Exercise intolerance in patients with McArdle's disease or mitochondrial myopathies

To assess respiratory and metabolic adaptations in patients with phosphorylase deficiency and mitochondrial myopathies using maximal exercise tests.

Five patients with McArdle's disease and five patients with mitochondrial myopathies performed the same incremental maximal exercise test. Their respiratory gas exchanges and the variation of the venous blood metabolites--lactate (LACT), pyruvate (PYR), alanine (ALA), ammonia (NH3)--were studied in comparison with the results of fourteen control subjects who performed the same test.

Compared with controls, the two groups of patients displayed a similar significant decrease of their maximal VO2. In McArdle's patients the limitation of the maximal oxygen consumption was associated with a low respiratory exchange ratio (RER), a high VE/VO2, and characteristic metabolic data: no rise of LACT and PYR, a decrease of ALA and an important rise of NH3. In mitochondrial myopathies low VO2 max were due to a leftwards shift, i.e. towards low powers of exercise, of LACT, PYR, NH3 and ALA values. However the pattern of increase of LACT, PYR and NH3, exponential, and of ALA, linear, as well as respiratory exchange ratios were similar to control values. In this case, the limitation of oxygen consumption was due to a lack of the usual substrate, pyruvate. Low respiratory exchange ratio demonstrated that the muscle metabolism had a tendency to shift to lipid oxidation.

These results suggest that patients with McArdle's disease may improve their muscle energy production by endurance training which enhances lipid metabolism, whereas in mitochondrial myopathies, the energy production by oxidation of pyruvate or lipids may be improved only by enzymatic substitution.