Myopia has become an international public health concern and creates a tremendous global economic burden. With the progression of myopia, the risk of ophthalmic diseases such as myopic macular disease, retinal detachment, glaucoma and cataract also increases[5–9]. Although interventions to control myopia (such as spending more time outdoors, orthokeratology and topical application of atropine can prevent the occurrence of myopia or slow its progression [10], the mechanism of the progression of myopia at various stages is still unclear. Retinal tissue is mainly supplied with oxygen and nutrition by choroid, and is vulnerable to myopia related fundus changes [11–13]. Fovea is the most sensitive part of vision, so the study of retinal blood flow parameters in macular area can provide clinical basis for diagnosis and treatment. In this study, we used OCTA to quantitatively analyze the relationship between the progression of myopia and macular blood flow, as well as the relationship with axial length in children and adolescents with myopia. As a new vascular imaging technology, OCTA can provide more detailed microvascular morphological information and stronger quantitative analysis capabilities in retinal and choroidal diseases. At present, it is commonly used in the diagnosis and treatment of retinal and choroidal diseases. In this study, we found that the area of FAZ that progresses quickly in children is smaller than that of slow progress, and in adolescents, the axial length with rapid myopia progress is longer than that with slow myopia progress. The change of SE was positively correlated with the change of axial length, but have no obvious correlation with the change of height, VD, PD and FAZ.
Previous studies have shown that the choroid may play an important role in the occurrence and development of myopia, but the mechanism of the choroid in the occurrence and development of myopia is still unclear [14]. During the occurrence and development of myopia, the choroid of the human eye becomes thinner. There are also some indications of changes in choroidal blood circulation, such as decreased intraocular perfusion pressure [15] and slowed choroidal vascular perfusion [16], etc. Research by Shih et al. showed that due to form deprivation, the choroidal blood flow of myopia in chicks was reduced. The decrease in flow is due to the expansion and thinning of the choroid in myopic eyes. It is also pointed out that the decrease in choroidal blood flow may be an adaptive response to the thinning of the retina caused by myopia [17], the normal high blood flow of choroid is very important for nourishment and oxygen supply to the outer retina, the choroid is the only source of oxygen and nutrients in the five layers of the outer retina, including the FAZ. The changes of vascular density, vascular structure and blood flow of choroid will directly affect the metabolism and function of retina, and cause pathological changes in the structure and function of retina[3]. Therefore, in the occurrence and progression of myopia, it is very important to monitor the morphology of the retina and the distribution of retinal blood vessels. The nutritional supply of FAZ comes from the choroid. And the value of FAZ can indirectly reflect the blood flow of the choroid. The blood flow of the choroid is abundant, and the area of FAZ is relatively large. In this study, the area of FAZ with rapid progression of myopia in children is smaller, and the area of FAZ with slow progression is larger, indicating that the progression of myopia is slow when the choroidal blood flow is abundant. For myopia prevention and control, we can try to increase choroidal blood flow to slow down the progression of myopia, such as the use of drugs that increase blood circulation or eye massage to increase blood circulation of the eyes, but there is no significant difference in the area of the FAZ of the adolescents in this study, that is, the old group. The author believes that this age group is at the peak of growth and development, the metabolism is relatively vigorous, and the choroidal blood flow is relatively rich, which has an impact on the area of FAZ. At the same time, the progress of myopia in adolescence has a certain relationship with growth and development. Researches such as Cheng Tianyu also believes that various hormones in the "Burst" may be the common internal motivation of physical development, sexual development, and refractive development [18], so there is no difference in the size of FAZ in the adolescent group.
