Biochemical defects in mitochondrial cytopathies: a new classification.

Current classifications of mitochondrial cytopathies neglect rigorous assessment of the literature which consists often of single case reports. In addition no attempt is made by many workers to distinguish between primary and secondary respiratory chain defects. Current biochemical classifications also fail to take into account the considerable number of patients who have ragged red fibre myopathies and lactic acidosis but who do not show demonstrable respiratory enzyme defects. For this reason a new approach is advocated with consideration of possible respiratory chain defects, probable primary respiratory chain defects, probable secondary respiratory chain defects, and mitochondrial cytopathies with normal respiratory chain enzyme function in vitro. Existing knowledge is reviewed under these categories to which are added further original observations.