Hallucinations, mainly of a visual nature, are considered to affect about one-quarter of patients with Parkinson's disease. They are commonly viewed as a side-effect of antiparkinsonian treatment, but other factors may be involved. The aim of this study was to determine the phenomenology, prevalence and risk factors of hallucinations in Parkinson's disease. Two-hundred and sixteen consecutive patients fulfilling clinical criteria for Parkinson's disease were studied. Demographic and clinical variables were recorded, including motor and cognitive status, depressive symptoms and sleep-wake disturbances. Patients with and without hallucinations were compared using non-parametric tests, and logistic regression was applied to significant data. Hallucinations had been present during the previous 3 months in 39.8% of the patients, and fell into three categories: minor forms, consisting of a sensation of a presence (person), a sideways passage (commonly of an animal) or illusions were present in 25.5% of the patients (an isolated occurrence in 14.3%), formed visual hallucinations were present in 22.2% (isolated in 9.3%) and auditory hallucinations were present in 9.7% (isolated in 2.3%). Patients with minor hallucinations had a higher depression score than non-hallucinators but did not differ in other respects. Logistic regression analysis identified three factors independently predictive of formed visual hallucinations: severe cognitive disorders, daytime somnolence and a long duration of Parkinson's disease. These findings indicate that, when minor hallucinations are included, the total prevalence is much higher than previously reported. A simple side-effect of dopaminergic treatment is not sufficient to explain the occurrence of all visual hallucinations. The main risk factor in treated patients is cognitive impairment, although sleep-wake cycle disturbances, and possibly other factors related to the duration of the disease, act as cofactors.

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To study prevalence of hallucinations in patients with Parkinson's disease (PD) during a 1-year period, and identify factors predictive of the onset of hallucinations in patients who were hallucination-free at baseline, 141 unselected outpatients with PD were evaluated prospectively for a set of demographic, clinical, and therapeutic variables and the presence of hallucinations during the previous 3 months. Patient groups were compared with nonparametric tests, and logistic regression was applied to significant data. Follow-up data were available for 127 patients. The hallucination prevalence rates (%) at the first and second evaluation were, respectively, 41.7 and 49.6 for hallucinations of all types (NS), 29.1 and 40.2 for minor hallucinations (i.e., presence or passage hallucinations, and illusions) (P = 0.02), 22.8 and 21.2 for formed visual hallucinations (NS), and 8.7 and 8.7 for auditory hallucinations (NS). Hallucinations rarely started or ceased during the study. The most labile forms were minor hallucinations, which developed in 20% of patients and ceased in 9%. During follow-up, 15% of patients started to hallucinate. Three factors, all present at the first evaluation, independently predicted the onset of hallucinations in patients previously free of hallucinations at baseline (odds ratio; 95% confidence interval): severe sleep disturbances (14.3; 2.5-80.9), ocular disorders (9.1; 1.6-52.0), and a high axial motor score (5.7; 1.2-27.4). Hallucinations have a chronic course in most parkinsonian patients. Factors predicting the onset of hallucinations point to a role of extranigral brainstem involvement and a nonspecific, facilitating role of ocular disorders.