Mitochondrial disorders are a group of heterogeneous diseases associated with abnormalities of the oxidative phosphorylation (OXPHOS), the most important source of energy for the cell. The number of mitochondrial syndromes and of identified causative genes is constantly increasing. Taken as a whole they are among the most frequent genetic diseases in humans at any age. The respiratory chain is the only metabolic pathway under double genome control and molecular genetics of these disorders is complicated by the existence of strict interactions between mitochondrial DNA and nuclear DNA. In childhood and infancy, clinical presentation differs from mitochondrial disorders with adult onset. The phenotypes are much more severe, often involving brain, frequently presenting as multisystemic disorders and seldom as isolated myopathy. Mutations in nDNA are more frequent than in adulthood.
The major phenotypes presenting in infancy are here correlated with genetic defects and biochemical data with the aim to facilitate diagnosis work-up.