Long-term, multicenter, multinational, observational, registry of patients with AQP4+ NMOSD that is designed to collect data on clinical outcomes and safety in patients prescribed Alexion C5 inhibitor therapies (C5IT). The registry will also collect data on patient reported outcomes (PROs), quality of life (QoL), and targeted AQP4+ NMOSD therapies used to provide evidence on the real-world impact of ALXN-C5IT on patients with AQP4+ NMOSD.

At the time of enrollment in the Registry, participant records will be queried for retrospective information about the participant's medical history and AQP4+ NMOSD treatment history for the time period beginning 1 year prior to ALXN-C5IT initiation through Registry enrollment. The duration of data collection for the Registry will be approximately 5 years from the day the last participant is enrolled.

Inclusion Criteria:

• Participant is ≥ 18 years of age at the time of enrollment in the Registry.
• Participant must have a confirmed diagnosis of AQP4+ NMOSD.
• At the time of enrollment in the Registry, participants must be receiving treatment with ALXN-C5IT for the purpose of chronic relapse prevention in a manner consistent with the local label. Specifically, they should have received at least 1 dose of eculizumab within 4 weeks prior to enrollment or at least 1 dose of ravulizumab within 12 weeks prior to enrollment.
• Participants must have both the following historical data available to be enrolled in the Registry: ALXN-C5IT dosing information since initiation and number and types of relapses from 1 year prior to ALXN-C5IT initiation through Registry enrollment.

Exclusion Criteria:

• Participants currently enrolled in an interventional clinical study for the treatment of AQP4+ NMOSD in which the intervention is a drug.

Participants with a diagnosis of AQP4+ NMOSD who are being treated with ALXN-C5IT at the time of enrollment.