Processing speed and distractibility in multiple sclerosis: the role of sleep

The effects of sleep disturbances on cognitive performance under distraction in multiple sclerosis (MS) are not known. The objective of the present study was to explore the association between an index of sleep (i.e. daytime somnolence), distractibility and the hallmark cognitive deficit in people with MS (i.e. processing speed).

MS participants (n=102) were recruited from outpatient neurology clinics. All participants completed the traditional Symbol Digit Modalities Test (SDMT). In addition, half the sample (n=52) completed a computerized version with distracters (c-SDMT) and half (n=50) without distracters. Excessive daytime sleepiness was explored with the Epworth Sleepiness Scale. Mood, anxiety and fatigue were also assessed. A linear regression analyses was conducted to determine predictors of cognitive performance.

Participants with excessive daytime sleepiness performed significantly slower on the distracter c-SDMT than those with normal sleepiness (p=0.01). No significant differences were found on the c-SDMT without distracters or the traditional SDMT. Depression and excessive daytime sleepiness predicted performance on the distracter c-SDMT, whereas only the former predicted performance on the traditional test.

Excessive daytime sleepiness in people with MS adversely influences their processing speed in the presence of real-world distracters. This finding suggests that a more complete understanding of processing speed deficits in people with MS should entail a closer look at sleep and the relationship with distractibility.