Patients with SCD and schizophrenia specifically had increased risk of developing Parkinson disease later in life.
Valtteri Kaasinen, MD, PhD
Data from a recent study suggest that schizophrenia spectrum disorder (SCD) increases the risk of Parkinson disease (PD) later in life.
Regional patients with PD had a 0.76% (n = 23) prevalence of previous SCD and throughout Finland, had a prevalence of 1.50% (n = 332). In control patients of the same age, the prevalence of SCD was 0.16% regionally and 1.31% nationwide. The odds ratio (OR) for developing PD after SCD was calculated to be 4.63 (95% CI, 1.76–12.19; P < .01) regionally and 1.17 (95% CI, 1.04–1.31; P <.01) nationwide.1
“Previous studies have recognized several risk factors for PD, including age, male sex, exposure to insecticides, and head injuries. However, the current understanding is that the development of PD is due to a joint effect of different environment, hereditary, and patient-specific factors,” lead author Tomi Kuusimäki, MD, Doctoral Candidate, University of Turku, said in a statement.2 “According to our results, a previously diagnosed psychotic disorder or SCD may be one factor that increases the risk of PD later in life.”
Kuusimäki and colleagues, including senior author Valtteri Kaasinen, MD, PhD, adjunct professor, department of neurology, University of Turku, analyzed regional data from 3045 patients with PD treated at Turku University Hospital between January 1, 2004, and July 31, 2019, and nationwide data from 22,189 patients with PD from the Special Reimbursement register. Logistic regression was used to compare PD diagnoses in patients with or without previous SCD.
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“The occurrence of PD and SCD in the same person has been considered rare because these diseases are associated with opposite alterations in the brain's dopamine system. Our study changes this prevailing conception,” Kuusimäki added in the statement.
The prevalence of previous SCD was higher in patients with PD than matched controls. The same held true for schizophrenia (SCZ) specifically, with a 0.46% (n = 14; OR, 4.68; 95% CI, 1.35–16.31; P = .02) prevalence in regional patients with PD and a 0.10% (n = 3) prevalence in controls. Nationwide, the prevalence of earlier SCZ in patients with PD was 0.65% (n = 144; OR, 1.09; 95% CI, 0.91–1.30; P = .34) as compared to 0.61% (n = 902) in patients without previous SCZ.
“The results of this study indicate that SCD increases the risk of PD later in life. The same association was seen in regional case–control data and in a nationwide nested case–control study with community‐dwelling Finnish individuals as a source population. Therefore, despite putatively opposite dopaminergic disease mechanisms, SCD does not decrease the risk of PD but rather seems to increase the risk,” Kuusimäki and colleagues wrote in their paper.
Furthermore, they added, this association was observed despite an increased mortality rate for those with severe SCZ diluting the observed effect. As such, they hypothesized that these results might be explained by the increased susceptibility of the brain dopamine system in the residual phases of SCD induced by a condition phase‐dependent mechanism that includes DA dysregulation. “Further studies are needed to investigate whether the severity of psychotic symptoms or the type or dosing of antipsychotic drugs impact the risk of PD,” they concluded.