Endomyocardial biopsies for early detection of mitochondrial disorders in hypertrophic cardiomyopathies.

Considering the high proportion of unexplained hypertrophic cardiomyopathies on the one hand and the occurrence of cardiomyopathies in several mitochondrial disorders on the other, we hypothesized that isolated hypertrophic cardiomyopathies in infancy could occasionally be the result of defects of oxidative phosphorylation. By means of a scaled-down technique, we were able to investigate oxidative phosphorylation on minute amounts of endomyocardial tissue (1 mg) in three patients with concentric hypertrophic cardiomyopathy (shortening fraction in diameter, 18% to 27%; normal mean +/- 1 SD, 33 +/- 3%) and in control subjects. Although the absolute respiratory chain enzyme activities in the endomyocardial biopsy specimens of the patients were within the low normal range, the determination of the activity ratios allowed us to ascribe hypertrophic cardiomyopathies to respiratory chain enzyme abnormalities in all three cases (complex I, two cases; multiple enzyme deficiency, one case). The respiratory chain enzyme activity ratios, which are normally constant irrespective of the tissue tested, were markedly abnormal in all three patients (cytochrome c oxidase/reduced nicotinamide-adenine dinucleotide cytochrome c reductase, 4.6 to 10.4; normal mean +/- 1 SD, 2.9 +/- 0.5). We conclude that mitochondrial disorders should be regarded as potential causes of hypertrophic cardiomyopathy in early infancy. Because cardiac catheterization is routinely performed for hemodynamic investigation of cardiomyopathies, we suggest that endomyocardial biopsies be considered as a tool for early detection of mitochondrial cardiomyopathies, especially in hypertrophic forms of the disease.