A mitochondrial metabolism disease characerized by a highly variable phenotype. Some patients have multisystem involvement of the brain, heart, muscle, liver, and kidneys resulting in death in infancy, whereas others have only isolated cardiac or muscle involvement with onset in adulthood and normal cognition. It has_material_basis_in homozygous or compound heterozygous mutation in the nuclear-encoded SDHA gene on chromosome 5p, the nuclear-encoded SDHAF1 gene on chromosome 19q, or the nuclear-encoded SDHD gene on chromosome 11q23.