Modulation of alertness by sustained cognitive demand in MS as surrogate measure of fatigue and fatigability.

OBJECTIVE: This study used reaction time (RT) as an objective marker of cognitive fatigue and fatigability in patients with multiple sclerosis (MS). METHOD: RT was measured in fifteen healthy controls and in thirty MS patients with cognitive fatigue identified with the Fatigue Scale for Motor and Cognitive Function (FSMC). Secondary fatigue was excluded through the Epworth Sleepiness Scale and the Beck Depression Inventory. RT was measured at rest (t1), following a 2.5 hour test session inducing high cognitive load (t2), and a one hour recovery period (t3). RESULTS: At rest mean RT was longer in patients than in controls (391 ms vs 205 ms). After exerting cognitive load (t2), RT in patients increased dramatically but remained unchanged in controls. After the recovery period (t3), RT returned to baseline levels in most patients. Patients further showed a significant correlation between RT and FMSC scores at t1, t2 and t3. CONCLUSION: RT performance is a suitable surrogate marker for assessing fatigue. RT is sensitive to cognitive load and the recovery from cognitive demand. It hence represents an objective index for fatigability which can inform the management and treatment of MS.

Induction of cognitive fatigue in MS patients through cognitive and physical load.

The objective of the study was to investigate whether cognitive fatigue in patients with multiple sclerosis (MS) is a spontaneous phenomenon or whether it can be provoked or exacerbated through cognitive effort and motor exercise. Thirty two patients with definite MS and cognitive fatigue according to the Fatigue Scale for Motor and Cognitive Functions (FSMC ≥ 22) performed attention tests (alertness, selective, and divided attention subtests from the TAP test battery for attention performance) twice during rest (baseline), and before and after treadmill training and cognitive load (a standardised battery of neuropsychological tests lasting 2.5 hours). Subjective exhaustion was assessed with a 10-point rating scale. Tonic alertness turned out to be the most sensitive test and showed significantly increased reaction times after treadmill training and after cognitive load. Patients' subjective assessment of exhaustion (10-point rating scale) and the objective test results were discrepant. In contrast, healthy control subjects (N = 20) did not show any decline of performance in the subtest alertness after cognitive or physical load. Data favour the concept that fatigue is induced by physical and mental load. Discrepancies between subjective and objective assessment offer therapeutic options. The common notion of a purely "subjective" lack of physical and/or mental energy should be reconsidered.