The cognitive performance of patients with multiple sclerosis during periods of high and low fatigue.

The objective of this study was to examine whether multiple sclerosis (MS)-related fatigue affects patients' cognitive performance. Thirty patients who had substantial fatigue in conjunction with MS and who reported marked diurnal variability in the severity of their fatigue were tested on two occasions: during a period of high fatigue and during a period of relatively low fatigue. The order of these sessions was counterbalanced across patients. During both sessions, patients completed a questionnaire concerning their present state of fatigue and a battery of neuropsychological tests of planning, selective attention, and paired associate learning. Although patients confirmed greater fatigue during the period of high fatigue and felt they had performed more poorly during this period, there were no differences in cognitive performance that could be attributed to fatigue. Instead, all subjects showed improvement from the first to the second session regardless of whether the latter entailed a period of high or low fatigue. In contrast to studies reporting fatigue-related declines in MS patients' cognitive performance, no differences in performance were found when MS patients were tested during periods of high versus low fatigue. These contrasting results, stemming from differences in experimental design, are discussed in terms of their implications for assessing cognitive function in patients with MS.