Directional OCT provides biomarkers of Parkinson’s Disease in the outer retina

An Australian study¹ reported that directional optical coherence tomography (OCT) images showed that Parkinson’s disease (PD) is associated with a thicker photoreceptor nuclear layer and a thinner layer of photoreceptor processes. These changes in patients with PD may serve as viable biomarkers, according to Jingjing Lin, PhD, and colleagues from the University of Melbourne, Melbourne, Victoria, Australia. First author Lin is from the Department of Optometry and Vision Sciences of the university.

“OCT imaging as a screening biomarker for PD has garnered attention^2-4 for its accessibility and integration in the community. Moreover, increasing evidence of PD-related pathologies in the retina adds to the impetus to investigate OCT as a scalable biomarker for PD,^5,6 the investigators said.

While OCT has been used previously to measure the retinal thickness, different OCT scanning protocols have led to discrepancies in measurements among various studies.

Lujan et al.⁷ developed directional OCT to obtain more accurate measurements of the outer nuclear layer (ONL). The investigators explained that “by modifying the scan direction such that the incident beam is perpendicular to the Henle fibers,⁸ the optical interface between the Henle fiber layer (HFL) and the ONL can be better identified.⁹ This approach has been used for ONL quantification in healthy people,^10,11 as well as those with albinism¹² and macular disease.^8,13 Therefore, in contrast with the previous literature on PD, which has only used standard on-axis OCT scans to assay the presumed ONL thickness,^4,14,15 directional OCT may provide a more accurate quantification of ONL thickness in PD.”

PD study methodology

The study included 14 patients with diagnosed PD and 18 healthy age-matched controls.

The investigators measured the participants’ true ONL (composed of photoreceptor cell bodies) rather than the conventional presumed or standard ONL (ONL + HFL), which includes photoreceptor processes) using directional OCT scans by altering the incident beam angle. Combined with standard line scans to obtain the HFL, true ONL and HFL thicknesses and the ellipsoid zone (EZ) reflectivity were measured and compared between groups using a mixed-effects analysis, they explained.

What the directional OCT scans showed

Lin and colleagues reported, “Directional OCT revealed that the 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).”

They concluded that directional OCT shows that PD is associated with a thicker photoreceptor nuclear layer and a thinner layer of photoreceptor processes, both of which cannot be detected using standard OCT alone.

They also pointed out that the EZ band is rich in mitochondria, and the lower reflectivity seen in PD may indicate metabolic changes in PD. “These detailed examinations of the outer retina facilitate a deeper understanding of PD-related photoreceptor changes,” they commented.

References

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15. Tran K, Lee PY, Finkelstein DI, et al. Altered outer retinal structure, electrophysiology and visual perception in Parkinson's disease. J Parkinsons Dis. 2024;14:167–80.