Cognitive Fatigability, not Fatigue predicts employment status in patients with MS three months after rehabilitation

Fatigue is potentially the most important factor causing unemployment in people with Multiple Sclerosis (PwMS). Widely accepted is a discrimination between fatigue as subjective sensation and fatigability as objective measure of change in performance. The aim of this study was to identify, whether cognitive fatigue or cognitive fatigability is a better predictor for employment status three months after discharge from a neurological rehabilitation center.

64 PwMS (mean age 48.9, 43 females, mean time since diagnosis 14.7 years, median Expanded Disability Status Scale (EDSS) 3.8), complaining of fatigue and reporting difficulties with their working capacity, participated in a cognitive loading task during inpatient rehabilitation. Reaction time performance was measured using a standardized alertness test (TAP-M). Tonic alertness was measured at 8 a.m., 11 a.m. and 2 p.m. Patients worked on a standardized test battery during the morning and after lunch to induce fatigability. All of them completed the Fatigue Scale for Motor and Cognition (FSMC), a standardized questionnaire to rate the trait component of cognitive and motor fatigue. Their employment status was rated within a standardized interview by phone three months after discharge from the clinic.

Mean cognitive fatigue according to the FSMC was 38.9 ± 7.4 and mean motor fatigue 41.0 ± 5.6, indicating severe cognitive and motor fatigue. 15 (88%) of 17 patients working fulltime had severe fatigue according to the FSMC. The cognitive subscale of the FSMC ("FSMC cognition") did not correlate (r_s = -.084, p = .512) and the motor subscale of the FSMC ("FSMC motor") correlated rather weakly but not significantly (r_s= -.220, p = .080) with the employment status. In contrast, there was a significant and medium correlation between alertness at 8 a.m. (alertness₁) and employment status (r_s = -.304, p = .014). Ordered logistic regression revealed that only alertness₁ and the alertness difference between afternoon and noon (alertness difference₃₂) predicted significantly the employment status. The FSMC motor and cognition subscales had no predictive value for employment.

Cognitive fatigability (tonic alertness at 8 a.m. or increase of reaction time during the afternoon) is more adequate to predict employment status in PwMS three months after discharge from the clinic than the subjective sensation of fatigue as determined by the FSMC.