A clinically isolated syndrome (CIS) is defined as the first clinical symptom suggestive of inflammatory demyelination of the central nervous systems (CNS) without evidence for dissemination in time and space that are both necessary for a diagnosis of multiple sclerosis (MS) . Natural history studies have shown that the majority of patients ultimately convert to clinically definite MS (CDMS) [2, 3]. Younger age at onset, male sex, type of clinical symptoms at onset , presence of oligoclonal bands (OCB) in cerebrospinal fluid (CSF) , and magnetic resonance imaging (MRI) T2-lesion load  are the most important prognostic factors for this conversion. CIS and MS are diagnosed using the McDonald criteria, which were recently revised in 2017, resulting in increased sensitivity and consequently lower incidence of CIS .
In randomized controlled trials, disease-modifying therapies (DMT) like interferon beta (IFN-beta) products and glatiramer acetate (GA) were shown to delay the conversion from CIS to CDMS significantly [7,8,9,10]. Consequently, IFN-beta and GA have been approved for CIS and are, thus, widely used in clinical routine .
However, prolonged periods of absence of disease activity may prompt consideration of DMT discontinuation in CIS patients and treating neurologists, especially if patients experience adverse events or syringe fatigue . While the benefit–risk profiles of IFNb and GA are generally favourable, side effects, matters of convenience, and even economic burden, including costs for health care systems, may be considered in the discussion whether to continue or to stop DMT .
Robust data supporting the discontinuation of DMT are scarce to date, with a lack of evidence especially in clinically stable CIS patients. Natural history data suggest that inflammatory activity declines with age  and there is some evidence in CDMS from observational databases [12, 15, 16] or unselected retrospective cohorts  with the limitation of considerable heterogeneity concerning both patient population as well as reasons for DMT discontinuation (ranging from pregnancy, lack of adherence, and side effects to stable disease course).
The primary goal of our study was to evaluate possible predictive factors for NEDA status after stopping DMT in a homogenous cohort of CIS patients, who started DMT after the very first clinical episode suggestive of MS and discontinued DMT after remaining free of disease activity for at least five years.
Patients and methods
This is a retrospective analysis of prospectively collected observational data. We recruited patients from the MS outpatient clinic of the Department of Neurology, Medical University of Vienna, diagnosed between 2001 and 2011 with a CIS according to McDonald criteria 2001 (see Fig. 1). The study was approved by the ethics committee of the Medical University of Vienna (EK 1203/2016).
Inclusion criteria were initiation of DMT (IFN-beta or GA) after a diagnosis of CIS was established. Even though the 2017 revision of the McDonald diagnostic criteria are the generally accepted criteria for MS diagnosis nowadays, for the purpose of this work, it is important to emphasize that we used the 2001 McDonald diagnostic criteria (applicable when the majority of the patients from our cohort was diagnosed). This version defined CIS as a monosymptomatic event suggestive of a first inflammatory demyelinating event with an acute onset reaching a peak within 14 days in the absence of objective clinical evidence of a second lesion and without evidence for dissemination in time and space as derived from typical MRI findings and eventually from positive oligoclonal bands in the cerebrospinal fluid (CSF) . The risk of misdiagnosis at this very early stage of the disease was minimized using the more conservative Barkhof MRI criteria in all patients, and CSF testing for oligoclonal bands (OCB). The Barkhof criteria consist of at least 1 gadolinium-enhancing lesion or at least 9 lesions on T2-weighted images, at least 3 periventricular lesions, at least 1 juxtacortical lesion and at least 1 infratentorial lesion. 3 of the 4 variables must be met .
After initiation of DMT, patients had to have no evidence of disease activity (NEDA-3) for at least five consecutive years before discontinuation of therapy . In this case, the option of DMT discontinuation was discussed with the patient extensively; however, the decision was primarily based on the patient’s individual choice. After discontinuation of DMT, patients underwent regular clinical examinations at least annually (clinical history with documentation of confirmed relapse and disease progression as measured by the Expanded Disability Status Scale (EDSS)) as well as annual MRI follow-ups for at least five consecutive years.
A relapse was defined as a typical symptom of an acute inflammatory demyelinating event lasting for at least 24 h, at least 30 days apart from the last episode, and without associated temperature increase or recent infection.
Furthermore, a confirmed sustained EDSS increase of 0.5 or more at 6-month follow-up as compared to baseline (i.e. time of treatment initiation) was defined as EDSS progression. MRI activity was defined as either a new or enlarging T2 lesion or a new contrast-enhancing lesion as compared to a prior MRI. NEDA-3 was defined as the absence of relapse, disease progression and MRI activity .
Patients were divided into two groups: the first group was labelled “evidence of disease activity” (EDA) and included those patients who suffered either a relapse, EDSS progression, and/or MRI activity during the 5-year follow-up period; the second group was called "NEDA" comprising patients with no evidence of disease activity (NEDA-3) within five years of follow-up. In addition, various age cut-offs were analysed based on the median of the group (31y) and current available data with regard to the NEDA rate (40y and 45y) .
The statistical evaluation of the collected data was done in SPSS (SPSS Inc. Version 26.0, Chicago, IL, USA). Continuous parametric variables were tested for normal distribution by Kolmogorov–Smirnov test. Categorical variables were expressed in frequencies and percentages, continuous parametric variables as either mean and 95% confidence intervals (95% CIs) or median and range as appropriate depending on normal distribution. The primary endpoint of the study was the comparison of EDA versus NEDA after a minimum follow-up of at least five years.
Univariate differences between patient groups in categorical variables were evaluated using cross-tabulation and chi-square test corrected by Fisher's exact test. Numeric variables were analysed by independent t-test or Mann–Whitney U test as appropriate depending on normal distribution.
Multivariate analyses were performed using binary logistic regression. Group comparisons regarding time to event (NEDA-3 status) were investigated using Kaplan–Meier curves and Cox regression models. The log-rank test was used for evaluation of significance. The significance level was set with a p < 0.05 (two-sided).