Psychosis in Parkinson's Disease: Looking Beyond Dopaminergic Treatments

Background: Parkinson's disease (PD) is the second most common neurodegenerative disorder worldwide. The symptoms of PD are characterized not only by motor alterations but also by a spectrum of nonmotor symptoms. Some of these are psychiatric manifestations such as sleep disorders; depression; cognitive difficulties that can evolve into dementia; and symptoms of psychosis, which include hallucinations, illusions, and delusions. Parkinson's disease psychosis (PDP) occurs in 18-50% of patients with PD. Treating PDP is challenging because antipsychotic drugs tend to be inefficient or may even worsen the disease's motor symptoms. Objective: This review aims to summarize the current understanding of the molecular mechanisms involved in PDP and recent innovative alternatives for its treatment. Methods: This is a narrative review in which an extensive literature search was performed on the Scopus, EMBASE, PubMed, ISI Web of Science, and Google Scholar databases from inception to August 2021. The terms "Parkinson's disease psychosis", "Parkinson psychosis," "neurodegenerative psychosis", and "dopamine psychosis" were among the keywords used in the search. Results: Recently, views on the etiology of hallucinations and illusions have evolved remarkably. PDP has been cemented as a multifactorial entity dependent on extrinsic and novel intrinsic mechanisms, including genetic factors, neurostructural alterations, functional disruptions, visual processing disturbances, and sleep disorders. Consequently, innovative pharmacological and biological treatments have been proposed. Pimavanserin, a selective 5-HT2A inverse agonist, stands out after its approval to treat PDP-associated hallucinations and illusions. Conclusion: Future results from upcoming clinical trials should further characterize the role of this drug in the management of PDP as well as other treatment options with novel mechanisms of action, such as saracatinib, SEP-363856, cannabidiol, electroconvulsive therapy, and transcranial magnetic stimulation.