Detailed Outer Retinal Assessment in Parkinson's Disease Using Directional OCT

This study aimed to compare outer retinal thickness and reflectivity using directional OCT between people living with Parkinson's disease (PD) and age-matched controls.

Fourteen neurologist-confirmed people living with PD and 18 age-matched healthy controls participated. To measure the true outer nuclear layer (ONL) (composed of photoreceptor cell bodies) rather than the conventional presumed or standard ONL (ONL plus Henle fiber layer [HFL], which includes photoreceptor processes), off-axis (directional) OCT scans were acquired by altering the incident beam angle. Combined with on-axis (standard) line scans to yield HFL, true ONL and HFL thicknesses as well as ellipsoid zone (EZ) reflectivity were measured and compared between groups using a mixed-effects analysis.

Directional OCT revealed that true ONL comprises 63% of the presumed ONL thickness. Compared with healthy controls, the PD group showed a thicker true ONL and a thinner HFL (P < 0.05), whereas the combined ONL and HFL thickness was not different (P > 0.05). The EZ band exhibited lower reflectance in PD participants than healthy controls at the fovea and parafoveal regions (P < 0.05).

Directional OCT reveals that people with PD have a thicker photoreceptor nuclear layer and a thinner layer of photoreceptor processes. These opposing effects are not detectable using standard on-axis OCT alone. The EZ band is rich in mitochondria, and a lower reflectivity in PD participants may be indicative of metabolic changes in PD. These detailed examinations of the outer retina facilitate a deeper understanding of PD-related photoreceptor changes.