Cognitive fatigue during a test requiring sustained attention: a pilot study

BACKGROUND: Fatigue is common in multiple sclerosis (MS), but difficulty quantifying fatigue severity has impeded studies of its characteristics, mechanisms, and therapeutics. Motor fatigue can be objectively measured as the decline in strength occurring during sustained contractions. Analogous declines in cognitive performance occur during tasks requiring sustained attention. OBJECTIVE: To objectively measure cognitive fatigue as a decline in performance during tests requiring sustained attention. DESIGN/METHODS: Patients with clinically stable MS (n = 20) and healthy controls (n = 21) with comparable age, gender, and education completed the Paced Auditory Serial Addition Test (PASAT) and the Digit Ordering Test (DOT) at two identical test sessions separated by 4-10 days, within a month after two practice sessions. Cognitive fatigue was quantified with two pre-specified methods for each test. The reliability of cognitive fatigue assessments was evaluated with intraclass correlation coefficients (ICCs) and construct validity was evaluated using correlations with measures of self-reported fatigue, cognition, and overall impairment/disability. RESULTS: MS patients had an average of 18.7 items correct on the first 20 items of the PASATand 17.8 correct on the last 20 items, quantified as 5.3-5.8% declines in performance using the different measurement methods (P = 0.01, rejecting the null hypothesis of zero mean decline). Although MS patients as a group demonstrated a similar decline at both sessions, ICCs were relatively low. Control patients did not demonstrate significant declines in performance during PASAT administration, but tests comparing declines in MS patients and controls did not demonstrate significant differences. Fatigue was not demonstrated using the DOT, and test-retest reliability was very poor. CONCLUSIONS: MS patients have objectively measurable cognitive fatigue during administration of the PASAT.