MOG and myelitis: In line with the fact that many MOG-IgG-positive patients develop ON and myelitis only successively, the mean observation period was longer in patients with a history both of ON and of myelitis at last follow-up (88.6 months; N = 22) than in patients with a history of either ON but no myelitis or myelitis but not ON (64.6 months; N = 28).