In patients who present with a clinically isolated syndrome (CIS), whose 
features are suggestive of multiple sclerosis (MS), fulfilling McDonald 2010 
magnetic resonance imaging (MRI) criteria for dissemination in space (DIS) and 
dissemination in time (DIT) enables a diagnosis of MS. While ⩾1 periventricular 
lesion is included in the 2010 DIS criteria, earlier McDonald criteria required 
⩾3 periventricular lesions to confirm DIS and recent Magnetic Resonance Imaging 
in Multiple Sclerosis (MAGNIMS)-recommended DIS criteria also require ⩾3 
lesions. We investigated the effect of varying the required number of 
periventricular lesions and found that the best combination of specificity and 
sensitivity for clinically definite MS was seen for ⩾1 periventricular lesion 
using both the McDonald 2010 and MAGNIMS 2016 criteria.