Introduction: Our knowledge of incidence and outcomes of MS relapses with specific symptomatology is limited. For example, optic neuritis is more common in early MS and the ability to recover deteriorates with longer disease duration. However, a comprehensive evaluation of multiple sclerosis relapse phenotypes, comprising clinical presentations, severity, impact and recovery, and capturing full spectrum of MS courses, duration and patient demography, has not yet been done. Aim: To identify patterns of clinical MS relapses, their impact on specific neuroanatomical locations and their associations with demographic and clinical parameters. Methods: Information about relapse symptomatology was collected prospectively in 17,555 eligible patients and 104,333 patient-years recorded in MSBase, an international observational MS registry. In a proportion of the relapses, information about relapse severity, impact on activities of daily living and recovery was available. Associations between relapse phenotype and patient characteristics were tested with a series of multivariable logistic regression models. Principal component analysis was conducted to assess the tendency of the specific relapse locations to be involved sequentially in individual patients. Results: Of 63,343 relapses, the majority affected pyramidal and sensory functions. Visual and brainstem relapses occurred more frequently in early disease and in younger patients. Sensory relapses were recorded mostly in earlier disease and less commonly in relapsing-progressive disease. Pyramidal, sphincter and cerebellar relapses were more common in older patients and in progressive disease. Women more commonly presented with sensory or visual symptoms, while men were more prone to pyramidal, brainstem and cerebellar relapses. Relapses were likely to recur within the previously affected locations (odd ratios 1.8 – 5, p = 10^-13), with pyramidal, sphincter and sensory relapses often converging within the same individuals (eigenvalue = 2.1, loadings 0.59-0.68). Sensory relapses had a lower impact on daily activities and together with visual and brainstem relapses showed better recovery than the other relapse presentations. Finally, relapse severity increased and the ability to recover decreased with age or more advanced disease. Conclusions: Patterns of clinical relapse symptomatology vary with respect to demographic and clinical factors, including age, sex, MS duration, course and stage
