Glaucoma is a neurodegenerative disease of the visual system and leading cause of irreversible blindness worldwide (1). Primary open-angle glaucoma (POAG), as the most prevalent form of glaucoma, is described as a multifactorial optic neuropathy that is chronic and progressive with a characteristic acquired loss of optic nerve fibers. In addition to increased intraocular pressure, many factors are involved in optic nerve damage, such as insufficient blood supply to the eye, vascular dysfunction, and breakdown of neurovascular coupling in glaucoma. Despite intense study, the pathogenesis of POAG is still not completely understood. There is also an urgent need for clinical discovery of new biomarkers and critical targets for early diagnosis and early treatment.

Platelets are anucleate cells that circulate in blood and are critical for the body's response to vascular injury, as well as preventing bleeding; emerging evidence suggests that platelet plays essential functions in several neurodegenerative conditions (2), including glaucoma, Parkinson's disease (PD), and Alzheimer's disease (AD). Platelet has been demonstrated to be involved in the pathomechanisms of glaucoma through animal model and clinical evidence (3–5). Williams et al. (3) confirmed the monocyte-platelet interactions in the optic nerve head and proposed targeting platelet aggregation and activation in glaucoma treatment. Nishijima et al. (4) observed that platelets actively interacted with retinal endothelial cells in the post-ischemic retina through P-selectin. Some observed an increased platelet aggregation in glaucoma, which negatively influences blood flow in the small branches of the short ciliary arteries supplying the optic disk (5). It is also vital to note that platelets show high expression of several proteins associated with the development of glaucoma and AD, such as the amyloid precursor protein (APP) and tau protein (6). Together this, it makes it interesting to consider the contribution of platelets to the pathomechanism of neurodegeneration.

Primary open-angle glaucoma is characterized by progressive loss of retinal ganglion cells and typical glaucomatous visual field defects. Retinal nerve fiber layer (RNFL) around the optic nerve head and in the macular region can be reproducible quantitative measured by optical coherence tomography (OCT). Our previous study reported that mean platelet distribution width (PDW) and mean platelet volume (MPV), two of which are platelet parameters implying platelet function and activation, were significantly elevated in patients with POAG and were positively associated with visual field mean deviation (MD) (7). However, whether platelet parameters are associated with retinal thickness, which represent structural change in glaucoma, is still unknown. We performed the following retrospective study to explore the possible correlation between platelet parameters and RNFL, ganglion cell complex (GCC) thickness, and cup/disk area ratio, evaluated by OCT in POAG.

This study was approved by the Ethics Review Committee of Eye, Ear, Nose, and Throat (EENT) Hospital, Fudan University. The design and implementation of this study adhered to the tenets of the Declaration of Helsinki. A written informed consent was obtained from all the patients.

Previously, we explored the relationship between platelet parameters and functional changes evaluated by visual field defects in POAG (7). In this study, further study on platelet and structural loss of retinal thickness found close correlations between PDW, MPV RNFL, and GCC thickness. In view of the fact that MPV or PDW reflects changes in platelet morphological and functional status, our findings provide clinical evidence to prove that platelet activation may contribute to glaucomatous optic neuropathy in POAG.

Platelet has been implicated in the pathogenesis of glaucoma through animal model and clinical evidence. Animal models have demonstrated that monocyte-platelet aggregates into the optic nerve head (ONH) are involved in early glaucomatous damage and that inhibition of platelet adhesion to the vessel wall can provide neuroprotection in glaucoma (3). Nishijima et al. (4) observed that platelets interacted actively with retinal endothelial cells in the post-ischemic retina through P-selectin expressed on the retinal endothelial cells. Several clinical studies found increased platelet aggregation (5, 11) and elevated PDW and MPV levels in POAG (7), indicating a prothrombotic state. Maric et al. (12) found higher MPV levels and nailfold capillary morphological vascular changes in patients with exfoliative glaucoma. Antiplatelet effect of antiglaucomatous eye drops was reported (13). In a word, altered platelet aggregability may trigger microthrombosis of the retinal capillaries and the short ciliary arteries, which result in vascular damage and generate defects in the microcirculation of the optic nerve head.

This study, using SD-OCT, an effective and reliable tool for monitoring structural changes of the retina, provided the information that the morphology and function of the platelet may be involved in the loss of retinal ganglion cells (RGCs). The evaluation of retinal thickness is as important as visual field testing for the evaluation of patients with glaucoma. It has been reported that RNFL loss of about 40 to 60% can occur before an appearance of visual field defect (14). In this study, structural assessments by OCT identified significant loss of disk rim area, average GCC thickness, and average RNFL thickness, in addition to elevated PDW and MPV levels, as the severity of POAG increased. Moreover, the Spearman's rank correlation analyses revealed significantly negative correlations between PDW, MPV levels, RNFL, and GCC thickness, in addition to positive correlations between PDW, MPV levels, and cup/disk area ratio.

Mean platelet volume and PDW are widely used for measuring the function and activation of platelet in the blood (15). To be specific, MPV is a marker of average platelet size and PDW represents the heterogeneity of platelet size. Increased MPV and PDW levels indicate more active and larger platelets that release more thromboxane A2 and aggregate more readily in vitro (16). In this study, significantly higher MPV and PDW levels and their correlation with visual field loss were found in POAG, which is consistent with our previous study (7). More importantly, we investigate the association between structural changes and platelet parameters. We found that PDW and MPV levels increased significantly as the RNFL and GCC thickness getting thinner and cup/disk area ratio getting larger, evaluated by OCT. To further explore the association between platelet parameters and retinal thickness, temporal, superior, nasal, and inferior quadrants of RNFL thickness were analyzed. All the four quadrants of RNFL thickness were found to be negatively associated with PDW and MPV, with the superior quadrant showing the strongest correlations with platelet parameters. These results demonstrated the agreement between platelet parameters and structural (OCT) and functional (VF) glaucomatous damage in POAG.

