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.