The SCP is observed as a defined silhouette morphology with a linear and continuous white shape against a black background in the OCTA image, which is lying in the retinal nerve fiber layer (RNFL) and the ganglion cell layer (GCL). The DCP is shown as a regular distribution around the FAZ with more complex multiple tiny radial and horizontal interconnections lying in the boundary plane between the inner nuclear layer (INL) and the OPL. Such differences in the location, size, and morphology between SCP and DCP may cause different impacts by various factors between them. We tried to analyze this using the SVD/DVD ratio in normal eyes and found that age, the BCVA, the axial length, and OCTA quality were significantly correlated with the SVD/DVD ratio.
Previous studies reported a retinal microvascular decrease in eyes with high myopia, which would result from the elongation of eyeballs.[9–11] In our study, the axial length was positively correlated with the SVD/DVD ratio, which could mean that the longer the axial length, the greater the DVD reduction than the SVD reduction. The DCP may be more susceptible to damage caused by axial elongation than the SCP. You et al. reported that longer axial length was significantly associated with the lower DVD, not with the lower SVD in their multivariate analyses, which is consistent with our study. This result would be associated with a previous study about the retinal layer thickness of high myopia. Kim et al. reported that in high myopia, both the inner and outer rings of the ETDRS grid were thinned in INL (where the DCP is located), but only the outer ring was thinned in the inner retinal layer including GCL and IPL (where the SCP is located), indicating that the range of the thinned INL was wider than tha of the thinned inner retinal layer. The density of the smaller vessels in the DCP is greater than that in the SCP, and these smaller vessels may be more vulnerable to elongation damage, potentially leading to a greater decreas of the DVD than SVD compared to normal eyes.[8 14] Further studies including more range of axial length including ≥ 26.0 mm are needed to confirm this hypothesis.
When analyzing OCTA images, OCTA quality is crucial, which ranges from 0 (no signal) to 40 dB (excellent quality) and is considered good if the value is between 15 and 25 dB. Similar concepts are signal strength or signal strength index in other devices. Previous studies found that these values affected the measurement of VD and its reliability.[1 2 12 15–17] You et al. reported that a lower SVD and DVD were significantly associated with a lower signal strength index, and it was the strongest impact on the measured retinal VD. However, to the best of our knowledge, no study has explored whether the SVD or DVD is more affected by quality values. We found that the SVD/DVD ratio was positively correlated with OCTA quality, indicating that the SVD is more directly affected by OCTA quality than the DVD. Although the exact mechanism is unclear, OCTA quality should be considered when comparing retinal VDs, especially of SCP.
Yu et al. found that the parafoveal flow index and vessel area density decrease with increasing age at a rate of 0.6% and 0.4% per year. Jo et al. also reported that age substantially affected VD in most of the peripapillary and macular areas. In our study, the older group showed a lower SVD and DVD, which is consistent with previous studies. Additionally, the SVD/DVD ratio was significantly correlated with age, which was lower in the older groups. This would result from different locations between SCP and DCP. The thickness of the inner retina including the RNFL, GCL, and IPL, which locates SCP, is known to decrease with increasing age. Previous studies reported the decrease in RNFL, GC-IPL with increasing age not only in diseased eyes but also in healthy eyes.[19–24] Additionally, the close relationship between the inner retinal layer thickness and the SVD has already been known through many studies.[25–28] Therefore, the SVD may interact with a thinning inner retina over time and decrease along with them. Whereas, the DCP lies on the INL and its thickness is relatively less affected by age. Therefore, the SCP could be more sensitive to changes in VD with age than the DCP.
Previous studies reported the relationship between the retinal VD and BCVA in patients with diabetes.[27 29 30] Samara et al. reported that the VD had a moderate negative correlation with logMAR visual acuity at the level of both the SCP and DCP in eyes with diabetic retinopathy. Additionally, You et al. reported that lower SVD and DVD were significantly associated with worse BCVA in a normal population. Our study showed a significant correlation between the BCVA and both the SVD and DVD in healthy eyes, which is consistent with a previous study. We also found that the SVD/DVD ratio was negatively correlated with the BCVA. The SVD located in the inner retina, which was more sensitive to ischemia and lack of oxygen and subsequently contributed to the photoreceptor cell damage and impaired BCVA in patients with diabetes, seemed to be correlated with BCVA more directly than the DVD also in healthy eyes.[30 31] Further studies are needed to confirm this result.
Our study has several limitations. First, the retrospective nature of the work inevitably introduces some selection bias. Second, since eyes with axial length ≥ 26.0 mm were excluded, additional studies including high myopia are needed to more accurately identify the relationship between axial length and the OCTA parameters as we mentioned above. The strength of this study was that we included a relatively large number of patients from a wide range of age groups, and enrolled OCTA images with OCTA quality ≥ 25 dB, allowing accurate analyses. Additionally, this is the first study to identify different impacts on the SCP and DCP by various factors via SVD/DVD ratio.
In conclusion, we investigated through the SVD/DVD ratio that various factors can affect SCP and DCP differently in healthy eyes and found that age, BCVA, axial length, and OCTA quality of OCTA images were significantly correlated with the SVD/DVD ratio. From these results, we were able to infer that age, the BCVA, and OCTA quality were more correlated with the SCP, while the axial length was more correlated with the DCP. Physicians should consider these results when interpreting OCTA images. Our results may also facilitate future OCTA analyses of retinal microvasculature according to retinal layer in eyes with various diseases.