In this study, we used OCTA to evaluate the macular vessel density in superficial and deep capillary plexuses in patients with MS and NMOSD. We found that SCP and GCC thickness were significantly lower in ON eyes of MS and NMOSD patients than in the controls, and the parameters strongly correlated to each other. Notably, such association was also observed in MS-ON eyes, indicating that former ON is not obligatory to cause ganglion cell loss and SCP reduction in this group.
In MS patients, the SCP's blood perfusion was more decreased in eyes with previous ON than in non-ON eyes. On the contrary, the DCP did not differ between ON + and ON- eyes but had significantly higher vessel density than controls. Although the DCP did not correlate with GCC thickness, the negative trend could be easily observed (Fig. 2a,b). Moreover, the discrepancy between SCP and DCP was strongly negatively correlated with GGC thickness and was significantly higher in ON eyes.
Nesper et al. reported that retinal vessels could actively adapt to the metabolic demand of retinal cells. The coupling mechanism allows for regulation of the retinal blood flow between plexuses, and, e.g., the higher DCP perfusion may result from vessel dilation or increased velocity of flow20. According to these observations, the relationship between SCP and DCP in MS eyes can be explained. SCP and DCP, respectively, supply the ganglion cell layer and inner nuclear layer. It was shown that after optic neuritis, the GCL becomes atrophic, whereas the INL remains unchanged, or its volume increases21,22. It was consistent with our findings that the reduced vessel density was observed only in SCP because of the lower metabolic demand of injured ganglion cells. Additionally, the redistribution of blood between plexuses through vertical anastomoses resulted in an increased density of DCP. The results of our study nicely demonstrate that the reduction of vessel density in SCP is secondary to ganglion cell loss in MS + ON and MS-ON eyes.
In contrary to MS-ON group, the NMOSD-ON eyes had comparable SCP, DCP, and GCC parameters to controls. In the studies of Huang et al. and Chen et al., the vessel density was measured only in SCP, and in NMOSD + ON and NMOSD-ON patients, they found it lower than in healthy eyes13,16. Kwapong et al. presented that, besides the reduced density of SCP, the density of DCP was also reduced in ON and non-ON eyes15. Moreover, the authors suggested that it might be evidence for subclinical primary retinal vasculopathy13,15. In our study, the only significant difference in vessel density was noted in SCP between NMOSD + ON eyes and controls. In accordance with other studies, the GCC thickness was significantly lower in ON eyes than in healthy eyes, and in non-ON eyes – was similar to controls13,16.
Comparing the features of MS + ON and NMOSD + ON patients, we made an interesting observation. Although the SCP and GCC thickness parameters are comparable (Table 3), the groups significantly differ in DCP, which is also well seen on the plots (Fig. 2a,d). While the DCP of MS + ON eyes tends to increase with the ganglion cell loss, in NMOSD + ON eyes, it tends to decrease. It shows that the blood flow distribution pattern between plexuses in NMOSD + ON is different from this previously described in MS. The increasing trend of DCP with GCC thickness loss is also observed in controls (Fig. 2c), and we may assume that the typical network of healthy vessels can adapt and act this way. Thus, the reduction of vessel density in DCP indicates the capillary loss in NMOSD + ON eyes. Although statistically insignificant, a weak tendency of DCP reduction is observed in NMOSD-ON eyes, suggesting that vascular loss may appear prior to optic neuritis, as Huang et al. and Kwapong et al. reported13,15. Inconsistent results were published on hemodynamics in the optic neuritis eye vasculature using ultrasound examination23,24. There are reports available on the effects of the upregulation of Th17 cells, that transforming growth factor-beta, which can cause myointimal fibrosis in NMO patients25. Moreover, the vascular reactivity, e.g., effects of nitric oxide, should be considered during the interpretation of results, however, no disturbances in cerebrovascular reactivity in MS patients were recently reported26.
The retinal capillary network evaluation showed the difference between superficial and deep capillary plexuses in MS and NMOSD patients. The compensatory vascular mechanism of DCP was only seen in MS eyes, indicating that vessel density of DCP in NMOSD patients was reduced. We think that the explanation may be found in the distinct pathogenic mechanism of NMOSD. Retinal capillaries of SCP and DCP are ensheathed by macroglial cells, i.e., astrocytes and Müller cells, contributing to the formation and maintenance of the inner blood-retinal barrier. While the astrocytes' bodies and processes are found exclusively in the nerve fiber layer, Müller cells' bodies are located in the inner nuclear layer, and they project processes through entire retinal thickness27. The high density of aquaporin-4 expressed on these cells is targeted by disease-specific IgG under inflammatory conditions. The T cells get access to the retina from SCP and DCP and open the BRB for the AQP4-IgG and complement. It was experimentally shown on an animal model that retinal damage may appear independently of optic neuritis28,29.
The limitation of our study is a small group of NMOSD patients. However, at the beginning of study enrollment, we had to exclude some patients who could not undergo ophthalmic examination because of visual and physical disability. Therefore, further studies with larger cohorts are necessary to confirm our observations.
In conclusion, we demonstrated for the first time that in the eyes of MS patients, the vascular changes are secondary to the decreased metabolic demand of atrophied ganglion cell layer, while in the eyes of NMOSD patients, the vasculopathy seems to be a primary process.