Muscle metabolic responses during dynamic in-magnet exercise testing:a pilot study in children with an idiopathic inflammatory myopathy

Rationale and Objectives: The clinical utility of supine in-magnet bicycling in combination with phosphorus magnetic resonance spectroscopy (31P MRS) to evaluate quadriceps muscle metabolism was examined in four children with juvenile dermatomyositis (JDM) in remission and healthy age- and gender-matched controls. Materials and Methods: Two identical maximal supine bicycling tests were performed using a magnetic resonance-compatible ergometer. During the first test, cardiopulmonary performance was established in the exercise laboratory. During the second test, quadriceps energy balance and acid/base balance during incremental exercise and phosphocreatine recovery were determined using 31P MRS. Results: During the first test, no significant differences were found between patients with JDM and their healthy peers regarding cardiopulmonary performance. The outcomes of the first test indicate that both groups attained maximal performance. During the second test, quadriceps phosphocreatine and pH time courses were similar in all but one patient experiencing idiopathic postexercise pain. This patient demonstrated faster phosphocreatine depletion and acidification during exercise, yet postexercise mitochondrial adenosine triphosphate synthesis rate measured by phosphocreatine recovery kinetics was approximately twofold faster than control (time constant 23 seconds vs 43 ± 7 seconds, respectively). Conclusions: These results highlight the utility of in-magnet cycle ergometry in combination with 31P MRS to assess and monitor muscle energetic patterns in pediatric patients with inflammatory myopathies.