[Features of premorbid status in patients with Rett syndrome].

Rett syndrome is a severe, progressive developmental condition of the nervous system. It is linked to the X chromosome and affects mainly girls aged 6 to 18 months. The syndrome is usually caused by pathogenic mutations in the MECP2 gene, which encodes a methyl-CpG-binding protein. The prevalence of the disease is 1:10.000 in the female population. This syndrome is characterized by a progressive course, loss of acquired motor, mental, and speech functions, and multisystem comorbidities. A case of Rett syndrome in a 3-year-old child, confirmed by molecular genetic analysis, is presented. The features of premorbid neurological status are described, including delayed speech development and muscle hypotonia, which require increased attention and additional molecular genetic testing for differential diagnosis, given the phenotypic heterogeneity of Rett syndrome.