(A and B) Comparisons of progressive RNFL loss between ON and NON eyes in 2 patient groups with different levels of demyelination in the optic nerve determined by asymmetry of mfVEP latency (n = 27, latency asymmetry ≥10 ms; n = 24, latency asymmetry <10 ms). Faster tRNFL thinning was only present in the patients with more severe optic nerve demyelination (latency asymmetry ≥10 ms). There was also borderline difference (p = 0.086) in pRNFL progression between ON and fellow NON eyes in patients with advanced demyelination; by contrast, no difference was observed in the latency asymmetry <10 ms group, which also supports an association between accelerated RNFL loss in the temporal disc area and the level of optic nerve demyelination. (C) There was a significant correlation between asymmetry of the tRNFL progression rate (%/year) and optic nerve demyelination, as determined by intereye asymmetry of mfVEP latency between ON and fellow NON eyes (r = −0.38, p = 0.007), which remained significant after controlling for age, sex, and disease duration by partial correlation (p = −0.37, p = 0.01). NON = non-ON; ON = optic neuritis; pRNFL = peripapillary retinal nerve fiber layer; RNFL = retinal nerve fiber layer; tRNFL = temporal RNFL; VEP = visual evoked potential.