Fatigue is a common and disabling symptom in those who suffer from multiple sclerosis (MS). Studies indicate that more than 75% of MS patients suffer from fatigue. 1-3 Two-thirds of MS patients rank fatigue as one of their three most debilitating symptoms. 1 Multiple sclerosis fatigue is particularly aggravated by heat and can significantly impair physical activity. 1 It is one of the main causes of work absenteeism in individuals with MS. 4 Yet fatigue is a lay term that remains subjectively defined and assessment of fatigue is therefore dependent on self-report. Perhaps as a result, the physiologic substrates of MS fatigue are not well-described. The subjective experience of fatigue is multifactorial and has been correlated with factors such as disability, depression, and pain. [5][6][7] Interestingly, no clear association has been established between MS fatigue and demographic factors, such as age and gender; lesion burden; or the clinical form of MS. 1,[7][8][9] ABSTRACT: Background: Fatigue is common and disabling in multiple sclerosis, yet its physiologic substrates remain poorly defined. The aim of this study was to determine the relationship between fatigue and an objective measure of alertness in MS patients. Methods: This study enrolled 49 consecutive MS clinic patients at two academic hospitals in Toronto. Alertness was assessed with the psychomotor vigilance test (PVT), a ten-minute reaction-time test that measures attention and is sensitive to sleep loss. Patients with visual impairment or arm weakness were excluded. Validated tools were used to assess fatigue, disability, mood, and pain. Results: The average age was 43; 65% were women. Median EDSS was 2.0 (range 0-7.5). Fifty-five percent reported a high impact of fatigue on their lives. Psychomotor vigilance test performance was worse than in an age-and sex-matched population, with a mean reaction time of 315 msecs and 3.98 lapses >500 msec (p<0.001). In a multiple regression analysis, fatigue was the most significantly correlated factor with mean PVT reaction time (p<0.05), and disability was also significantly correlated (p<0.01). Mood and pain did not correlate with the PVT. Eighteen (37%) reported often experiencing restlessness in their legs at night. Conclusion: Subjective fatigue and disability were associated with poor performance on alertness testing in MS patients. This research highlights a potential role for psychomotor vigilance testing in providing a standardized assessment tool for an important aspect of MS-related fatigue.RÉSUMÉ: La fatigue dans la sclérose en plaques est associée à une diminution de la vigilance psychomotrice. Contexte : La fatigue est un symptôme fréquent et invalidant chez les patients atteints de la sclérose en plaques (SP) et sa cause physiologique demeure mal définie. Le but de cette étude était de déterminer la relation entre la fatigue et une mesure objective de vigilance chez des patients atteints de la SP. Méthode : Nous avons inclus 49 patients consécutifs de cliniques de SP situées dans deux hôpitaux ac...