Visual hallucinations in Parkinson's disease: a review and phenomenological survey

Objectives - Between 8% and 40% of patients with Parkinson's disease undergoing long term treatment will have visual hallucinations during the course of their illness. There were two main objectives: firstly, to review the literature on Parkinson's disease and summarise those factors most often associated with hallucinations; secondly, to carry out a clinical comparison of ambulant patients with Parkinson's disease with and without visual hallucinations, and provide a detailed phenomenological analysis of the hallucinations. Methods - A systematic literature search using standard electronic databases of published surveys and case-control studies was undertaken. In parallel, a two stage questionnaire survey was carried out based on members of a local branch of the Parkinson's Disease Society and followed up with a clinical interview. Results - The review disclosed common factors associated with visual hallucinations in Parkinson's disease including greater age and duration of illness, cognitive impairment, and depression and sleep disturbances. The survey comprised 21 patients with visual hallucinations and 23 without. The hallucinators had a longer duration and a greater severity of illness, and tended to show more depressed mood and cognitive impairment. The typical visual hallucination in these patients is a complex visual image experienced while they are alert and have their eyes open. The image appears without any known trigger or voluntary effort, is somewhat blurred, and commonly moves. It stays present for a period of seconds or minutes. The content can be variable within and between hallucinators, and includes such entities as people, animals, buildings, or scenery. These features resemble those highlighted in hallucinations in the visually impaired (Charles Bonnet's syndrome). Conclusion - A consistent set of factors are associated with visual hallucinations in Parkinson's disease. The results of the phenomenological survey and those of visual hallucinations carried out in other settings suggest a common physiological substrate for visual hallucinations but with cognitive factors playing an as yet unspecified role.