Schizophrenia may increase risk for Parkinson’s disease

Schizophrenia spectrum disorder was linked to an increased risk for Parkinson’s disease later in life, according to study results published in Movement Disorders.

“It has been proposed that [schizophrenia] and [Parkinson’s disease] could coexist in some patients because of differently affected dopaminergic system regions that act independently: overactive dopaminergic transmission in the nigrostriatal and mesolimbic pathways in [schizophrenia] and decreased dopaminergic transmission in the nigrostriatal pathway in [Parkinson’s disease],” Tomi Kuusimäki, MD, of Turku University Hospital in Finland, and colleagues wrote. “On the other hand, given the opposite effects in the [brain dopamine] system, it could be hypothesized that the [two] diseases seldom coexist, assuming that major symptoms of one condition would neutralize the other via opposite dopaminergic effects in the brain.”

The investigators aimed to evaluate the risk for Parkinson’s disease following diagnosis of a schizophrenia spectrum disorder. On the regional level, they conducted a retrospective record-based case-control study that included 3,045 individuals with Parkinson’s disease who were treated between 2004 and 2009 in southwestern Finland. On the national level, they used registers in a nested case-control study to examine 22,189 Finnish patients who received a clinically confirmed diagnosis of Parkinson’s disease between 1996 and 2015.

The researchers included Parkinson’s disease patients with previously diagnosed schizophrenia spectrum disorder. From both data sets, they derived comparable non-Parkinson’s disease control groups.

Results showed a prevalence of 0.76% in regional data and 1.5% in nationwide for prevalence of earlier schizophrenia spectrum disorder among Parkinson’s disease patients. Age-matched controls had a prevalence in regional data of 0.16% and in national data of 1.31%. After schizophrenia spectrum disorder diagnosis, the odds ratio for Parkinson’s disease was 4.63 (95% CI, 1.76-12.19) in regional data and 1.17 (95% CI, 1.04-1.31) in the national data.

“The results indicate an increased risk [for] hypodopaminergic [Parkinson’s disease] after being diagnosed with hyperdopaminergic [schizophrenia spectrum disorder],” Kuusimäki and colleagues wrote. “This increase could be associated with risk-altering dopamine receptor antagonists or disease-related neurobiological effects that increase vulnerability. Further studies are needed to investigate whether the severity of psychotic symptoms or the type or dosing of antipsychotic drugs impact the risk of [Parkinson’s disease].”