[XVIII Post-ECTRIMS Meeting: Review of the New Developments Presented at the 2025 ECTRIMS Congress (Part I)]

The 18th edition of the post-European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS) meeting was held on 17-18 October 2025 in Madrid, bringing together Spanish neurologists specialising in multiple sclerosis (MS), who presented the most relevant advances discussed at the ECTRIMS congress held a few days earlier in Barcelona. The aim of the meeting was to summarise the latest developments in the pathogenesis, new diagnostic criteria and biomarkers of MS. This article is the first of a two-part series (I and II) that reviews recent data on cell trafficking into the central nervous system, highlighting the functional heterogeneity of the brain endothelium, the molecular mechanisms regulating leukocyte infiltration and the differential role of macrophages and resident microglia in tissue damage and chronic inflammation. The evidence supporting the role of Epstein-Barr virus as a necessary risk factor in MS is presented, together with the therapeutic and preventive implications from studies of antiviral therapies, cell-based therapies and vaccines. Regarding diagnosis, the new criteria for neuromyelitis optica spectrum disorders are addressed, with particular attention to the molecular heterogeneity of double-seronegative forms, as well as the update of the magnetic resonance imaging (MRI) guidelines following the 2024 revision of the McDonald criteria. Advances in the neuroimaging of chronic active inflammation, quantitative spinal cord imaging and the concept of spinal cord reserve are highlighted. The growing role of artificial intelligence applied to MRI for the early prediction of progression and treatment response is also reviewed. Finally, advances in remyelination mechanisms and in biomarkers are presented, underscoring the value of multimodal and longitudinal approaches for prognostic stratification and treatment monitoring. Overall, the data reviewed reflect a change into a more integrated and predictive understanding of MS, with direct implications for early diagnosis, treatment personalisation and the design of future clinical trials.