Mitochondrial enzyme deficiency causing exercise limitation in normal-appearing adults

Deficiencies of mitochondrial enzymes cause a number of severe neurologic syndromes in pediatric patients. Isolated myopathy secondary to enzymatic deficiency only rarely has been recognized and reported in adults. Three normal-appearing patients with unexplained dyspnea on exertion, tachycardia, and fatigue were studied with progressive incremental and constant load exercise testing with hemodynamic and gas exchange measurements. Subsequently, muscle biopsies were performed and enzymatic profiles determined. Exercise testing suggested altered cellular energy metabolism. Oxygen transport and gas exchange at rest and with exercise were normal. During exercise, ventilation increased excessively for the work performed, but it was appropriate for carbon dioxide production. Cardiac outputs increased significantly (196 to 305%), heart rates reached maximum levels, and lactic acid levels increased (385 to 833%) during exercise at 25 W. Muscle morphology was normal, but mitochondrial enzyme assays demonstrated one or more deficiencies in all three patients. Mitochondrial enzymatic deficiency can be a cause of unexplained exercise symptomatology and limitation in adults. Our experience would suggest the incidence of the disorder may be greater than that previously recognized or expected. Symptoms of exercise limitation associated with dyspnea, tachycardia, or muscle fatigue should alert the clinician to a possible abnormality in cellular energy metabolism.