A longitudinally extensive optic nerve lesion (more than half the length of the nerve) (n = 6) and/or involvement of the optic chiasm (n = 4), two findings previously considered typical for AQP4-IgG-positive NMO [29], were present during acute ON in 8/26 (30.8 %) cases with available data. This evidence concerns the gene AQP4 and neuromyelitis optica.