The VD and FAZ area are OCTA parameters used for quantitative analysis of retinal microvasculature status . The importance of the VD and FAZ on DME in the SCP and DCP has been confirmed in many studies [14, 18, 19]. Al of these studies investigated the macula globally. In the present study, we investigated vascular status and the association with structural morphology of the macula in a sectoral manner. A 3 mm ETDRS macular grid evaluation of our study revealed that the VD was significantly lower in the superior and nasal quadrants in the SCP and the temporal quadrant of the DCP in DME eyes compared to the corresponding quadrants in healthy eyes. The FAZ area was significantly greater (both superficial and deep) in DME eyes than in control eyes. The sectoral analysis of the thicknesses according to 3 mm ETDRS grid sectors revealed that inferior, superior, nasal and temporal quadrant thicknesses were significantly greater in DME eyes. ELM disruptions detected in each quadrant were 6 eyes (23.1%) inferior, 5 eyes (19.2%) superior, 5 eyes (19.2%) nasal and 7 eyes (26.9%) in temporal scans. EZ disruptions detected in each quadrant were 11 eyes (42.3%) inferior, 11 eyes (42.3%) superior, 10 eyes (38.5%) nasal and 13 eyes (50%) in temporal scans in DME eyes.
In diabetic eyes without clinically detectable DR, the VD of both superficial and deep capillary plexuses was lower than that in healthy controls, and eyes with a worse DR stage showed larger FAZ areas and lower VD values [20, 21]. Moreover, the risk of DR progression was found to be associated with a larger FAZ area and lower VD on the DCP, whereas the risk of DME development was found to be associated with lower VD on the SCP [18, 22]. There is growing evidence that the DCP is affected earlier than the SCP in DR because of the terminal architecture of the deep capillaries [19, 23]. However, the rationality and priority of the SCP or DCP on the formation of the DME remain unclear [18, 19]. To contribute to understanding the pathogenesis of DME, we conducted sectoral analysis and found that the VD was significantly lower in the superior and nasal quadrants in the SCP and temporal quadrant of the DCP in DME eyes. The VDs in the SCP appeared to be affected in more quadrants, although the decrease in the temporal quadrant of the SCP did not reach significance because of the small sample size (P: 0.058). The importance of our results may be that the SCP was affected more in advanced DR with DME. The SCP includes larger vessels (arterioles and venules) beside the capillaries. Additionally, it was found that the diameter of large vessels steadily increased from diabetes without retinopathy to more severe stages of diabetic retinopathy in SCP . Vessel length and vessel caliber affect vessel density . This outcome may result in a limited decrease in the VD, although capillary dropout occurs in the SCP. We assumed that our DME eyes had advanced DR because almost all had a history of lfk and intravitreal injection. Our correlation analysis between the VD of the SCP and DCP revealed that there were no correlations in the corresponding quadrants. This result may indicate that the SCP and DCP are affected independently from each other. On the other hand, there is probably a compensatory relationship between the SCP and DCP; thus, there may be dependence . We suggest that in DR, the DCP may first be affected but compensated by a relatively normal SCP to prevent the formation of macular edema. However, if the decrease in the VD in the SCP exceeded the compensation of the DCP in advanced DR (involvement in more quadrants, such as our results), DME occurred. To the best of our knowledge, such a finding has not been mentioned before in the literature. Longitudinal studies are needed to confirm these results.
By the automated quantitative approach, VD calculations may confound because of cystoid spaces in the regions counted as nonperfused areas . Many researchers have excluded these areas in the images to calculate the VD, but according to the results of the present study, we suggest that cystoid spaces actually occur in vascular rarefaction areas. Moreover, some studies on VD measurements found that the VD did not change after treatment, although anatomical relief was observed [6, 28]. That finding supports our consideration. Nevertheless, pushed capillaries (vessel displacement) that are due to cystoid spaces may affect the calculation of the VD [29, 30]. In our DME eyes, the VD in any of the SCP quadrants was not affected by the presence of cysts in the corresponding quadrants of the SCP or DCP. The VD in any of the DCP quadrants was not affected by the presence of cysts in the corresponding quadrants of the DCP, but the presence of cysts in the nasal and temporal quadrants of the SCP affected the VD in the corresponding quadrants of the DCP. This result may be due to the distortion effect of cysts in the SCP on the capillaries of the DCP. We did not measure the actual area of the cysts because it requires different image processing, and we only evaluated the presence of the cysts in the corresponding quadrants. Future studies that include the measurement of the actual area of cysts may provide a better explanation for this effect. Mané et al. found that cystoid spaces were often located within areas of nonperfusion, and cysts were more visible and more numerous in the deeper retina . Consistent with that finding, the majority of our patients had cysts in the DCP, especially in the temporal quadrant, where there was a significant decrease in the VD, but the association could not be proven statistically. The actual area occupied by cysts may be meaningful for that association.
When considering correlations between the VD in quadrants and sectoral retinal thicknesses in DME eyes, all VDs in the DCP were negatively correlated with thicknesses, but only the nasal quadrant VD in the SCP was positively correlated with nasal thickness when the corresponding quadrants were evaluated. These findings may reflect that edematous thickening is especially linked with a decrease in the VD of the DCP, but we cannot say that there is the same link between the VD of the SCP and thickening because of the possible compensatory changes in the SCP, as mentioned above.
