Diplopia in Parkinson’s disease: visual illusion or oculomotor impairment?

Introduction Approximately 20% of patients with Parkinson’s disease (PD) experience diplopia; however, the cause of the diplopia is unclear. We aimed to explore the association of diplopia, and its subtypes, with oculomotor abnormalities, impaired vision, and visual hallucinations, in patients with PD. Methods This exploratory study included 41 PD patients, recruited from two general hospitals, of whom 25 had diplopia and 16 did not have diplopia, as well as 23 healthy controls (HCs). We defined subtypes of diplopia as selective diplopia, i.e., diplopia of single objects, and complete diplopia, i.e., diplopia of the entire visual field. All participants underwent a full orthoptic and ophthalmologic examination. Results PD patients with diplopia had a high prevalence of oculomotor abnormalities (84%), impaired vision (44%), and visual hallucinations (44%), compared to PD patients without diplopia (33%, 6%, and none, respectively, p < 0.01), and compared to HCs (23%, 9%, and none, respectively, p < 0.01). Oculomotor abnormalities were equally prevalent in both subtypes of diplopia (selective and complete), whereas impaired vision was predominantly found in patients with selective diplopia. Moreover, only patients with selective diplopia had visual hallucinations. Conclusions In PD patients, diplopia may be indicative of oculomotor or visual impairments. Hence, it is worthwhile to refer PD patients with diplopia to an orthoptist and an ophthalmologist for evaluation and, possibly, treatment of diplopia. Furthermore, in the case of selective diplopia, the neurologist should consider the presence of visual hallucinations, which may require the adjustment of the patient’s medication.