Cognitive impairment in patients with Parkinson’s disease: profiles and implications for prognosis

Abstract

Objectives: The main objective of the present thesis was to explore cognitive impairment in patients with Parkinson’s disease (PD), focussing on non-demented patients and elucidating cognitive profile, the course and predictive power of cognitive impairment for the development of dementia. Methods: The sample of 139 patients with PD was drawn from an epidemiological study of PD performed in Rogaland County, Norway in 1993. In 1997, the survivors performed a baseline evaluation consisting of neurological, psychiatric and neuropsychological assessments. They were re-assessed 4 years later with the same test-battery. A group of 38 healthy elderly controls performed the same neuropsychological test battery at baseline, in order to obtain normative data. PD was diagnosed according to explicit and generally accepted criteria based on clinical examination. The cognitive assessment consisted of a battery of neuropsychological tests assessing visual memory (Benton Visual Retention Test, multiple choice version), visuospatial abilities (Judgement of Line Orientation), and executive functions (Stroop Word Test), as well as of two screening instruments: the Mini Mental Status Examination (MMSE) and Dementia Rating Scale (DRS). Dementia in PD was diagnosed according to Diagnostic and Statistical Manual of Mental Disorders, 3rd edition, revised (DSM- III-R) criteria based on a semi-structured clinical caregiver-based interview and the results of the neuropsychological tests. The diagnosis of mild cognitive impairment (MCI) was made according to a modified version of the criteria proposed by Petersen et al 2001. Results: Fifty-five percent of the PD patients without dementia were cognitively impaired. Impairment on the Stroop Word Test was associated with an increased risk for incident dementia. Thirtyeight PD patients (52.7%) who fulfilled the criteria for MCI were identified. The progression rate to dementia after 4 years was 60% in subjects with MCI, compared to only 20% among the cognitively intact PD patients. Finally, cognitive heterogeneity was found both in patients with MCI and with dementia: In both groups, the majority exhibited a predominantly executive impairment (i.e. “subcortical” cognitive profile), although a considerable proportion had a predominantly memory impairment (i.e. “cortical” cognitive profile). Conclusions: These findings show that cognitive impairment is common even in non-demented patients with PD, and that MCI is an early manifestation of the dementia in PD. The cognitive profiles indicate that in most patients with PD, fronto-subcortical changes are the main contributing factor to dementia, whereas in other patients, cortical and hippocampal changes predominate.