Piredda R, et al. - In this systematic review, researchers analyzed the current reports on cognitive and behavioural symptoms in monogenic forms of Parkinson disease (PD) and identified recurring patterns of clinical manifestations in those with specific mutations. In order to retrieve observational studies of monogenic PD, a systematic literature search was performed. Using standardized templates, data pertaining to cognitive and psychiatric manifestations were extracted. Of the 1,889 citations retrieved, 95 studies were involved in PD-related gene mutations: 35 in SNCA, 35 in LRRK2, four in VPS35, 10 in Parkin, three in DJ1 and eight in PINK1. Depression was the most frequent symptom with respect to psychiatric symptoms, occurring in 37.5% of PINK1 cases and 41.7% of VPS35 and LRRK2 cases. The co-occurrence of cognitive decline was observed with visual hallucinations. Higher rates of cognitive impairment result in widespread accumulation of Lewy bodies, indicative of SNCA, PINK1 and DJ1 mutations. Likewise, in SNCA mutations, a higher degree of visual hallucinations is observed, probably because of the more widespread accumulation. In these patients, the lower α-synuclein pathology rates for LRRK2 and Parkin may underpin the more benign course of the disease.