Clinically isolated syndrome (CIS) is a single episode of neurological symptoms caused by inflammation and demyelination in the central nervous system, which is similar to MS. MS can sometimes be diagnosed after a single attack, on the basis of specific findings on MRIs. When this is not the case, the diagnosis may be CIS. Patients with CIS are at risk of further disease activity or relapses, and thus their diagnosis may change to MS. It is important that these patients are thoroughly assessed and closely monitored by an experienced clinician.