Relevance. Studies on the prevalence of orphan diseases, which undoubtedly include opticoneuromyelitis or Devick's disease, have a number of features and difficulties. The lack of data in the forms of state statistics and the lack of clinical patient registers make it difficult to understand the potential of such diseases. If initially opticoneuromyelitis was considered as a special form of multiple sclerosis, in the clinical picture of which optical neuritis (lesions of the optic nerves) and myelitis (lesions of the spinal cord with the formation of para - and tetraparesis, impaired sensitivity to the conductor type and pelvic disorders) prevailed, then with the identification of a specific biomarker – antibodies to aquaporin-4, these conditions were isolated into an independent nosological unit. The practical significance of the differentiation of the two demyelinating diseases lies in the fact that the approaches to their treatment are fundamentally different: drugs that change the course of multiple sclerosis (PITRS) can worsen the condition of patients with ZSONM, provoking the development of exacerbations in them, so an important stage was the formation of diagnostic criteria for ONM. Goal. Conducting an epidemiological study on the prevalence and morbidity of patients with orphan disease – diseases of the opticoneuromyelitis spectrum (opticoneuromyelitis, Devick's disease) (ZSONM) with an analysis of patients of each type by age and gender, age of disease onset, analysis for the presence of antibodies to aquaporin-4, the presence of oligoclonal antibodies in serum and cerebrospinal fluid, the main clinical manifestations, treatment: basic immunosuppressive therapy, anti-B-cell therapy, anti-IL6-therapy, therapy with glucocorticosteroids. Materials and methods. The study included 142 patients who were on inpatient treatment in the Federal State Budgetary Institution "Scientific Center of Neurology", in the intercircular departments of multiple sclerosis in Moscow, in the regional centers of demyelinating diseases of the Leningrad, Novosibirsk region, the Republics of Mari El, Tatarstan, and the Khanty-Mansi Autonomous Okrug. All patients met the diagnostic criteria of the 2015 ZSONM, on the basis of which the diagnosis was made. The analysis was carried out according to the criteria described in the purpose of the study. Results. ZSONM is an urgent problem of neurology, as it mainly affects people of young, working age, the share of patients from 18 to 49 years of age totaled 77%. Women are more often ill, and family planning issues remain relevant in patients with ZSONM. Most patients are seropositive for antibodies to aquaporin-4, but a fairly high percentage of patients with ZSONM are seronegative (33%), which leads to the search for other biomarkers of demyelinating diseases. It is also necessary to conduct a comprehensive clinical, laboratory and neuroimaging examination of each individual patient in order to diagnose in accordance with the current diagnostic criteria, since otherwise up to a third of patients may receive incorrect diagnoses.