To describe and compare the pattern of optic chiasm involvement in patients with myelin oligodendrocyte glycoprotein antibody associated optic neuritis (MOG-ON) with aquaporin-4 antibody associated optic neuritis (AQP4-ON).

MOG-IgG associated disease (MOGAD) has been demonstrated to be a distinct entity from AQP4-IgG positive neuromyelitis optica spectrum disorder (NMOSD).  Optic neuritis is often the presenting symptom in NMOSD.  It has been previously reported that chiasmal involvement is seen in at least 50% of AQP4-ON compared to a minority of patients with MOG-ON. 

A retrospective review of all patients evaluated at Mayo Clinic for diagnosis of ON with positive serum testing for either MOG-IgG or AQP4-IgG by cell-based assay at Mayo Clinic laboratories in Rochester, Minnesota.  Inclusion criteria included the availability of MRI brain/orbits at the time of ON.   These were reviewed for optic chiasm involvement and the morphology of the chiasmal involvement.

One-hundred and seventeen patients (77 AQP4-ON and 40 MOG-ON) were identified that fulfilled inclusion criteria.  Among patients with AQP4-ON, 20% (15 of 77) were found to have chiasmal involvement.  Of these, 14 of 15 were present in the first ON attack.  Isolated chiasmal involvement was observed in 27% (4 of 15).  Among patients with MOG-ON, 15% (6 of 40) had chiasmal involvement, which were all present in the first ON attack.  Of these, 17% (1 of 6) had isolated chiasm involvement. 

In this large cohort, chiasmal involvement of ON in MOGAD and AQP4-IgG positive NMOSD occur at more similar frequencies than have previously been reported, emphasizing the need for appropriate diagnostic testing when this clinical feature is present.