OBJECTIVESSeveral treatments have been approved for the prevention of attacks in AQP4-IgG+ neuromyelitis optica spectrum disorder (NMOSD). However, because of the rarity of the disease, little is known concerning how to switch from one treatment to another in case of lack of effectiveness or side effects. In this article, we report a severe attack in a patient with NMOSD after switching from eculizumab to satralizumab.METHODSA 44-year-old woman with NMOSD was treated with azathioprine and then rituximab, without optimal control of the disease. Eculizumab was initiated with clinical efficacy. Two years later, after the onset of rheumatoid arthritis and because of difficult venous access, a switch to satralizumab was proposed.RESULTSAfter switching, a severe attack occurred 11 weeks after the last eculizumab infusion. Severe tetraplegia was related to a new extensive cervical lesion associated with a tumefactive lesion of the corpus callosum. The patient was treated with 10 infusions of methylprednisolone and 10 plasma exchanges. Eculizumab was reintroduced 20 days after symptom onset. Three months later, mild improvement was observed.DISCUSSIONIn clinical practice, in case of intolerance or side effects, anticomplement therapy should be switched to another NMOSD treatment with caution because of a high risk of relapse.CLASSIFICATION OF EVIDENCEThis case report provides Class IV evidence that eculizumab should be stopped with caution and switched to another treatment immediately. This is a single observational study without controls.