Enlargement of FAZ areas at the level of both the SCP and DCP in DR and diabetic maculopathy has been proven in many studies, consistent with our study [5, 21]. In terms of the correlation between the FAZ area and sectoral VDs, there were no associations in the SCP, but the nasal and temporal quadrants of the DCP VDs were negatively correlated with the FAZ area of the DCP in the present study. A possible explanation for this finding may be that the FAZ area of the SCP increases because of edematous distortion. However, the increase in the FAZ area of the DCP may be due to capillary dropout, especially in the nasal and temporal quadrants of the DCP, in our patients. It is supposed that the accumulation of fluid in the macula causes distortion and enlargement of the FAZ area [23, 31]. Normally, in the inner retinal layers, interstitial fluid is strictly controlled by the retinal capillary plexus and Müller cells. When fluid production exceeds fluid reabsorption, macular edema forms . Excessive extravasation of fluid from the vascular bed to the interstitial space may cause the formation of cystoid spaces and tissue distortion . We could not find any association between sectoral thicknesses and the FAZ area of the SCP, but the FAZ area of the DCP was correlated positively with both quadrants and central thicknesses. In our DME eyes, the FAZ area of the SCP was not affected by the presence of cysts in any quadrants of the SCP or DCP. The FAZ area of the DCP was not affected by the presence of cysts in any quadrant of the SCP but was affected by the presence of cysts only in the superior and nasal quadrants of the DCP. Again, this result may be due to the distortion effect of cysts on the capillaries in the DCP but nevertheless probably because of capillary dropout. We observed that the cysts in the DCP had greater dimensions, so a distortion effect of the cysts may be evident in the DCP. The expansion of the cystoid spaces may cause disruption of the wall of the cysts that contain Müller cells. Müller cells have important structural and functional duties. Müller cells that span from the internal limiting membrane (ILM) to the ELM provide the scaffold of the retinal layers. Müller cells wrap the retinal capillaries and have a tight relationship with the vascular bed functionally and morphologically. Therefore, it seems that the association between the presence of cysts and vascular status is a vicious circle.
Moon et al. showed the importance of the VD and FAZ areas of the DCP on the ELM and EZ . Consistent with these findings, we found that ELM disruptions were not affected by the corresponding VDs in the SCP, but nasal and temporal quadrant ELM disruptions were affected by VDs in the DCP in the corresponding quadrants. ELM disruptions in each quadrant were not affected by the FAZ area in the SCP but were affected by the FAZ area in the DCP. In our sectoral analysis, we could not find any association between EZ disruptions and the VD of either the SCP or DCP when the corresponding quadrants were evaluated. Again, we did not find any association between EZ disruptions and the FAZ area of either the SCP or DCP. All these results may confirm that ELM is primarily affected by the DCP, especially in the nasal and temporal quadrants, but that cannot be said for EZ. The presence of EZ disruption was approximately two times more frequent according to ELM disruption in our subjects. In a recent study, Lei et al. reported that EZ disruptions were found in 40% of diabetic eyes . Our sectoral results were comparable, but their subjects had 57% DME, while all of eyes in our study group had DME. Therefore, their results are relatively higher according to ours. That may be due to devices used. They used SD-OCT; however, we used SS-OCT, which was confirmed to have better deep resolution. Some researchers suggested that outer retinal disruptions were correlated with SCP changes, while some reported DCP partially contributed [13, 14, 24]. DCP provides approximately 15% of the oxygen supply of the photoreceptors, while the outer one-third of the retina is mainly supplied by the choriocapillaris . On the other hand, Borrelli et al. reported an association between the EZ and the choriocapillaris . Therefore, the EZ may be mainly dependent on the choriocapillaris rather than inner retinal capillary plexuses. Evaluation of the choriocapillaris was not within the scope of this study, and further studies are warranted to obtain more detailed information about the association between the EZ and choriocapillaris.
There are some limitations in this study. 1) This was a retrospective cross-sectional study, which may have caused ascertainment bias. 2) The small sample size was due to the strict inclusion and exclusion criteria; poor quality of the images and the presence of epiretinal membrane were the most common exclusion reasons. 3) The inclusion of both patients with differing previous treatments and treatment-naive DME patients with different durations. 4) We performed all measurements on 6x6 mm scans because we imaged all patients in a standard manner with a 6x6 mm scan due to automated sectoral calculations in our clinical settings. If the 3x3 mm scans give automated sectoral analysis, they may provide better measurements . On the other hand, the suspended scattering particles in motion (SSPiM) may cause an overestimation of DCP vessel density in eyes with DME when 3-mm OCTA scans are used for analysis, but that effect may be less when 6-mm scans are used . For these reasons, we used a 6 mm scan as the standard in all our patients, even for 3 mm area calculations. 5) The quantification structural analysis and the area of the FAZ may be affected by subjective judgment. For these reasons, all image processing was performed by the same person to reduce the interrater bias. It was stated previously that age and sex were correlated with both FAZ and VD of the SCP and DCP layer in both diabetic and healthy subjects . We created the control group with the same age and sex distribution to eliminate these effects.
In conclusion, The VDs in the SCP appeared to be affected in different quadrants according to the DCP in advanced DR with DME. There were no correlations in the corresponding quadrants between the SCP and DCP in terms of VD. This result may indicate that the SCP was affected in more quadrants than the DCP and probably independently from each other. There was no correlation between the FAZ area and sectoral VDs in the SCP, but the nasal and temporal quadrants of the DCP VDs were negatively correlated with the FAZ area of the DCP. The presence of cysts may not be related to the vascular status of the SCP, probably due to large vessels in the SCP, but may be partially related to the vascular status of the DCP. ELM disruptions were associated with the vascular status of DCP but not SCP. EZ disruptions were not associated with the vascular status of the SCP and DCP. These results may confirm that the ELM is primarily affected by DCP, especially in the nasal and temporal quadrants, but that cannot be said for the EZ. Longitudinal studies are needed.