Background: Radiologically isolated syndrome (RIS) is defined as white matter lesions fulfilling DIS criteria for multiple sclerosis (MS) in individuals without MS signs and symptoms. Disease-modifying therapy (DMT) may reduce the probability of developing clinically definite MS in high-risk individuals, as recently shown in two phase 3 studies including patients from Europe and the United States.

Objective: To describe the frequency and long-term outcomes of patients with RIS.

Methods: Observational, longitudinal retrospective and prospective study in patients with an MS spectrum phenotype receiving regular clinical care at an MS referral centre in Chile. Magnetic Resonance (MR) and clinical visits were performed every 6 months or in the event of a suspected relapse. We assessed the characterization of patients who developed a clinical event, new asymptomatic MRI lesions or remained with no evidence of disease activity (NEDA). DMT outcomes were also evaluated as exploratory analysis, including No Evidence of Disease Activity 3 (NEDA3) and Progression Independent of Disease Activity (PIRA).

Results: Out of 848 subjects, we identified 28 asymptomatic patients with MR findings consistent with RIS (3.3 %), with a median follow-up of 72 months (range 1-157). The cohort was 82 % female, with a median age at RIS diagnosis of 34 years (range 20-54), and 13 % had a family history of MS. Clinical conversion occurred in 25 % (7/28) of patients, with 71 % developing relapsing-remitting MS and 29 % developing primary-progressive MS, in a median of 21 months. The median time to the first attack was 15 months (range 3-38) for relapsing-remitting MS patients, while the median time to primary-progressive MS diagnosis was 104 months (range 50-157). Asymptomatic new lesions were observed in 57 % (16/28) of patients, with a median time of 26 months. NEDA3 was achieved in 67 % of patients who started DMT after developing a new asymptomatic lesion, compared to 43 % of those who started after clinical conversion.

Conclusions: RIS is an infrequent diagnosis, but early treatment during the prodromal asymptomatic phase appears to be associated with improved outcomes.