Exercise testing as a diagnostic entity in mitochondrial myopathies
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Exercise intolerance is one of the most common symptoms in patients with mitochondrial myopathies (MM). At the whole body level, this is characterized by a reduction in maximal oxygen consumption (VO2max) with an excessive carbon dioxide production (VCO2), increased rating of perceived exertion and a hyperdynamic circulatory response at a given exercise intensity. Fewer patients with MM display overt muscle atrophy and weakness even in the absence of a peripheral neuropathy. At the level of the skeletal muscle, the abnormal exercise response in MM patients is characterized by an increase in; delivery of oxygen relative to extraction (reduced myoglobin or hemoglobin desaturation), lactate production, phosphocreatine hydrolysis and time of post-exercise PCr and ADP recovery. Classically, the characterization of exercise intolerance is performed using cycle ergometry with measurements of VO2, VCO2, respiratory exchange ratio (RER = VCO2/VO2), heart rate, minute ventilation, rating of perceived exertion, and cardiac output (where available). Exercise protocols to maximum or for a given time period at a set workload can differentiate MM from controls with a sensitivity of 0.63-0.75 and a specificity of 0.70-0.90. Modified hand-grip exercise protocols, especially if coupled with simultaneous measurements of myoglobin/hemoglobin desaturation (near infra-red spectroscopy) or venous oxygenation, can achieve similar or higher levels of sensitivity and specificity. Similarly, exercise coupled with muscle phosphocreatine/Pi ratios, PCr, pH or ADP recovery kinetics, determined using magnetic resonance spectroscopy are useful in differentiating MM, but are limited by availability, experience and cost. In summary, aerobic exercise testing with some measurement of oxygen consumption can be performed in most institutions and can provide valuable information in the both the work-up of patients with suspected MM as well as in the monitoring of therapy in such patients.
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