and peak skeletal muscle (vastus lateralis) fractional O2 extraction, as estimated by near-infrared spectroscopy (NIRS), increased significantly with training both in MM and in McA. Thus, training induced an increase of exercise tolerance at least in part due to a reduction of the impaired fractional O2 extraction by skeletal muscles. Moreover, training significantly speeded the V’O2 kinetics, even though only in the patients who had presented, before training, markedly slow V’O2 kinetics (i.e. sign of the most pronounced metabolic impairment). Surprisingly, the improvements in exercise tolerance obtained by the training program did not determine an increase in the habitual level of physical activity evaluated a couple of months after the termination of the training program. Overall, the results of the studies reported in this thesis demonstrate that, within a translational approach, a combination of traditional and more innovative functional evaluation methods can effectively detect the functional improvements of patients with metabolic myopathies following pharmacological and/or exercise interventions, yielding insights also on the mechanisms of the improvements at the pathophysiological level. Thus, functional evaluation of oxidative metabolism by non−invasive methods could be usefully employed in the diagnostic process of metabolic myopathies, in the follow−up of patients, and in the evaluation of the effects of therapies and/or rehabilitation interventions. Moreover, the analysis of the physiological and bioenergetics adaptations to exercise in patients with metabolic myopathies represents an interesting model to investigate and elucidate, in vivo, the regulation of basic physiological processes.