Retinal Architecture in Parkinson's Disease with Rapid Eye Movement Sleep Behaviour Disorder: Insights from a Scoping Review.

Background: Parkinson's disease (PD) is the leading age-related neurodegenerative disorder with a deposition of α-synuclein-containing Lewy bodies. Idiopathic REM sleep behaviour disorder (iRBD) can occur a decade prior to motor symptoms onset in PD. The retina acts as a window to the brain and its structural changes, along with RBD, may serve as a prodromal marker for PD. Summary: We evaluated the existing scientific evidence on structural retinal alterations in subjects with iRBD and PD with and without RBD. The selected four studies were observational and investigated the structural retinal layer thickness in iRBD patients, PD patients who likely had RBD (probable RBD), PD lacking RBD, and healthy individuals. Findings reported thinning of the retinal ganglion cell layer, nerve fibre layer (RNFL), outer and inner plexiform layers, inner and outer nuclear layers, reduced ganglion cell complex thickness, and peripapillary RNFL. Additionally, one study reported functional changes, including diminished contrast sensitivity and visual acuity in both the iRBD and PD groups. Key Message: This scoping review highlights significant thinning of retinal layers in RBD subjects in the context of PD. Retinal imaging serves as a biomarker in the early detection of neurodegeneration.