The association of cognitive reserve with motor and cognitive functions for different stages of Parkinson's disease

Introduction: The cognitive reserve (CR) theory has been proposed to account for the mismatch between the degree of neuropathological changes and clinical outcome in dementias. Recently, it has also been applied to Parkinson's disease (PD) with promising results, but mostly just focusing on separate proxy measures of CR, such as education, working and leisure time activities, instead of adopting a more comprehensive approach. Using the Cognitive Reserve Index questionnaire (CRIq), this study examined the association of CR with motor functions and cognition in patients with medium-low (1–9 years) and medium-high (> 9 years) PD duration. Methods: Fifty patients with PD underwent a neurological and a neuropsychological assessment, comprised of: Unified Parkinson's Disease Rating Scale- section III, Mini-Mental State Examination, Clock-Drawing Test, Rey auditory verbal learning test (immediate and delayed recall trials), Digit Span Forward, Corsi Span Forward, Frontal Assessment Battery, Raven's Colored Progressive Matrices, WAIS similarities subtest, Phonemic Fluency, Semantic Fluency and CRIq. Results: PD patients with a higher CRIq score showed a reduced motor impairment and a better global cognitive performance when compared to PD patients with a lower CRIq score, with an advantage especially observed on executive functions and short-term memory. The CR effect was even enhanced in the case of longer disease duration, as observed when considering the overall neuropsychological tests performance and non-verbal abstract reasoning in particular. The results obtained when considering education, as a single proxy measure of CR, provided no additional findings, nor did they reveal all the effects yielded by the adoption of the CRI score. Conclusion: Our results support the beneficial role of CR against motor and cognitive dysfunctions in PD and suggest that its protective role may be mostly manifested at the later stages of the disease. A theoretical framework able to explain the different impact of CR on Alzheimer Disease and PD is discussed. Finally, our results stressed the importance of using a comprehensive measure of CR instead of focusing on just one of its proxies.