The Incidence of Optic Neuritis worldwide: the ON-COLOUR Study

Introduction: Due to its significant impact on disability, optic neuritis (ON) is most frequently investigated in the context of demyelinating diseases, and especially multiple sclerosis (MS). Objectives/Aims: We aimed to report the incidence of ON worldwide and beyond occurrence in demyelinating diseases. Methods: Literature search was performed in accordance with PRISMA guidelines, and included articles published between January 2010 and March 2025, written in English and indexed in PubMed. To identify relevant studies, the following keywords sequence was used: ((optic neuritis[Title]) AND (incidence[Title/Abstract])). Results: A review of 56 articles on ON incidence identified 10 relevant studies in Europe, Americas and Asia, in both pediatric and adult populations. The highest mean incidence rate of ON is reported for Canada, with 75.9 [95% CI: 72.1–79.1]/100,000 person-years. The U.S. Armed Forces reported 8.1, while Olmsted County, Minnesota, recorded 4.0 [95% CI: 3.2–4.7]; in this study, 9% of ON cases were associated with NMO and MOG antibodies, and 57% with MS. A study from Barcelona reported a rate of 5.36, higher than that in the United Kingdom – 4.5 [95% CI: 4.3–4.6]. In contrast, the lowest rates were recorded in South Korea, ranging between 2.34 [95% CI: 2.20–2.49] and 3.29 [95% CI: 3.28–3.30]. A Danish study reported an incidence of 3.28 [95% CI: 2.44–4.31], with 39% of patients developing MS during the follow-up. In the pediatric population, the lowest incidence was observed in the United Kingdom – 0.77 [95% CI: 0.64–0.90], while Asian countries, such as South Korea and Taiwan, recorded rates of 1.04 [95% CI: 1.01–1.07] and between 1.79 and 2.46, respectively. Conclusion: The incidence of ON widely varies by world regions, age group, and sex. Rates are higher in women than men the highest peak being in, middle-aged adults, and are lower in children. Western countries report higher incidence rates compared to Asian countries. Several factors, including healthcare access, changes in diagnostic criteria, and environmental conditions are hypothesized to explain these differences. Future research should further explore the ON-MS relationship, and, for more recent times, the potential influence of external factors like the COVID-19 pandemics.