Case Report: Progressive visual decline after optic neuritis associated with microcystic macular oedema.

A 39-year-old man was diagnosed with multiple sclerosis (MS) following unilateral optic neuritis (ON). His vision improved with corticosteroid treatment, but residual deficits remained (visual acuity at initial discharge: 0.7). Ocrelizumab was started, without any confirmed relapses or activity on magnetic resonance imaging (MRI) ever since. Nevertheless, his vision gradually deteriorated (0.3). Optical coherence tomography (OCT) detected microcystic macular oedema (MMO) at month 6, which showed clear progression over 5 years. MMO is rare, and longitudinal data on its evolution are scarce. In our case, a progressive MMO course was accompanied by retinal neuroaxonal thinning and gradual visual decline, despite clinical and MRI stability under ocrelizumab. This case highlights that late or progressive visual deterioration after severe ON may be associated with evolving MMO, which can be monitored by longitudinal OCT.