Our study shows generally satisfactory intra-device correlation coefficients of the widely commercially available SD-OCT and SS-OCT in normal Chinese children population, regardless of the refractive status. However, a statistically significant intra-device difference exists in the measurements. We also noted a marked superiority in the quality to visualise the choroid-sclera interface in SS-OCT (Fig. 2) compared to SD-OCT (Fig. 1), same as the finding previously reported in other studies. 9, 28
In a study of 35 healthy adult eyes, Matsuo et al 11 found a high intra-device correlation coefficient between SD-OCT and SS-OCT, which is consistent with our current study. Excellent correlation was also noted in other similar studies in normal adults. 10, 12, 15, 16 In contrast to our findings, Matsuo et al 11 and Ikuno et al 29 revealed a thicker measured choroidal thickness from SS-OCT than from SD-OCT, which were both manually determined. The differences may be attributed to the automated segmentation of SS-OCT in our study. Zafar et al 15 observed that the mean subfoveal choroidal thickness measured manually by SD-OCT and SS-OCT was greater than that automatically determined SS-OCT. They reported that a considerably higher choroid-sclera interface was identified by automatic software. Nevertheless, in our current study, automatically segmented images were further inspected and the border was manually adjusted if required, so the difference may not be fully explainable.
Another postulation from an earlier study by Michalewski et al 30 was the difference in the measurement method. Manually determined SD-OCT choroidal thickness measurements were based on focal measurements whereas that of automatically determined SS-OCT were calculated from the average thickness in a circular area with a diameter of 1000µm. As choroidal layer becomes thinner with the distance away from the fovea, focal measurements near fovea tend to overestimate the thickness and this may not be fully compensated by averaging focal measurements from different spots. 30
In a pediatric study on choroidal thickness measurements by SS-OCT, Xiong et al 31 reported subfoveal choroidal thickness of 272 ± 61µm, 283 ± 63µm and 269 ± 61µm for healthy children aged 6, 7 and 8 respectively. Similar studies done on Chinese school-age children also reported consistently thicker choroid than our subjects (251.84 ± 47.12µm). 26, 32–34 Of note, our previous study showed that Hong Kong has a higher prevalence of myopia than other Chinese cities. The notable difference could be attributed to the more intensive pre-school education, along with frequent near-work activities and lack of outdoor time due to crowded living places in Hong Kong compared to other regions of China. 22 Given the widely accepted postulation of the negative relationship between choroidal thickness and myopia, 26, 35 it is conceivable that the measured choroid is thinner in our study population.
In addition, we observed a statistically significant difference between the measurements obtained from two devices, which differs from previous study on healthy adult eyes. 10–12, 15, 16 Hanumunthadu et al 36 reported thicker large choroidal vessel wall and thinner medium vessel wall at subfoveal area in children eyes compared to adult eyes. A denser and high-flow vascular network was also found in children eyes. We believe that the age-related variation in choroidal vasculature, in particular thicker and more crowded vessels, may result in a higher tissue density. It may affect the accuracy in automatic detection of borders of choroidal layer in SS-OCT. Furthermore, lowered penetrance may affect the visualisation of the lower borders, predominantly in SD-OCT with shorter wavelength.
As for topographical variation, our results are consistent with previous paediatric studies showing thickest and thinnest choroid layers in temporal and nasal regions respectively. Similarly, parafoveal sectors are thicker than perifoveal sectors as observed in these studies. 26, 31–34 Our study further demonstrates poorer ICCs in nasal sectors compared to other sectors. We believe that the thin nasal choroidal layer results in a larger percentage error. Also, its proximity to the optic nerve may account for its higher variability compared to other regions. This also explains the less favourable validation data of conversion equations in nasal sectors.
In subgroup analysis, we observed similar ICCs in hypertrophic and emmetropic eyes, but poorer ICCs in myopic eyes in all regions. We believe that the inter-individual topographic variation of choroidal thickness during myopic growth may account for the difference. It is generally believed that choroid is stretched towards temporal direction when eyeball grows axially during myopic change. 37–40 Chui et al 41 proposed the ‘slippage’ hypothesis that difference in the rate and extent of stretching of tissue layers (retina, choroid and sclera) may result in slippage between ocular tissues during axial elongation. The ongoing process of scleral stretching gives rise to dynamic topographic asymmetry in choroidal thickness. In this regard, there is an inter-individual topographic variation of choroidal thickness depending on individual’s myopic progression. Unlike in the circular measurements by SS-OCT, focal measurements by SD-OCT may not truly reflect the variation and therefore results in a greater discrepancy.
Our study, to the best of our knowledge, is the first to compare the choroidal thickness measurements by two generations of OCT instruments in children. We directly compared measurements by different devices on the same subject.
Nevertheless, limitations of this study should be acknowledged. As mentioned above, SD-OCT choroidal thickness measurements were derived from focal measurements on different linear radial scans while that of SS-OCT were derived from regional measurements in a circular area. The discrepancy may be significant in children with greater topographical variation in choroidal thickness. Of note, most of these children are myopic, who are not the main focus of our current study.
In conclusion, we have shown a high consistency in choroidal thickness measurements between the two generations of OCT device in healthy paediatric eyes. With a satisfactory ICC, it is justifiable to directly compare two sets of measurements for monitoring choroidal thickness changes in population-based epidemiological research study. Nevertheless, it should be noted that choroidal thickness measured by SD-OCT was statistically significantly greater than that measured by SS-OCT, especially in myopic children. Therefore, it is not recommended to interchange the two OCT-results when assessing disease progression in individual patient in clinical practice.