Anticipating Clinical Onset in Multiple Sclerosis: Challenges and Opportunities.

Multiple sclerosis (MS) is a chronic, immune-mediated disorder of the central nervous system, increasingly recognized as a disease continuum that begins years before the first neurological event. Genetic susceptibility, environmental exposures, and silent neuroinflammation contribute to early disease activity. Recent studies have highlighted a preclinical phase that includes both a biological stage, characterized by elevated biomarkers such as serum neurofilament light chain up to 10 years before onset, and a prodromal phase, marked by subtle but measurable symptoms. Population-based cohorts consistently show increased healthcare use, higher prevalence of psychiatric and cognitive disturbances, fatigue, pain, and gastrointestinal disorders years before diagnosis, which may represent prodromal symptoms. Radiologically isolated syndrome (RIS), defined by incidental demyelinating lesions in asymptomatic individuals, represents the visible form of this phase and provides a unique opportunity to study the transition to clinical disease. Approximately half of RIS patients develop MS within a decade, with predictors including younger age, male sex, CSF oligoclonal bands, and spinal cord involvement. Recent randomized controlled trials demonstrated that early use of disease-modifying therapies in RIS significantly reduces conversion risk. Defining the preclinical and prodromal phases of MS offers a major opportunity to refine risk stratification, enable earlier intervention, and ultimately prevent or delay the onset of clinically definite MS.