Comorbid schizophrenia and Parkinson’s disease: A case series and brief review

Traditionally, schizophrenia is considered to be a result of dopaminergic hyperactivity while dopaminergic deficiency underlies Parkinson's disease (PD). This opposing pathophysiology makes comorbid schizophrenia and PD seemingly impossible; however, they do coexist rarely in clinical practice. We present four patients with paranoid schizophrenia diagnosed in their youth who developed parkinsonian symptoms on a stable regimen of quetiapine or clozapine after several years. The diagnosis of comorbid schizophrenia and PD was made mainly according to clinical observation. In addition, dopamine transporter (DAT) imaging with F-18-FP-CIT PET was done in two patients, which showed normal DAT density. It is believed that dopaminergic dysfunction in distinct dopaminergic pathways may explain the coexistence of these two disorders.