Relationship between extrapyramidal symptoms and serum anticholinergic levels in treated chronic schizophrenics

Serum anticholinergic levels were measured by radioreceptor assay (RRA) in schizophrenic patients treated for drug-induced Parkinsonism; 68 patients stabilized on a single neuroleptic and an anticholinergic antiparkinsonian agent (benzhexol, benztropine or procyclidine) were assessed using the extrapyramidal side effects (EPS) scale prior to each blood sampling. Serum anticholinergic levels showed a significant inverse correlation with EPS but did not appear to be dose-related in any of the three anticholinergic drug groups. Percentage binding to proteins was significantly less with benztropine than either benzhexol or procyclidine. Serum-free anticholinergic levels correlated significantly with total serum levels in the benzhexol and procyclidine groups but not in the benztropine group. At serum levels above 4.5 pmol/ml atropine equivalents, EPS was significantly less than at levels below that. We discuss the implications of this finding and suggest practical clinical applications of measurement of serum anticholinergic levels. Serum neuroleptic and serum prolactin levels did not correlate with either dose or serum level of anticholinergics.