Cognitive impairment is common in the course of Parkinson’s disease (PD) and displays a continuum from subjective cognitive impairment to dementia. Illuminating the pathophysiological processes associated with the continuum may help create follow-up and new treatment approaches. In this context, large-scale intrinsic connectivity networks are widely investigated to elucidate the neural processes underlying PD and are promising as non-invasive biomarkers. This systematic review aims to examine the alterations in large-scale intrinsic connectivity networks in the continuum of PD-associated cognitive impairment. ScienceDirect, Web of Science, and PubMed databases were searched with the specified keywords. The studies obtained as a result of this review were investigated by the PRISMA criteria, which were taken as a basis for the systematic review and writing of meta-analyses. A total of 974 studies were obtained from three databases. Twenty studies were included in the systematic review based on predetermined eligibility criteria. Among the large-scale connectivity networks examined in these studies, it was found that sensory-motor networks decreased their connectivity in the continuum of PD-associated cognitive impairment, and there were conflicting results in terms of cognitive networks. Well-designed longitudinal studies are needed to clarify the alterations in the intrinsic connectivity networks in the PD cognitive impairment continuum. In these studies, it is necessary to define the cognitive disorder groups well, to control the connectivity changes that may occur due to dopaminergic treatment, and to evaluate Parkinson’s patients with subjective cognitive impairment and dementia within the continuum.

Background and purpose: Studies have found that up to 73% of COVID‐19 patients experience hyposmia. It is unclear if the loss of smell in COVID‐19 is due to damage to the peripheral or central mechanisms. This study aimed to explore the impacts of COVID‐19‐induced hyposmia on brain structure and cognitive functions.Methods: The study included 36 hyposmic (h‐COV) and 21 normosmic (n‐COV) participants who had recovered from mild COVID‐19 infection, as well as 25 healthy controls (HCs). All participants underwent neurological examination, neuropsychiatric assessment and Sniffin’ Sticks tests. High‐resolution anatomical images were collected; olfactory bulb (OB) volume and cortical thickness were measured.Results: Addenbrooke's Cognitive Examination—Revised total and language sub‐scores were slightly but significantly lower in the h‐COV group compared to the HC group (p = 0.04 and p = 0.037). The h‐COV group exhibited poorer performance in the Sniffin’ Sticks test terms of discrimination score, identification score and the composite score compared to the n‐COV and HC groups (p < 0.001, p = 0.001 and p = 0.002 respectively). A decrease in left and right OB volumes was observed in the h‐COV group compared to the n‐COV and HC groups (p = 0.003 and p = 0.006 respectively). The cortical thickness analysis revealed atrophy in the left lateral orbitofrontal cortex in the h‐COV group compared to HCs. A significant low positive correlation of varying degrees was detected between discrimination and identification scores and both OB and left orbital sulci.Conclusion: Temporary or permanent hyposmia after COVID‐19 infection leads to atrophy in the OB and olfactory‐related cortical structures and subtle cognitive problems in the long term.