Succinate Dehydrogenase Deficiency

BACKGROUND: Partial succinate dehydrogenase deficiency (15% to 50% of normal reference enzyme activity) in skeletal muscle causes mitochondrial myopathy with various symptoms, for example, brain involvement, cardiomyopathy, and/or exercise intolerance. The deficiency may be isolated or may coexist with other respiratory-chain enzyme defects. The histopathologic assessment of succinate dehydrogenase activity in muscle biopsies of patients with suspected mitochondrial myopathies has focused on the finding of increased staining, usually in ragged-red fibers, rather than on reduced staining. OBJECTIVES: To determine the prevalence of muscle succinate dehydrogenase deficiency among patients with respiratory-chain defects and to determine whether the reduced activity is present histochemically and is comparable to the quantitative reduction found in muscle homogenates. PATIENTS AND METHODS: One hundred eight muscle biopsies were evaluated from patients with suspected mitochondrial myopathies by qualitative histochemical analysis and quantitative biochemical analyses of respiratory-chain enzymes using standard methodologies. RESULTS: Fifty-two patients had defects in respiratory-chain complexes; of these patients, 12 (23%) had partial deficiencies in succinate dehydrogenase activity either alone or together with reductions in other enzymes. The reduced activity was detectable histochemically in muscle biopsies with residual enzyme activity of up to 34% of the normal reference activity, while 2 biopsies with higher residual activity (49% and 68% of normal) could not be distinguished from normal biopsies. CONCLUSIONS: Of the patients with respiratory-chain enzyme defects, 23% had partial deficiencies of succinate dehydrogenase activity in muscle biopsies. This reduction could be detected histochemically in biopsies in most cases. The marked prevalence of succinate dehydrogenase deficiency among patients with respiratory-chain defects and its detection initially by histochemical analysis are important findings.