This study shows that in the adolescent group those with rapid progression of myopia the axial length is longer than those with slow progression of myopia, and the change of SE is positively correlated with the change of the axial length. The elongation of the axis of the eye is the most cause of myopia, and is closely related to the progression of myopia[19–21] .Studies have shown that the thinning of the retina caused by the elongation of the axial length which will reduce its oxygen demand, thereby reducing blood circulation[ 22], which affects choroidal blood flow, and at the same time, the decrease in blood circulation will cause the reduction of scleral blood supply and oxygen supply. The study of Hao WUA et al. proposed that scleral hypoxia is a new target for the progression of myopia [23], so the longer axis of eye the relative faster progresses of myopia, but most of the participants in this study have mild to moderate myopia, especially in the young age group, who are basically mild to moderate myopia, and all the axial length were less than 26mm, so the impact on scleral hypoxia is not as great as that in the old age group with longer eye axis, which may be the reason why there is no difference in the eye axis between the observation group and the control group in the young group. Studies such as Fan and Saw have shown that axial elongation is highly correlated with the progression of myopia [24–25], which was consistent with the positive correlation between the change of SE and the change of axial length in this study.
This study found that the speed of myopia progression is not related to the VD and PD of the superficial microvessels in the macular area, which is consistent with the study by Xiao Feng-qi and others researchers ,they also found that the blood flow density of the deep retinal microvessels in the macular region was lower in high myopia than in mild myopia [26], the only source of oxygen and nutrients in the outer five layers of the retina is the choroid. The blood flow density of the deep retinal microvessels in the macular region reflects the choroidal blood flow, which is consistent with the reduction of choroidal blood flow in the progression of myopia mentioned earlier. The Angio-Plex OCT Angiography (Carl Zeiss Meditec, Dublin, CA) has no value for deep blood flow density, so there is no comparison of deep blood flow density in this study. Mo et al. [27] used OCTA analysis and found that the density of superficial microvessels in the foveal area adjacent to the macular in the pathologic myopia (PM) was reduced. In this study, most of the participants were mild to moderate myopia, so the conclusion was inconsistent with Mo et al. In this study, there was no significant correlation between change of SE and the change of superficial vascular density VD, PD in the macular area, and the change of FAZ, change of height. The study by Xiao Feng-qi et al showed that there was no significant difference in the blood flow of the superficial retinal capillaries in the macular area among different myopia groups [26]. In this study, subjects followed up the progress of myopia, but the follow-up time was short, and the maximum progress of myopia was only 1.0D. Therefore, there weren’t significant changes in VD and PD of the superficial retina in the macular area. Similarly, FAZ changes are not obvious. Multiple studies have shown that there is a significant correlation between the increase in height and the length of the eye axis [28–29]. However, unlike the consistent conclusions of multiple studies on the relationship between height and eye axis length, the relationship between height and refractive degree is still controversial. Some studies have found that the higher the height, the more refractive degree tends to myopia[30–31], the higher the myopia rate[32–33], but there are still many studies that have not found a significant correlation between height and refractive degree, which may be due to corneal compensation thinning counteracts the effect of axial elongation, leaving the refractive degree unchanged [28–29,34].
Through this study, we have learned that the area of FAZ with fast myopia progression in children is smaller than that with slow myopia progression, and the axial length with fast myopia progression in adolescents is longer than that with slow myopia progression. This provides an early warning indicator for our myopia prevention and control. In the stage of children, we can predict the trend of myopia progression by checking the OCTA of the macular area to understand the area of the FAZ. In the stage of adolescent, we can predict the trend of myopia progression by checking the eye axis. If there is a reminder that myopia is progressing fast, In myopia prevention and control, more effective prevention and control methods should be adopted, or a variety of methods should be used for myopia prevention and control. However, this study has some limitations. Firstly, the sample size is too small. Secondly, the selected subjects are all children and adolescents aged 8–14 in the region, which cannot represent the entire population. Thirdly, it is assumed that FAZ area and axial length can possible to provide a sensitive and "quick predictive index" of myopia progression. Because of the lack of benchmark data comparison, further research should be done to predict the standard value of the corresponding age group in order to better predict the progression of myopia through this method in clinical work. And better propose the prevention and control measures of myopia.