Is there any correlation between alpha-synuclein levels in tears and retinal layer thickness in Parkinson's disease?

Purpose To determine the total alpha-synuclein (alpha Syn) reflex tears and its association with retinal layers thickness in Parkinson's disease (PD). Methods Fifty-two eyes of 26 PD subjects and 52 eyes of age-and sex-matched healthy controls were included. Total alpha Syn in reflex tears was quantified using a human total alpha Syn enzyme-linked immunosorbent assay (ELISA) kit. The retinal thickness was evaluated with spectral-domain optical coherence tomography. The Movement Disorder Society-Unified Parkinsons Disease Rating Scale (MDS-UPDRS), Non-Motor Symptoms Scale (NMSS), and Montreal Cognitive Assessment (MoCA) were used to assess motor, non-motor, and cognition. Results In PD, total alpha Syn levels were increased compared to control subjects [1.76pg/mL (IQR 1.74-1.80) vs 1.73pg/mL (IQR 1.70-1.77), p < 0.004]. The nerve fiber layer, ganglion cell layer, internal plexiform layer, inner nuclear layer, and outer nuclear layer were thinner in PD in comparison with controls (p < 0.05). The outer plexiform layer and retinal pigment epithelium were thicker in PD (p < 0.05). The total alpha Syn levels positively correlated with the central volume of the inner nuclear layer (r = 0.357, p = 0.009). Conclusion Total alpha Syn reflex tear levels were increased in subjects with PD compared to controls. PD patients showed significant thinning of the inner retinal layers and thickening of outer retinal layers in comparison with controls. Total alpha Syn levels positively correlate with the central volume of the inner nuclear layer in PD. The combination of these biomarkers might have a possible role as a diagnostic tool in PD subjects.