Fractional anisotropy helps to differentiate the optic nerve impairment between neuromyelitis optica spectrum disorders and multiple sclerosis.

• NMOSD-related optic nerve impairment is extensive, often greater than half of the optic nerve, with the most significant involvement of the posterior segment of the optic nerve. • MS-related optic nerve impairment is more limited than NMOSD, and anterior and middle optic nerve involvement is common. • Optic nerve DTI is a convenient and effective imaging tool that can help characterize NMOSD and MS.