Cerebral disease with more prominent left sided cerebral involvement may be more closely associated with psychotic phenomena; a comprehensive review of cerebral laterality in relation to psychosis has suggested that a special, although unclear, pathophysiology may be at work.1

A review of patients with treated Parkinson’s disease found an overall incidence of psychiatric side effects of 20% (range 10–50) in 908 patients treated in major studies.2 Psychoses—that is, hallucinations and delusions—occurred with a frequency of 4% and were more likely to occur with concurrent dementia, increasing age and use of higher dose of levodopa. It is assumed that the main precipitant of psychotic phenomena in Parkinson’s disease is dopaminergic excess secondary to treatment. Cognitive impairment, which has been reported to occur in as many as 29% of patients with Parkinson’s disease, associated with increased age and older age at onset, may also increase vulnerability to psychosis.

Asymmetry in Parkinson’s disease is said to remain unchanged over time,3 and patients with unilateral onset of Parkinson’s disease manifestations have greater degeneration of the contralateral substantia nigra at postmortem examination.4 If predominantly left sided pathology increases the vulnerability to psychotic phenomena then initial right sided predominance of parkinsonian symptoms and signs might be a predictor of increased vulnerability. If initial right sided …