¹Okehazama Hospital Fujita Mental Care Centre, Psychiatry, Toyoake, Japan, ²Misatopia Ogura Hospital, Psychiatry, Azumino, Japan

Background: Central anticholinergic agents (biperiden, trihexyphenidyl) are used to treat drug-induced parkinsonism induced by antipsychotic drugs. However, these drugs induce peripheral autonomic side effects such as thirstiness, dysuria, and constipation, as well as central side effects such as cognitive dysfunction and increased risk of delirium in the elderly. Although their use has decreased with the advent of atypical antipsychotics, many patients continue to use these medications without any thought of their side effects.

Aims & Objectives: It is recommended that dosage should be reduced over time to avoid adverse events such as anticholinergic rebound and worsening of psychiatric symptoms. However, there is still insufficient evidence regarding the dose at which the risk of adverse events increases and the rate of dose reduction. In this study, we examine the relation between the dose and reduction speed of central anticholinergic agents and the incidence of adverse events associated with dose reductions.

Method: Patients admitted to a psychiatric ward with a diagnosis of schizophrenia who were taking biperiden or trihexyphenidyl and who did not present drug-induced extra pyramidal symptoms were included in the study. The speed of dose reduction and incidence of adverse events in patients who completed the end of anticholinergic tapering were examined retrospectively from the medical records. The study was approved by the Ethics Committee of Misatopia Ogura Hospital.

Results: Thirty-one patients with a mean age of 68.4±9.3 years were included in the study. The dosage of antipsychotic drugs was 423.6±320.3 mg of chlorpromazine equivalent, and that of biperiden equivalent was 3.3±2.0 mg. The tapering of anticholinergic drugs was completed over 29.3±19.3 days. Adverse events such as anxiety, insomnia, worsening of psychiatric symptoms, parkinsonism, and anticholinergic rebound occurred in 20 patients. The incidence of adverse events did not differ when the patients were divided into groups with the dose reduction rate of 0.85 mg/week as the threshold, and there was no correlation with the dose of antipsychotic drugs (r=0.27). On the other hand, there was a correlation between the dose before reduction and the occurrence of adverse events (r=0.83).

Discussion & Conclusions: Adverse events occurred at a rate of 64.5% when the anticholinergic dose was reduced by 0.85 mg/week in biperiden equivalents. There was no correlation with antipsychotic dosage. Various adverse events appeared at higher doses before reduction. Caution should be exercised when reducing doses of anticholinergic drugs at high doses.