Can retinal nerve fiber thickness be A biomarker for clinical stage and cognitive destruction in parkinson's disease?

Aim: It was aimed to investigate the possible relationship between retinal nerve fiber thickness and disease stage, disease symptom severity, quality of life and degree of cognitive impairment in idiopathic Parkinson's disease (IPD), and to examine whether optical coherence tomography can be an early indicator of the clinical stage of the disease and cognitive impairment. Material and method: The study was followed by 40 patients with a diagnosis of IPD from the Neurology Department of Zonguldak Bülent Ecevit University Faculty of Medicine, and 24 control groups with appropriate characteristics. After screening, it was continued with 34 patients and 21 controls. Participants' demographic characteristics, hand dominance, comorbid disease history, and medications were recorded. In the clinical evaluation of the patient group; Movement Disorder Society‐ Unified Parkinson's Disease Rating Scale, Hoehn and Yahr grading were used. To evaluate cognitive functions in the patient and healthy group; Mini-Mental State Examination, Addenbrooke's Cognitive Examination Battery-Revised (ACE-R) and Digit Span Test were used. Quality of life was evaluated with the World Health Organization Quality of Life Scale, depression status was evaluated with the Geriatric Depression Scale-15, and excessive daytime sleepiness was evaluated with the Epworth Sleepiness Scale. Retinal nerve fiber layer (RNFL) thickness was evaluated by optical coherence tomography (OCT). Results: There was a significant thinning in the inferior and nasal quadrant RNFL thickness; significant decrease in memory, fluency, and language scores in the patient group. Total ACE-R, attention, memory, and fluency score were found to associated with the right nasal fibers. Language functions were found to be related to the right temporal fibers. As a different scale, a positive correlation was found between the N/T index (index obtained by nasal and temporal retinal fiber ratio), which is one of the OCT findings, and attention from cognitive functions. Age was found to be a factor affecting RNFL and cognitive functions. Although there was no significant relationship between the interocular RNFL difference and cognitive impairment, this difference was more significant in patients with right-sided dominant parkinsonian symptoms. Conclusions: Optical coherence tomography can be used to show and monitor neurodegeneration and multisystemic involvement in IPD. Age can be a confounding and limiting factor in cognitive and retinal assessments. Cognitive involvement and OCT findings may be correlated. Our study is specific in terms of examining the relationship between the side dominance of the findings, interocular difference and cognitive evaluation. It is thought that the neurodegenerative process may be related to the dominant hemisphere. However, prospective, longitudinal, and large-scale studies should be performed for more detailed evaluation. Keywords: Optical coherence tomography, cognition, Parkinson's disease, retinal nerve fiber thickness