Clozapine for Quetiapine-Refractory Psychosis in Parkinson's Disease: A Long-Term Single-Center Retrospective Study

Background: Hallucinations and delusions are severe long-term complications of Parkinson's disease (PD). Clozapine is the only antipsychotic with proven efficacy in PD psychosis (PDPsy) available outside the United States but apprehensions about its adverse event profile result in a substantial underuse of clozapine. Objectives: To investigate the practical use and long-term efficacy of clozapine in severe psychotic disorders in PD. Methods: In this retrospective study, the author used data collected over a period of 20 years and included all PD patients under his care, who were treated with clozapine for psychotic disorders. Results: During the observation period, 41 PD patients (38 with PDPsy, 2 with psychotic depression, and one with schizoaffective disorder) were started on clozapine. They had responded poorly or only transiently to quetiapine. An overnight switch to clozapine was tolerated in most. Maximum clozapine doses ranged from 12.5 to 150 mg (72.9 ± 29.9 mg). A significant reduction in psychotic symptoms was achieved in 2 days to 6 months. Among the cases tolerating clozapine, 10 had a full, 25 had a good, 3 had a moderate, and 2 had a poor clinical response to clozapine. Treatment lasted up to 12 years. The long-term response was full or good in 23, moderate in 3, and poor in 2 patients. Conclusions: Clozapine is often effective in the treatment of psychotic disorders in PD including PDPsy poorly or only transiently responding to quetiapine. Side effects including agranulocytosis are manageable in the majority of cases. Clozapine treatment should not be delayed if other measures against PDPsy prove ineffective.