Study of the Clinical and Radiological Impact of Ravulizumab in People With Neuromyelitis Optica Spectrum Disorder

This is an observational study to: * evaluate the on-treatment clinical performance of ravulizumab in relation to the pre-treatment time period (time period prior to exposure), * enhance knowledge regarding conventional MRI outcomes in people with NMOSD treated with ravulizumab, * identify factors suggestive of subclinical disease progression through conventional MRI sequences, * determine if treatment with ravulizumab, impacts longitudinal 3D conformational MRI measures at the dorsal medulla and other regions of the CNS, and * identify biomarkers (e.g., serum neurofilament light chain (sNfL), conventional and novel MRI markers, etc.) related to disease activity.