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'O-2/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'O-2 was measured breath-by-breath throughout the incremental test. The V'O-2/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'O-2/Work-Rate slope (mLO(2).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.