Selective diplopia in Parkinson's disease: a special subtype of visual hallucination?

Diplopia is sometimes reported by patients with Parkinson's disease (PD) without apparent oculomotor disorders. We assessed clinical features and associated oculomotor and perceptual performance in 14 patients (6 male, 8 female) with PD with a peculiar type of selective diplopia. Duplication of images was confined to single objects or persons, occurred repetitively, and lasted few seconds in all subjects. Frequency of episodes ranged from several episodes per day to three episodes per year. In six of seven subjects undergoing comprehensive ophtalmological examination, subtle ocular disorders (heterophoria, strabism, etc.) were found. Nine of 14 patients were suffering from current or previous visual hallucinations and 3 more patients developed hallucinations within 3 years of diplopia onset. Selective diplopia of isolated single objects and persons in PD is possibly related to hallucinosis and minor ocular disturbances seem to be a triggering factor for this peculiar type of misperception.