Stability of intraindividual variability as a marker of neurologic dysfunction in relapsing remitting multiple sclerosis

Background: Impairments in information processing speed are common in multiple sclerosis (MS), with affected individuals demonstrating slower responses and more intraindividual variability (IIV) in their performance on timed tasks. Evidence suggesting that IIV provides novel information about cognitive deficits in MS is accumulating; however, little is known about the stability of IIV across multiple assessments. In this study, we investigated IIV in response speed in persons with MS across 6 monthly sessions using the Attention Network Test-Interaction (ANT-I). Method: Individuals with relatively mild relapsing remitting MS and healthy controls completed the ANT-I at 6 monthly intervals. Clinical assessments (Sessions 1 and 6) and conventional magnetic resonance imaging (MRI) studies (Sessions 1-6) were examined for individuals with MS. Results: The MS groups clinical and neuroimaging measures were stable during the 6-month period. Individuals with MS were slower and more variable in reaction time performance on the ANT-I than were controls. Differences in IIV between groups were maintained across the 6 sessions, with IIV demonstrating less susceptibility to across-session practice effects than mean latency scores. Conclusions: IIV provides a stable measure of cognitive performance in mildly affected persons with MS who are clinically and radiologically stable. Further studies exploring its utility as a clinical outcome are warranted.