Unexplained exertional limitation: characterization of patients with a mitochondrial myopathy.

Exercise intolerance is a common complaint, the cause of which often remains elusive after a comprehensive evaluation. In this report, we describe 28 patients with unexplained dyspnea or exertional limitation secondary to biopsy-proven mitochondrial myopathies. Patients were prospectively identified from a multidisciplinary dyspnea clinic at a tertiary referral center. All patients were without underlying pulmonary, cardiac, or other neuromuscular disorders. Patients underwent history, physical examination, complete pulmonary function testing, respiratory muscle testing, cardiopulmonary exercise testing, and muscle biopsy. Results were compared with a group of normal control subjects. The estimated period prevalence was 8.5% (28 of 331). Spirometry, lung volumes, and gas exchange were normal in patients and control subjects. Compared with control subjects, the patient group demonstrated decreased exercise capacity (maximum achieved V˙ o 2 67 versus 104% predicted; p < 0.0001) and respiratory muscle weakne...