How would you treat someone who has both schizophrenia and Parkinson's disease? Would treating the schizophrenia worsen the Parkinson's and vice versa?
One of the primary neurotransmitter deficits in schizophrenia is excess dopamine levels in the ventral striatum, while Parkinson's disease is associated with loss (death) of dopamine neurons, which causes a reduction of available dopamine.
As pointed out, the antipsychotic medications which are dopamine receptor antagonists used to treat schizophrenia may well exacerbate the effects of already low dopamine levels found in Parkinson's disease.
Co-occurrence of schizophrenia and Parkinson's disease in the same individual is quite rare, and the treatment of such cases has not been studied systematically. Treatment of these cases can be quite challenging.
Interestingly, there are a number of case reports in the literature that indicate that the atypical antipsychotic medication clozapine can be an effective treatment for the psychosis associated with schizophrenia, while at the same time helping to improve the physical symptoms associated with Parkinson's, such as the tremor and dyskinesia. Unlike typical antipsychotics, such as haloperidol, and some atypical antipsychotics, such as risperidone, clozapine is relatively unique in its lack of producing Parkinsonian-like symptoms, even at high doses. This is in part because clozapine binds relatively weakly to the D2 dopamine receptor compared to most other antipsychotics.
Studies on the treatment of psychosis associated with Parkinson’s (which often emerges as hallucinations in the context of dopamine agonist therapy used to treat Parkinsonian tremor) can also be used as a guide to address the question. Clozapine has been studied in randomized trials and demonstrated as the most consistently effective treatment for psychosis in Parkinson’s. Less well studied but also frequently used is quetiapine.