Unilateral optic neuritis and thinning of retinal layers in the affected and unaffected eyes

Purpose: Multiple sclerosis (MS)-related optic neuritis (ON) causes thinning of inner retinal layers. It remains unclear whether unilateral MSON also affects the unaffected contralateral eye. The purpose of this study was to compare macular retinal layer thicknesses in MS eyes with unilateral optic neuritis (MSON), their unaffected contralateral eyes and MS participants without a history of ON (MSnON). Methods: This cross-sectional screening study included 101 MSON and 106 MSnON participants. Retinal layer thicknesses were measured using optical coherence tomography scans at the standardized zones of the macula (central circle, inner ring and outer ring) and compared between the groups. Results: The unaffected contralateral non-ON eyes of MSON participants had thinner inner retinal layers including a thinner retinal nerve fibre layer (p values, 0.003–0.009), a thinner ganglion cell layer (p values, <0.001–0.006) and a thinner inner plexiform layer (p values, 0.004–0.012) compared to MS participants without a history of ON. Affected MSON eyes had thinner inner retinal layers compared to both unaffected fellow eyes and to MSnON participants (p < 0.001 for all comparisons). Additionally, in MSON eyes, the inner nuclear layer and outer retina were thicker at the inner and outer rings compared to contralateral eyes (p < 0.001 for all comparisons), but not when compared to the MSnON participants. Conclusion: We recommend bilateral examination, OCT imaging and follow-up for MS patients with unilateral acute ON to monitor also the contralateral eyes, which present with thinner inner retina layers than MSnON participants' eyes.