Currently, about half a million people are living with the disease in the U.S. — a number that is likely to increase as the population ages, because Parkinson’s is more common among older adults.
By contrast, schizophrenia spectrum disorders — a range of conditions characterized by psychosis — tend to develop in young adulthood and are thought to be caused by an overactive dopamine system in the brain.
So whereas Parkinson’s can be treated with a drug called levodopa, which boosts dopamine levels, schizophrenia is often treated with drugs that block the activity of dopamine.
The opposite effects of these two illnesses on the dopamine system have given rise to the long-standing belief that people with schizophrenia are much less likely to develop Parkinson’s later in life.
A large retrospective study by researchers at the University of Turku in Finland and the University of Eastern Finland in Kuopio casts serious doubt on this belief.
“The occurrence of Parkinson’s disease and schizophrenia in the same person has been considered rare because these diseases are associated with opposite alterations in the brain’s dopamine system,” says doctoral candidate Tomi Kuusimäki from the University of Turku, who led the research. “Our study changes this prevailing conception.”
The researchers analyzed the medical records of 3,045 people with Parkinson’s treated between 2004 and 2019 in southwestern Finland, and 22,189 patients entered in a Finnish national register between 1996 and 2015.
They compared the prevalence of preexisting diagnoses of schizophrenia among the Parkinson’s patients with diagnoses among age-matched controls without Parkinson’s.
Nationally, the prevalence of schizophrenia spectrum disorders among people with Parkinson’s was 1.5% compared with 1.31% among the controls.
Regionally, the prevalence was 0.76% among people with Parkinson’s compared with 0.16% among the controls.
Despite the apparent differences in prevalence between the national and regional data, this suggests that having a schizophrenia spectrum disorder increases a person’s risk of developing Parkinson’s in later life.
The researchers say the increased risk may stem from prolonged use of drugs that block dopamine receptors, known as dopamine receptor antagonists or antipsychotics.
The drugs may change the brain’s dopamine system in a way that increases a person’s chances of developing Parkinson’s disease.
Alternatively, over time, schizophrenia may directly damage the dopamine system in such a way that a person becomes more vulnerable to Parkinson’s.
“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],” say the researchers in their paper.
The authors of the paper observe that the relationship between schizophrenia and Parkinson’s is further complicated by overlapping features of the two types of condition.
In schizophrenia, problems with movement — known as parkinsonism — are a common side effect of the first generation of antipsychotic drugs.
People with Parkinson’s, on the other hand, often develop signs of psychosis.
The researchers write that they tried to avoid confusing these overlapping effects with the underlying conditions themselves in their analysis.
However, they note:
“Advanced [Parkinson’s disease] is often associated with psychotic symptoms, and parkinsonism may be drug-induced in patients with schizophrenia spectrum disorder […]. Therefore, in register studies, it is difficult to reliably differentiate concurrent [schizophrenia and Parkinson’s] diagnoses in a patient at a certain point.”