Wide‐field choroidal thickness and vascularity index in myopes and emmetropes

To the Editor: We read with interest the recent study conducted by Yazdani et al. investigating regional variations in different choroidal morphological parameters in young adult emmetropic and low myopic subjects. The authors noted that low myopic patients had significantly lower stromal choroidal thickness (CT) and higher choroidal vascularity index (CVI) values than emmetropes. They concluded that choroidal thinning in low myopic patients could be assigned to the changes in the stromal layer of the choroid rather than the luminal layer. We intend to congratulate the authors on their extensive research. However, the conclusion of the study suffers from some shortcomings. The methodology of the study conducted by Yazdani et al. does not support the study's conclusion, and the author did not mention any rational conclusion and clinical application(s) of their findings. Although the title refers to myopia, the target population were low myopic healthy adults with no manifestations of pathological findings. The main focus of the study was simply on the changes of the choroidal components in low myopic patients between 20– 38 years old. In practice, myopia progression mainly occurs before 21 years of age, particularly in children and teenagers, and then stabilizes. In addition, knowing CVI and other component changes in this age group (20– 38 years old) has no clinical value for practitioners. Previous studies on CVI in myopic patients primarily concentrated on choroidal components in high myopia due to its sightthreatening nature. 7 In a study conducted by Alshareef et al. with almost the same methodology, the authors evaluated the subfoveal stromal and luminal area of patients with nonpathologic highmyopia compared with the emmetropic subjects. They found a thinner CT and mainly attributed it to the thinner stromal area rather than the luminal area. Evaluation of choroidal parameters in low myopia may be valuable in the paediatric population as reported by Li et al. They evaluated widefield variations of the luminal and stromal area of low myopic paediatric population (age range between 8– 15 years old), and found a negative association between stromal area and the degree of myopia. As a result, despite the authors claim that their findings would be a reference point for future studies on myopia progression in children, it could not be the case. Despite the knowledge of diurnal variations in measurements of choroidal components, the authors performed their examinations in an extended period (between 13:00 to 18:00). According to the study by Kinoshita et al., significant diurnal variations were found for some choroidal components, including the luminal area with the maximum and minimum values found at 6:00 and 15:00, respectively. Therefore, as the data collected at 15:00 were recorded between 13:00 and 18:00, the examinations of the luminal area are somewhat debatable. These results have been supported by Singh et al. that reported significant diurnal fluctuation in subfoveal and temporal peripapillary CVI measurements in healthy subjects. A more restricted examination time, i.e., 2 or 3 h, has previously been suggested to neutralize the effect of diurnal fluctuations in such studies. Another issue jeopardizes the experimental design of this research paper. Unfortunately, the authors did not check some influencing factors such as systolic and diastolic blood pressures and body mass index in the case selection process of the study. The algorithm authors proposed as normative data for overall CVI of low myopic patients overlooked the differentiation between the choroidal large and mediumsized vascular layers (Haller's and Sattler's layers, respectively). It has been reported that myopic refractive error could affect these sublayers of the choroid independently. The authors also did not include the choriocapillaris layer that is an essential component of the choroid. This microvascular layer can result in secondary vision loss in myopic patients. In conclusion, future longitudinal and controlled studies on a paediatric population with high myopia are warranted to ascertain the causeeffect relationship between myopia progression and changes in different elements of the choroidal structure. In addition, the role of CVI as a novel choroidal metric can be helpful in the clinical management process of progressive myopia.