Figure 3 shows it directly that patients with POAG with higher PDW and MPV levels had significantly thinner RNFL and GCC thickness, as well as enlarged cup/disk area ratio, which suggested that platelet activation may contribute to the loss of RGCs and change of ONH in glaucoma. Several studies have reported the role of platelet in neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, multiple sclerosis, and Huntington's disease (17). Platelets have the ability to rapidly adhere to the endothelium cells, become activated and secrete bioactive mediators, thus contributing to blood–brain barrier permeability, allowing the entry of white blood cells and producing cerebrovascular inflammation, which plays significant roles in neurons loss in neurodegeneration (18).

As age is known to be associated with RNFL loss (19), this was overcome by considering age as a covariate in our linear regression model. The multiple linear regression analysis further confirmed that RNFL and GCC thicknesses were independently associated with platelet parameters such as PDW and MPV (P < 0.05) in patients with POAG after adjusting for age, gender, body mass index, and intraocular pressure. Thus, measurement of platelet parameters may be a useful tool to predict structural damage in POAG, which is expected to be a simple, convenient, and efficient approach to supplement ophthalmic examination in glaucoma. Moreover, considering that platelet activation appears to be important to glaucomatous damage, targeting the regulation of platelet function may offer novel therapeutic interventions in glaucoma, especially in POAG.

It is increasingly accepted that the pathogenesis of glaucoma is multifactorial because of the fact that glaucomatous optic neuropathy occurs at all the levels of IOP and that patients progress regardless of interventions to regulate IOP. The exact mechanism of glaucomatous damage due to platelet hyperaggregation is not clear. It has been speculated that microcirculatory defects, such as vasoconstriction and retention of blood, damage endothelial cells, leading to subendothelial collagen exposure, which triggers platelet aggregation, subsequently causing the ischemic injury of the optic nerve (5, 20). Here is another question. What may cause the hyperresponsiveness of platelets? A possible cause of platelet hyperreactivity in patients with glaucoma may involve the pigment epithelium-derived factor (PEDF), which is a multifunctional secreted protein that mediates neuroprotection and inhibition of angiogenesis in the retina (21). PEDF is significantly reduced in glaucomatous eyes (22), which may lead to increased platelet aggregability and the establishment of a thrombogenic state.

Last but not least, platelet aggregates may promote an elevated IOP by blocking the physiological pores of Schlemm's canal. Platelet is fundamental in the function of the inner wall of Schlemm's canal (23). Disruption or alteration in platelets' function may result in extensive pore occlusion and decreased aqueous humor drainage (24).

The strengths of this study include the following points: (1) the first study for systematic evaluation on the relationship between platelet parameters and the structural, functional changes in POAG; (2) the first study to demonstrate directly that platelet parameters' close relationships with the thickness of the peripapillary retinal nerve fiber layer and macular ganglion cell complex, using OCT as a clinically useful and reliable tool; and (3) the strict inclusion criteria for this study and the detailed and comprehensive clinical examination performed by glaucoma specialists for all the enrolled subjects. Our results are expected to bring new inspiration for the study of biomarkers and novel targets in glaucoma.

Although this study was the first study to investigate the relationship between platelet parameters and the structural, functional changes in POAG, we acknowledge that this study had some limitations. First, this was a single-center, retrospective study involving a relatively small sample size due to the strict inclusion criteria. Further studies with larger sample sizes and multicenter settings are required to further confirm our findings. Second, due to the inherent deficiencies of retrospective methods, we cannot judge the association between platelet parameters and disease progression. This question is worth further exploration based on the prospective approach. Third, antiglaucoma therapy has been shown to prevent or delay the development of retinal ganglion cell damage. Although all the subjects received antiglaucoma therapy in this study, this study did not collect detailed information on medicine usage for data analysis. Further study will be suggested to focus on the mechanism of platelet activation in glaucomatous optic neuropathy through animal models or pathological experiments.

In conclusion, this study found significantly negative association between PDW, MPV levels and RNFL, GCC thickness, as well as positive association between PDW, MPV levels, and cup/disk area ratio in patients with POAG. Our results suggested that platelet activation may contribute to glaucomatous optic neuropathy.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by the Ethics Review Committee of Eye and Ear, Nose, Throat Hospital (EENT), Fudan University. The patients/participants provided their written informed consent to participate in this study.

YM and SL performed the statistical analysis, drafted this article, and interpreted the data. MS contributed to data collection, literature search, and statistical analysis. XS critically revised this article. SL and WC were involved in the design of this study and supervised this project. All authors contributed to the article and approved the submitted version.

This work was supported by the Excellent Physician—Excellent Clinical Researcher Plan (SYA202004), the Shanghai Municipal Commission of Health and Family Planning (201840050), the State Key Program of National Natural Science Foundation of China (81430007), the subject of major projects of National Natural Science Foundation of China (81790641), the Shanghai Committee of Science and Technology of China (17410712500), and Shanghai Science and Technology Committee Foundation Grant (19411964600).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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