**4. Discussion**

We studied the CVI changes across the entire depth of the choroid in both eyes of patients affected by unilateral CSC. In the CSC eyes, the CVI increased as the distance from RPE increased to reach a peak (0.500) in the medium depth of choroid and then reduced towards the CSI (0.495). On the contrary, the mean CVI of fellow eyes tended to reduce from RPE to the CSI (0.501; 0.493; 0.467). The control group showed a different trend, with the lowest average vascular density in the medium layer (0.506; 0.492; 0.506).

Previous studies analyzed the CVI changes in healthy eyes, showing the highest average vascular density in the outer level or Haller's layer [19–22]. Sohrab et al. analyzed only three choroidal sections of en face scans and calculated the vessel density on the basis of a preselected threshold of red, green and blue (RGB) intensity. The authors showed a different average vascular density in choriocapillaris (76.5%), Sattler's layer (83.6%) and Haller's layer (87.2%) [19].

In another study with a cohort of 30 healthy eyes, the CVI values were 53.16%, 51.38% and 55.69%, respectively, at the level of choriocapillaris, medium choroidal vessel and large choroidal vessel layers [22].

The en face CVI of patients affected by acute or chronic CSC was noted to increase as the distance from Bruch's membrane increased. Patients with acute CSC had the point of maximum vascularity (48.35% ± 2.06%) at 75% depth of CT, while those with chronic CSC reached the peak vascularity level at 50% of the choroidal depth, with a CVI of 48.70% ± 1.32% [23].

In our cohort, the variation in CVI between choriocapillaris, superficial, medium and deep level of the choroid were not significant for both eyes. These results are in contrast to those previously reported [23]; a possible reason is the different choroidal segmentation method applied. Indeed, Wong et al. compared choriocapillaris and various choroidal depths of CVI (25%, 33%, 50%, and 75%). Moreover, we have observed no significant difference between the whole and various CVI layers of CSC and fellow eyes, which suggests a similar vascular architecture in both affected and fellow eyes. This could support the theory of a bilateral involvement of CSC, previously revealed by many studies [3–6,8,28–31] and found to increase with a longer follow-up [3–6].

Another aspect to consider is that CSC, as a pachychoroid condition [32], is characterized by an increase in the size of Haller's vessels, which may compress the inner layers

and determine a similar vascularity throughout the CT [32,33]. In fact, in severe cases the choriocapillaris and intermediate caliber vessels could be so attenuated that the Haller's layer would occupy a significant proportion of CT [32].

Choroidal thickness analysis suggests that choroidal thickness in eyes with CSC is larger than that in age-matched control eyes and fellow eyes [30,31,34,35]. Considering the multiple factors that could influence the choroidal thickness (age, axial length, refractive error, blood pressure, time of the day), there is no definitive threshold for defining an eye as having pachychoroid [32]. Nevertheless, according to a previous study that considered 395 μm as a sensitive value to diagnose the "pachychoroid" disease, subfoveal CTs of affected and fellow eyes were increased [36].

Interestingly, our study shows how the choroid plays an important role in the pathogenesis of CSC but that it is not the only player. In fact, other than the similar vasculature and the pathological choroidal thickness, there were no signs of CSC in the fellow eye group. In this respect, it is recognized that other factors, such as the RPE, could play defensive roles against high choroidal hydrostatic pressure [37].

The strength of our study is that we provided a measure in vivo of the vascularity across the depth of the choroid, showing some similarities and differences between study eyes and fellow eyes of patients affected by CSC. The study had several weaknesses. First, was the small sample size; indeed, the strict criteria for unilaterality of the disease led to the exclusion of many cases. Considering this limitation, the research should be considered as just a preliminary study that could not provide any definite conclusion. Second, the single time measuring of CVI does not take account of the choroidal variations based on blood pressure and time of day [15]. Lastly, two further limitations arise from the arbitrary cut-offs in identifying the choriocapillaris and the manual identification of the other points of measurement.

#### **5. Conclusions**

In this preliminary study, the en face CVI of both eyes of patients affected by CSC showed no difference between affected and fellow eyes. The trend of changes in CVI for CSC and fellow eyes showed no statistical difference in the choroidal layer comparison. On the contrary, healthy eyes showed a significant difference in CVI across the depth of the choroid.

**Author Contributions:** Conceptualization, C.I., J.C. and E.P.; data curation, O.B., K.K.V. and M.A.R.; formal analysis, M.P. and F.T.; investigation, F.T., G.D., E.S.P., O.B., K.K.V. and M.A.R.; methodology, F.T., C.I. and J.C.; writing—original draft, F.T. and E.P.; writing—review and editing, C.I., G.G., J.C. and E.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of Comitato Etico Indipendente—Azienda Ospedaliero Universitaria di Cagliari (NP/2022/3119).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patients to publish this paper.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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