Clinical presentations and laboratory investigations in respiratory chain deficiency.

Respiratory chain deficiencies have long been regarded as neuromuscular diseases. In fact, oxidative phosphorylation, i.e., ATP synthesis by the respiratory chain not only occurs in the neuromuscular system, indeed, a number of non-neuromuscular organs and tissues are dependent upon mitochondrial energy supply. For this reason, a respiratory chain deficiency can theoretically give rise to any symptom, in any organ or tissue, at any age with any mode of inheritance, due to the twofold genetic origin of respiratory enzymes (nuclear DNA and mitochondrial DNA).