Optic nerve diameter in early-stage idiopathic Parkinson's disease: A prospective observational study.

Idiopathic Parkinson's disease (IPD) is a neurodegenerative disorder affecting both motor and non-motor pathways, including the visual system. Although retinal changes have been proposed as biomarkers of neurodegeneration, data on optic nerve involvement in early IPD are limited. This study aimed to compare optic nerve sheath diameter (ONSD) between patients with early-stage IPD and healthy individuals. In this prospective observational study, 100 patients with IPD and 50 healthy controls were recruited between June 2021 and November 2025. All participants underwent comprehensive ophthalmologic examination and B-scan ultrasonography. ONSD was measured 3 mm posterior to the globe using a standardized technique. Patients with ocular, orbital, or systemic conditions affecting the optic nerve were excluded. Disease severity was assessed using the Unified Parkinson's Disease Rating Scale Part III and the Hoehn-Yahr staging system. Statistical analyses were performed using non-parametric tests, with a significance level of P < .05. The median disease duration in the IPD group was 24 months, and most patients were classified as Hoehn-Yahr stage 1 or 2. There were no significant differences between patients with IPD and controls in right or left ONSD measurements. Subgroup analyses according to Hoehn-Yahr stage also showed no stage-dependent differences. Optic nerve sheath diameter appears preserved in patients with early-stage IPD and does not significantly differ from that of healthy controls. These findings suggest that ultrasonographic ONSD may have limited value as a structural biomarker in early disease stages. Further studies including advanced stages and multimodal imaging approaches are needed.