Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease of the central nervous system, usually presenting as clinically isolated syndrome (CIS). The course of MS following first symptoms is unpredictable, as approximately 30% of patients with MS have a benign course and don't develop significant disability while another 20-30% progress to severe disability within a relatively short time period. In this context, it is difficult to counsel an individual patient and choose the best treatment option at time of diagnosis. For these reasons, prognostic markers that could be used to predict future disease course are extremely useful. The only cerebrospinal fluid (CSF) prognostic biomarker currently used in clinical practice are oligoclonal bands (OCB) that can predict conversion from CIS to clinically definite MS, although this observation is not consistent. However, OCB analyses are qualitative with issues in reproducibility and a limited dynamic range. CSF immunoglobulin (Ig) free light chains (FLC) are a quantitative measure of humoral response in CSF that has showed greater sensitivity and specificity than OCB for confirming diagnosis of MS. Moreover, in few recent studies they seem to have also a prognostic value, predicting conversion from CIS to clinically definite MS and correlating with the Expanded Disability Status Scale (EDSS). Optic Neuritis (ON) can be a first clinical relapse of MS and is particularly interesting because it may constitute a suitable clinical model for neuroprotection studies, as visual function can be measured with quantitative methods, including Visual Evoked Potential (VEP) and Optical Coherence Tomography (OCT). The investigators aim to better explore the utility of CSF Ig FLC as potential prognostic biomarker for MS, and to predict the recovery of visual function after ON, as model of MS relapse. The investigators will study its potential correlation with MS relapses, with changes in several functional outcome scores, exploring physical disability, fatigue, behavior, cognition, upper and lower extremity function, and with MRI disease activity. For a subgroup of patient, the investigators aim to explore its potential correlation with in vivo measures of demyelination and neuronal and axonal loss after ON, as model of potential recovery after MS relapse. The investigators aim also to compare the prognostic value of Ig FLC with Neurofilament light chain (NfL), a potential prognostic biomarker wider studied in MS.