Seeing Through the Fog: Visual Symptoms in Parkinson?s Disease and Dementia

Introduction: Non - motor symptoms such as dementia and visual hallucinations are critical determinants of long - term outcomes and quality of life in Parkinson's disease (PD). This study was motivated by the need to better understand these symptoms, particularly the visual disturbances associated with PD and Parkinson's disease dementia (PDD). Aim: A central aim of this study was to characterise the spectrum of visual symptoms experienced by individuals with PD and PDD. In addition to complex visual hallucinations already recognised for their prognostic significance we investigated a broader range of phenomena, including illusory misperceptions, presence and passage hallucinations, and diplopia (double vision). Another major objective was to define key metrics of visual exploration strategies during visuocognitive tasks, and to examine their relationship with cognition, visual symptoms, and motor function. We also evaluated the usefulness of retina - specific assessments in exploring the potential contribution of retinal dysfunction to visual impairment in PD. Result: Our findings indicate that not all visual symptoms share a common pathophysiological mechanism. We propose that hallucinations should be categorised into distinct phenomenological groups to improve understanding of their causes and predictive value in future longitudinal research. Additionally, we found that patients with perceptual difficulties exhibited significantly less efficient visual exploration strategies, highlighting the interplay between cognitive deficits and eye movement patterns in PD. Retinal structure, as measured by optical coherence tomography (OCT), did not show significant alterations in PD, suggesting that caution is needed before using OCT as a biomarker for the disease. Nevertheless, our neurophysiological findings point to the retina as a potential origin of diminished visual acuity in PD, despite no marked differences in central or peripheral retinal responses between PD patients and controls.