Neuromyelitis optic spectrum diseases (NMOSD) are a group of rare neuroimmunological diseases involving mainly the optic nerves and spinal cord, to a lesser extent the brain, and causing severe exacerbations that lead to persistent disability of patients. For many years, opticoneuromyelitis was considered a prognostically unfavorable variant of the course of multiple sclerosis (MS), however, in 2004, specific autoantibodies to aquaporin-4 were found in such patients, which made it possible to isolate NMOSD into a separate group of demyelinating diseases other than MS. Due to similar clinical signs and the predominantly remitting course of diseases, it is often difficult to make a correct diagnosis and, accordingly, prescribe effective therapy, which often leads to incorrectly selected therapy with incorrect diagnosis. In some cases, this leads to a worsening of the course of NMOSD. We present a case of late diagnosis of NMOSD that confirms the development of exacerbation in the patient 2 months after the first course of therapy with alemtuzumab prescribed as a highly effective therapy for highly active remitting MS. Timely diagnosis of NMOSD makes it possible to exclude such cases.