Background. Neuromyelitis optica spectrum disorder (NMOSD) has been masked under other diagnoses for many years. Modern diagnostic criteria expand diagnosis possibilities, but they need approbation in real clinical practice. The purpose was through the analysis of a clinical case and literature search to analyze the algorithm for making a diagnosis of neuromyelitis optica based on the new clinical guideline “Diagnosis and treatment of the neuromyelitis optica spectrum disorders” developed in 2023 in Ukraine, to identify marker signs that exclude alternative diagnoses, to analyze the effectiveness of patient treatment in dynamics. Materials and methods. We used the bibliosemantic method, as well as methods of systematic and comparative analysis. We presented the results of our own clinical experience and conducted a dynamic analysis of a clinical case of neuromyelitis optica. Results. Over the past six years, 23 patients diagnosed with neuromyelitis optica were treated at the neurological department of the Lviv Regional Clinical Hospital: 1 patient in 2018, 5 in 2019, 3 in 2020, 6 in 2021, 8 in 2022. We examined the clinical case of a 52-year-old patient who sought medical help at the Department of Neurology of Danylo Halytskyi Lviv National Medical University in 2023. We analyzed the patient’s path to the diagnosis of neuromyelitis optica with the dynamics of changes on magnetic resonance imaging from 2019, considering current diagnostic criteria in which neuromyelitis optica was included in the single term NMOSD. The presence of even one of the six main clinical manifestations, in combination with the presence of AQP4-IgG, gives grounds for making a diagnosis of NMOSD. We carried out a differential diagnosis with other pathologies. Based on this, we identified marker signs to exclude multiple sclerosis, acute disseminated encephalomyelitis, infectious myelitis, and other diseases. Diagnostic aspects of NMOSD and treatment options were considered. An analysis of the effectiveness of the patient’s treatment in dynamics was carried out. Conclusions. The new Ukrainian clinical guideline for neuromyelitis optica spectrum disorder, which is based on international consensus diagnostic criteria, improves the standardization of the approach to patients with NMOSD and can be effectively used in real clinical practice. In our clinical case of a patient with myelitis without optic neuritis, the presence of AQP4-IgG and careful differential diagnosis with other pathologies play a key role in the diagnosing.