Background. Hereditary metabolic diseases (HMDs) are monogenically inherited diseases caused by gene mutations, which control the synthesis of polypeptides (proteins) that perform various functions (structural, immune defense, enzymatic catalysis, transport). The classification of HMDs is varied. Among them are violations of the formation and utilization of energy - mitochondrial diseases (MDs). A special case of MD is the Leigh syndrome, which is discussed in this article.
Description of the clinical case. The article presents a clinical observation of the onset of a mitochondrial disease (Leigh syndrome, subacute necrotizing encephalomyelopathy) in a boy aged 6 months.
Conclusion. In clinical practice, it is necessary to be aware of HMDs, in particular mitochondrial pathologies, which are rare (the total incidence of MDs is 1:5000–1:10,000 liveborn infants). In this example, the “search” for a mitochondrial disease was prompted by increasing neurological symptoms combined with lactic acidosis.

Keywords: subacute necrotizing encephalomyopathy, Leigh syndrome, mitochondrial diseases, lactic acidosis, metabolic disorders