Subjective feeling of fatigue and cognitive performances: a specific link in Multiple Sclerosis

Introduction. Fatigue is one of the most frequent and debilitating symptom reported by patients with Multiple Sclerosis (MS). Yet, the link between cognitive impairments and subjective feeling of fatigue remains unclear. This study aims to investigate the associations between cognitive performance and subjective measures of both physical and mental fatigue. Stepwise regression analyses were conducted in order to determine, correspondingly, the contribution of physical and cognitive fatigue scores, as well as depression and anxiety scores, on patients' cognitive performance.
Methods. 38 MS patients (EDSS: 1 to 6) and 28 healthy controls, matched on age and sex, were included in this study. Cognitive and physical fatigue scores were obtained from the Modified-Fatigue Impact Scale (M-FIScog and M-FISphys), and anxiety and depression level were determined from the HAD scale. Cognitive performance was assessed for working memory (Working Memory Index (WMI) from the WAIS III), verbal episodic memory (sum of the 5 learning trials of the CVLT), visual episodic memory (immediate recall score of the 10/36 test), processing speed (Processing Speed Index (PSI) from the WAIS-IV, Symbol Digit Modality Test (SDMT), denomination and reading speed at the Stroop task) and executive functioning (interference score of the Stroop task). In order to take participants' age distribution and education levels into account, cognitive scores were converted into standard scores.
Results. For MS patients, M-FIScog scores explained variance in performance for PSI (R² :0.12, p<0.05), and for denomination (R² : 0.17, p<0.05) and interference (R² :0.32, p<0.0005) scores of the Stroop task. Regarding the control group, no variable was significant for each 8 cognitive scores. M-FISphys, depression and anxiety scores were retained by none of the significant model.
Discussion. These results stress the need to dissociate cognitive and physical fatigue in MS. Only cognitive fatigue appeared as an explanatory variable of cognitive performance. Moreover, the influence of cognitive fatigue seems to be MS-specific for task relying on processing speed, and accentuated when the task requires executive control.