Using directional OCT in 14 PD patients versus 18 controls, the study separated true ONL from Henle fiber layer and found a thicker photoreceptor nuclear layer, a thinner photoreceptor process layer, and reduced ellipsoid zone reflectivity in PD, changes not detectable with standard OCT.

By revealing photoreceptor structural shifts and lower mitochondria-rich EZ reflectivity, this noninvasive imaging approach offers a potential biomarker for metabolic/mitochondrial involvement in PD that could support patient stratification, monitoring, and therapeutic hypotheses targeting…

PURPOSE: 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. METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.