This study has provided a comprehensive assessment of the topographical variations in choroidal thickness, including the luminal and stromal area and choroidal vascularity index in Chinese children with high hypermetropia.
Choroidal thickness is different between the two groups and the significantly higher in the high hyperopia group. A significant difference was found in the following spots, including sub-foveal, Nasal 0.5, Nasal 1.5, Nasal 3.0, Temporal 0.5
Choroidal thickness is the main parameter used to obtain information about the choroidal layer and has been examined in children and adolescents in many studies19–22. In healthy pediatric populations, the mean sub-foveal from these studies ranged from 245 to 361 µm20–22. Read et al.23 reported that the mean of the sub-foveal choroidal thickness of 330 ± 65 um in Australian children (4 ~ 12 years) with spherical equivalent refractive errors (SER) (+ 1.25 ~- 0.50 D). Takafumi Mori and Yukinori Sugano also found a similar result in preschool Japanese children24. Additionally, Zha Yi et al.25 reported that the sub-foveal choroidal thickness of 328.12 ± 65.93µm in 4–12 years old Chinese children with SER was between − 1.75D to + 0.63D. Our study in normal children had very close values (325.03 ± 56.67 µm)for age-matched normal children. Our results support that variations of choroidal thickness measurements for normal children between studies might result from age, ethnic differences, and different measurement techniques.
The value of choroidal thickness in evaluating the degree of refractive errors or amblyopia had also been depicted in some studies19, 24–26. The results are roughly the same in different cohorts: the myopic eyes had thinner sub-foveal choroidal thickness, and the hyperopic eyes and hyperopic amblyopic eyes had thicker choroidal thickness than emmetropic eyes and myopic eyes. However, few studies focused on the structural characteristic of the choroid in high hyperopia subjects. In the current study, both the subfoveal choroidal thickness and topographical variation in high hyperopic children were described. Consistent with the existing conclusions, we found that the sub-foveal choroidal thickness in high hyperopia children was significantly thicker than that in the emmetropic control group. The average choroidal thickness in the hyperopia group in our study was similar to the results of previous studies, but they seemed to be lower than those in the hyperopic amblyopia children with lower SER, which might verify the previous speculation that hyperopia was associated with subfoveal ChT, whereas amblyopia had no significant independent effect on subfoveal ChT in our study population25. The pathophysiologic mechanisms involved need further verification. Additionally, the topographic features of the choroid in the hyperopia group in this study were almost the same as those in controls: the choroid was the thickest in the sub-foveal region and the thinnest in nasal regions which approach the optic nerve(Fig. 3). These findings were also consistent with previous results of children either with different refractive status or with amblyopia, indicating the relatively stable choroid contour in pediatric populations19, 25.
In normal eyes, choroidal thickness in children can be affected by various factors, such as age, gender, diopter, and axial length. Previous studies reported a significant association between the A.L. and choroidal thickness27–29. Nagasawa et al. and Bidaut-Garnier reported that ChT was negatively correlated with axial length30, 31. Li et al. reported that a thinner subfoveal choroidal thickness correlated with a longer A.L. but no other factors, such as age, sex, and SER in myopic children32. Read et al. found that choroidal thickness increased in children with normal axial elongation, whereas children undergoing faster A.L. elongation tended to exhibit less thickening and a thinning of the choroid in some cases33. In the current study, a thinner SFCT was also found to be negatively correlated with longer A.L. and larger age but positively correlated with the sphere in normal and hyperopic children. However, the causal relationship between choroidal thickening and hyperopia remains controversial.
Since CT could be affected by sorts of variables and could not represent the choroid's subtle structural changes, we cannot assess the choroid structural changes and blood supply only by C.T. measurement. Recently, Agrawal et al.17 assessed the L.A., S.A., and TCA through EDI-OCT images and proposed a new parameter- CVI to assess the choroidal vascular structure. In their study, they found that CVI was less affected than the sub-foveal choroidal thickness and suggested CVI be a more robust marker of choroidal diseases. Subsequently, other studies have also demonstrated less variability of CVI. In our study, we found that the A.L. was significantly correlated with SFCT but not the CVI, which was consistent with the previous results, further indicating that CVI might be an alternative metric for the evaluation of choroidal disorders.
On the other hand, a shorter A.L. tended to be associated with a bigger L.A., S.A., and TCA. Li et al. reported that changes in luminal areas might directly influence choroidal thickness, as blood vessels represent the main component of the choroid32. Meryem et al.34 found that TCA and S.A. were higher in hyperopia than in both emmetropic and myopic eyes. In the present study, L.A. and TCA values and CVI of the high hypermetropia eyes were significantly larger than that of the normal control eyes suggesting an increase in blood flow and blood vessel area in eyes with shorter A.L.s. It is still unknown whether the thickening of the choroid in hyperopia is caused by the increase of stromal area or the enlargement of the vascular luminal area. Multi-center studies with large sample sizes are needed for further validation.
Age is also an important factor affecting choroid structure. Previous studies found that the choroidal thickness tended to decrease with increasing age in adults27, 28 Fujiwara et al. found that choroidal vascular density had a negative association with age in healthy adults35. Ruiz-Medrano et al.36 found that the choroidal stromal area was not affected by age in a healthy population that included normal children. In our study, stromal area, luminal area, and total choroidal area were found to decrease significantly with increasing age.
Our study has several limitations. First, the sample size is relatively small, and the high hyperopes are all Chinese children. Second, the OCT measurement of each subject in our study was performed at a randomized time at their convenience. Further studies with a large population and unified examination time are needed for more positive and robust results.