AQP4 and neuromyelitis optica: Isolated ON is one of the major clinical signs in NMO as well as in multiple sclerosis (MS).3 The ON attacks in NMO are more severe than those in multiple sclerosis (MS), and can lead to unilateral or bilateral blindness.3 In addition, the lesions and involvement in optic chiasm and tracts in NMO are more extensive in comparison to those in MS in the MRI.4 Serum anti‐AQP4 antibody‐positive patients with isolated ON are recently classified as having NMO spectrum disorder (NMOSD), because they have a high risk of eventual conversion into definite NMO.5, 6