Diplopia in Movement Disorders: A Systematic Review of the Literature

Introduction: Although the reported frequency of diplopia is between 10 to 40% of patients with Parkinson's disease (PD) and other movement disorders, it remains one of the most undiagnosed non-motor symptoms. Furthermore, it has a major impact on the quality of life of these patients. The aim of this study is to systematically review the literature regarding the frequency, causes, and implications of diplopia in movement disorders. Methodology: An electronic search was conducted in March and June 2023 using the PubMed database in order to identify appropriate studies. Studies that were written in English, that represented observational, analytical studies, and case reports, and that provided information regarding diplopia in movement disorders were included in the systematic review. Results: A total of 686 articles were identified out of which 43 met the inclusion criteria. The studies included in the systematic review ranged from descriptive studies (case reports and case series) to analytical-observational studies (cross-sectional studies, prospective and retrospective cohort studies, and case-control studies). In Parkinson's disease, the incidence of diplopia ranged from 10 to 38%. In these patients, diplopia was linked to the presence of visual hallucinations and cognitive decline but also to convergence insufficiency and the presence of motor fluctuations. Cases of diplopia secondary to deep brain stimulation were also reported. Diplopia was associated with longer disease duration and worse motor and non-motor scores. Diplopia was also reported in other movement disorders such as multiple system atrophy (frequency as high as 18%) and progressive supranuclear palsy (frequency as high as 39%) and was associated with increased mortality and shorter duration in life span. Conclusions: Diplopia occurs in up to 38% of patients with movement disorders and has a negative impact on their health-related quality of life. Treating physicians should actively ask about diplopia and other ophthalmological symptoms, as many patients do not spontaneously report them. The pathophysiology of diplopia is complex, and it involves heterogeneous peripheral and central mechanisms. The management of these patients should involve a multidisciplinary team of health professionals in order to provide appropriate, tailored management.