Whether Eculizumab reduces relapse frequency in patients with relapsing NMO. The number of attacks during the one year treatment period will be compared to the number of attacks that occurred prior to initiation of eculizumab treatment. For patients with more than 2 year disease duration, the average number of attacks in the preceding 2 years will be calculated. For patients with less than 2 years disease duration the number of attacks in the preceding year will be used.