DRUG INFORMATION QUARTERLY Treatment of psychosis in Parkinson's disease

usually centres on improving dopaminergic func tion in the nigro-striatal pathways. Levo-dopa and selegiline increase the amount of synaptic dopamine available; apomorphine, bromocriptlne and pergolide are agonists at dopamine receptors. Symptoms may also be improved by redressing the balance of dopaminergic and cholinergic activity. Benzhexol, procyclidine and orphenadrine, among others, are antagonists at muscarinic receptors. All of these drugs have been associated with psychiatric phenomena. This is not surprising since they can be said to mimic pathological processes: schizophrenia may be associated with increased dopamine activity; dementia with decreased acetylcholine activity. If psychosis occurs and is severe or long-lived, then drug withdrawal may be contemplated. Generally, the last anti-Parkinsonian drug to be prescribed before the problem arose should be stopped first. Otherwise the following order for stopping drugs should usually be followed: anticholinergics, selegiline, amantadine, dopamine agonists (Quinn, 1995). After this, the dose of levo-dopa should be gradually reduced. Some success may be gained by manipulating the timing and size of levo-dopa doses. In particular, giving levo-dopa after food may slow its absorp tion and diminish adverse psychiatric effects. Treating psychosis by these methods can be problematic, as withdrawing the offending agent may lead to loss of motor control. Treating movement disorders associated with Parkinson's