Pattern of cognitive impairment in patients with Parkinson's disease and psychosis: A critical review

Psychosis is one of the debilitating non-motor symptoms (NMS) of Parkinson's disease (PD). Cognitive impairment is considered to be a risk factor for emergence of psychosis in PD. Early detection of relevant cognitive impairment may serve as a predictor for development of psychosis, with implications for prevention and early intervention. However, the exact pattern of cognitive impairment associated with psychosis is not clear. In this article, we aim to critically review the literature on case-control studies in PD patients with and without psychosis in order to understand the pattern of cognitive impairment in those with psychosis. Majority of studies conducted till date have focused on executive and visuospatial functions. Despite some inconsistencies, most of the studies found significant impairment in these domains in PD patients with psychosis compared to those without psychosis. Studies assessing for other cognitive functions such as attention, language and memory in PD patients have also found worse performance in those with psychosis. Although there is enough evidence to suggest that PD patients with psychosis have poor cognitive functioning, it is unclear if these deficits are generalized or specific. The available evidence, which is primarily in the form of cross-sectional studies assessing for specific cognitive deficits, is not adequate to indicate a clear demarcating pattern of cognitive deficits, which differentiates PD patients with and without psychosis. Longitudinal studies with extensive cognitive assessment are warranted.