Clinical Disease Activity With Long Term Natalizumab Treatment

The primary objective of the study is to retrospectively investigate the proportion of participants free of new or enlarging fluid-attenuated inversion recovery (FLAIR) lesions over time in approximately 300 Relapsing-Remitting Multiple Sclerosis (RRMS) participants with regular MRI follow-up, who have received natalizumab ≥24 month from two different observational cohorts: 1) approximately 230 participants from the Czech Republic; and 2) approximately 70 participants from Belgium. The secondary objectives of this study are as follows: Brain volume change by various measures; Changes in the number and volume of magnetic resonance imaging (MRI) lesions; No evidence of disease activity (NEDA) with and without brain volume change.