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Multiple sclerosis (MS) results in cognitive impairment in approximately half of affected individuals 1 . These deficits can occur very early in the disease course; sometimes even as the initial presenting symptom before a formal diagnosis can be made 2,3 . Although much individual variability exists, domains affected often include: attention and information processing speed, memory, visual perception and executive functioning 4 . Studying these deficits is important given the strong negative impact on quality of life 5 . Cognitive deficits often progress over time, although the rate of progression is slow, with changes being detectable in only a subset of individuals, and only after intervals of at least three years 6,7 . Amato et al 8 examined both cross-sectional and longitudinal studies. On the basis of their review they concluded that cognitive dysfunction is highly prevalent and that secondary-progressive subtypes may develop greater cognitive ABSTRACT: Objective: To determine if different methods of evaluating cognitive change over time yield measurably different outcomes. Methods: Twelve cognitively impaired patients with clinically definite Multiple sclerosis (10 relapsing-remitting, 2 secondary progressive) underwent neuropsychological testing (baseline, 6, 12 months). Data was analysed using: t-tests evaluating group differences on individual tests, group differences in composite scores, reliable change analyses at the level of the individual, and comparisons regarding number of tests failed at each time point. Results: Group t-tests on individual tests yielded no change. When tests were grouped according to theoretical constructs, analyses revealed change in processing speed. Reliable change estimates revealed that 16% of the sample deteriorated. When change was measured with respect to the number of domains affected at each time point, 58% of the sample deteriorated on at least one subtest. Conclusions: Methodology has a significant impact on interpretation of longitudinal data. In the same group of subjects, traditional group analyses documented no change in individual test scores or change on a single composite score. Analyses of individual results documented change from 16 to 58% of the sample. Advantages and disadvantages of each method were discussed. Findings have implications for interpretation of longitudinal studies.RÉSUMÉ: Changements cognitifs significatifs dans la sclérose en plaques : la méthode d'évaluation importe. Objectif : Le but de l'étude était de déterminer si différentes méthodes utilisées pour l'évaluation longitudinale des changements cognitifs donnent des résultats dont les différences sont mesurables. Méthode : Douze patients ayant une atteinte cognitive, porteurs d'une sclérose en plaques (SP) cliniquement certaine (10 cas de SP rémittente récurrente, 2 de SP secondaire progressive), ont subi des tests neuropsychologiques (évaluation initiale et 6 et 12 mois plus tard). Les données ont été analysées au moyen du test t pour évaluer les différences de groupe pour chaque test...

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