Neuropsychological Features of Mild Cognitive Impairment in Parkinson’s Disease: A Critical Review of the Literature

Abstract

Motor features traditionally define Parkinson’s disease (PD), but non-motor characteristics such as cognitive impairment and dementia have been gradually documented as a core part of PD. Mild cognitive impairment, as a pre-dementia phase of cognitive dysfunction, is recognised as common in non-demented PD patients. In any case, before Parkinson’s Disease-Mild Cognitive Impairment (PD-MCI) diagnostic criteria proposed by the Movement Disorder Society (MDS) Task Force in 2012, there was no agreement in the scientific community about definition, clinical features and evolution of PD-MCI. It explains why epidemiological data reported by many studies provide contrasting results. Nevertheless, many investigations have pointed out that a large amount of PD patients without dementia present a specific neuropsychological impairment, mainly characterized by executive, visuospatial and memory deficits. This review focuses on mild cognitive impairment in PD (PD-MCI). To clarify the characterization of PD-MCI neuropsychological profile, a MEDLINE literature search was carried out. Data were extracted from studies published since 1980. The neurocognitive profile of PD patients was heterogeneous. Cognitive impairment was common in PD even at the time of diagnosis and before drug treatment and it was associated with functional impairment. Primary findings reported that single domain impairment with executive dysfunction was more common than multiple domain impairment, but, nowadays, those results are largely criticized. Moreover, visual-space abilities deteriorated very early during the pathology. Patients with posterior cortical deficits seemed to be associated with the development of Parkinson’s Disease Dementia (PD-D), whereas patients with front striatal deficits were not, hence suggesting the presence of two distinct cognitive syndromes with different aetiologies and prognoses. However, several questions regarding PD-MCI criteria remain currently unanswered. More attention must be paid to the definition of best cut-off scores and to accurate, specific and sensitive tests for discriminating between patients with and without PD-MCI. Further investigations should define the exact prevalence of PD-MCI across different sites/cohort types, estimate PD-MCI subtypes prevalence and even link the clinical phenotype (akinetic-rigid vs tremor-dominant) to the different neuropsychological profiles. Furthermore, research needs to focus on detecting the neuropsychological predictors of PD-MCI conversion into PD-D. Poor efficacy of non-pharmacological intervention to delay the onset of PD-D in PD-MCI patients is also discussed.