Smooth pursuit and memory saccades are impaired in early-stage Parkinson’s disease patients

Introduction: Parkinson's disease (PD) is a progressive neurodegenerative disease caused by degeneration of dopaminergic neurons in substantia nigra pars compacta (SNc). One of the most prevalent symptoms is eye movement impairment, presenting in 75% of PD patients, which have fragmented and hypometric smooth pursuit movements with prolonged latency. We aimed to investigate differences in smooth pursuit, reflexive, and memory-guided saccades and antisaccades between patients with early-stage PD and healthy controls. Methods: We conducted a cross-sectional study with idiopathic PD patients in early stage of disease (Hoehn and Yahr stage 0, 1 and 2) and healthy controls. The impairment of smooth pursuit, saccades, antisaccades, and memory-guided saccades was evaluated with eye-tracker analysis using a battery of tests. Results: Forty-two subjects with early-stage idiopathic PD and 50 healthy controls participated in the study. There were no statistically significant differences in age, gender, years of education, or cognition between the groups. Early-stage PD patients showed impairment in velocity, phase, and range of motion of smooth pursuit eye movements, as well as impaired precision and recollection performing visually guided memory saccades. Consequently, there is also a reading dysfunction, with slower reading speed and longer eye fixations. No significant differences were found regarding reflexive saccades and antisaccades between these two groups. Conclusion: Results suggest that impaired smooth pursuit movements, memory-guided saccades and reading functions are present in early-stage PD, even without other expressed motor symptoms. These findings could potentially contribute to the development of new and non-invasive diagnostic biomarkers in PD.