Optic nerve MRI
MRI findings in ON can include T2-hyperintense lesions, nerve swelling, and gadolinium enhancement of the affected optic nerve on T1-weighted imaging. In MOG-ON, optic nerve lesions are frequently extensive, also termed longitudinally extensive ON (LEON), affecting more than half of the pre-chiasmatic optic nerve length (40,85,86). Moreover, MOG-ON predominantly affects the anterior part of the optic nerve. This can help with differentiating MOG-ON from AQP4-ON, which is also often extensive, but predominantly affecting the posterior part of the optic pathway (including the optic chiasm) (40,87–89). In contrast, MS-ON typically involve shorter segments of the optic nerve compared to both MOG-ON and AQP4-ON. Meanwhile, bilateral ON is specifically more frequent in MOG-ON than in MS-ON, and in paediatric patients have been associated with higher MOG-IgG titers (38,40). Another characteristic feature described in MOG-ON is perineural or periorbital gadolinium enhancement in the orbital soft-tissue that is not typically found in MS-ON (51,85,86,90–92).