Untangling the neurovascular clues in the retinas of patients with Parkinson disease

(Image credit: AdobeStock/Jevgenij)

A study that investigated retinal neurovascular signatures in patients with Parkinson’s Disease (PD) found that some retinal parameters differed significantly compared with healthy controls,¹ according to Wenwen Zhu, MD, and colleagues. Zhu is from the Department of Ophthalmology, Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Miami, and the Department of Ophthalmology, Affiliated Hospital of Shandong Second Medical University, Weifang, Shandong, China.

In addition to the effects of PD on the brain, research has reported that PD also affects extracerebral structures such as the retina, which is an extension of the central nervous system and shares many anatomic and physiologic characteristics with the brain,^2,3 the investigators explained.

However, there currently is no consensus of what specific changes take place in the retinas of patients with PD. For example, they cited studies that reported significant retinal thinning in patients with PD, particularly in the retinal nerve fiber layer (RNFL) and ganglion cell-inner plexiform layer (GCIPL).^4-7 While other studies did not identify differences in retinal tissue volume (RTV) between PD and healthy controls,^8,9 and some reported increases in the total macular thickness or GCIPL thickness in patients with PD.^10,11 There also is no agreement among studies on the effects of PD on the retinal vascular density (RVD).^9,12-15 These findings are of particular interest, because retinal perfusion status has been linked to the properties of cerebral perfusion.^16,17

Other studies have reported decreases in cerebral blood flow in the frontal, parietal, and temporal lobes of patients with PD, alongside increased cerebral blood flow in the bilateral hippocampi, red nuclei, right substantia nigra, thalamus, and much of the cerebellum.¹⁸

What’s missing is the knowledge regarding the contribution of these abnormalities in brain perfusion to PD.^19-22 

The retina may hold the answers.

Because of these knowledge gaps, the authors cited that a robust understanding of retinal microcirculation in PD patients is necessary. “Although decreased RVD may indicate hypoperfusion in the retina in PD,^9,15,23-26 it is unknown how retinal blood flow (RBF) interacts with RVD and RTV in PD patients. Furthermore, no studies, to the best of our knowledge, discuss how these three variables may interact during neurodegenerative disease. Elucidating how these variables, and thus the retinovascular signature of PD, change will add to the growing body of literature that seeks to use retinal imaging to diagnose or stage PD,"^26-28 they commented.

They speculated that the retinovascular signature may provide a valuable noninvasive tool in future analysis of PD and reveal new pathological mechanisms underlying its characteristic neurodegeneration.

Study of neurovascular parameters

The study under discussion included 15 patients with clinically diagnosed PD and 18 age-matched healthy controls. Both groups underwent measurement of the RBF using a retinal function imager. Optical coherence tomography angiography was performed to assess the RVD, retinal vessel length density (RVLD), and corresponding RTV. Retinal tissue perfusion (RTP) was computed as the RBF normalized by RTV and retinal capillary function (RCF) as the RBF normalized by RVLD, the authors recounted.

Zhu and colleagues reported, “The patients with PD exhibited significantly lower RBF, RTP, and RCF than the healthy controls (P < 0.001). The RVD, RVLD, and RTV did not differ significantly between the groups (P > 0.05).”

They also found that the RBF, RVD, RVLD, and RTV did not share a linear relationship with one another (P > 0.05).

Zhu and colleagues concluded, “This study is the first to investigate retinal neurovascular signatures and provides novel insights into the interplay between retinal microstructure, microvasculature, and microcirculation in PD. Multimodal retinal imaging may offer a valuable window into the disease mechanism of PD. Future research is needed with larger, longitudinal studies to investigate whether interventions targeting vascular dysfunction could improve retinal and cerebral health in PD.”

References

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