Objective: To investigate the coincidence of Parkinson’s disease (PD) and schizophrenia spectrum disorders (SCD).

Background: PD is characterized by a progressive loss of mesostriatal dopamine (DA). The DA hypothesis of schizophrenia (SCZ), on the other hand, has linked hyperactive DA system to positive symptoms of SCZ. Due to the opposite dopaminergic effects, coexistence of the two conditions in the same individual has been considered a rarity.

Method: We performed a case-by-case review of patient records in combination with a national register study. Regionally, we studied all 3045 PD patients diagnosed 2004-2019 at Turku University Hospital district in Finland. PD patients with previously diagnosed SCD (ICD-10 code F2*; separate analysis for SCZ F20*) were included. Nationally, we used Finnish register data which included patients aged ≥35 years who received drug reimbursement for PD 1996-2015 (n=21683). Patients were excluded if the PD or F2* diagnosis was considered incorrect or uncertain, if F2* diagnosis was made after the age of 60 years or less than 6 years before the PD diagnosis. Comparable non-PD control groups were derived from regional (R) and national (N) data. Logistic regression was used to compare prevalences of diagnoses (SAS 9.4. for Windows).

Results: In PD patients, the prevalence of earlier SCZ was 0.5% (R) and 0.7% (N) and the prevalence of SCD was 0.8% (R) and 1.4% (N). In age-matched non-PD patients, corresponding prevalences of earlier SCZ were 0.1% (R) and 0.6% (N); and 0.2% (R) and 1.2% (N) for earlier SCD. [Table 1.] Odds ratios (OR) for PD were 4.68 (1.35-16.31, p=0.02) for SCZ and 4.63 (1.76-12.19, p<0.01) for SCD in regional data. Corresponding ORs in national analysis were 1.07 (0.90-1.28, p=0.45) for SCZ and 1.16 (1.02-1.31, p=0.02) for SCD.

Conclusion: Our results suggest that a diagnosis of SCD increases the risk of PD in later life. The same association was seen both in case-control individual data and in national population-level register data. Therefore, despite the opposite dopaminergic disease mechanisms of PD and SCZ, the coexistence is not decreased but paradoxically increased. Further, the results were detected although the increased mortality of severe SCZ patients dilutes the effect. We postulate that the result could be explained by increased vulnerability of the DA system due to dysregulated DA system in SCZ with lower tonic DA release but oversensitive phasic DA release.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/increased-risk-of-parkinsons-disease-in-patients-with-schizophrenia-spectrum-disorders/