In this study, we compared bilateral CT values among several points in the macular, nasal peripapillary, and peripheral areas using wide-field SS-OCT in subjects with uncomplicated pachychoroid. The degree of symmetry was generally high in the macular area (except nasal point 4 and temporal point 3) and low in the nasal peripapillary and peripheral areas. The interocular difference in CT increased gradually from the macular area to the nasal peripapillary and temporal areas (p < 0.001, one-way ANOVA), and the interocular difference in CT between the macular and nasal peripapillary area showed a near significance (p = 0.058) according to the post hoc Bonferroni test. This was assumed to be due to the low interocular symmetry of CT for nasal point 4 and temporal point 3 in the macular area.
According to a search of the PubMed database, no studies have analyzed interocular symmetry and differences in CT in uncomplicated pachychoroid patients. In particular, there have been no wide-field SS-OCT studies on the interocular symmetry of CT in the nasal peripapillary and peripheral areas. Most previous studies on the interocular symmetry of CT were mainly confined to foveal and parafoveal areas in healthy subjects rather than those with uncomplicated pachychoroid.2, 22–25 Therefore, this study is important in that it is the first to compare the interocular symmetry of CT among the nasal peripapillary, peripheral and macular (foveal and parafoveal) areas using wide-field (16-mm) SS-OCT in subjects with uncomplicated pachychoroid.
Most of the studies mentioned above using Early Treatment Diabetic Retinopathy Study (ETDRS) maps or other methods reported that interocular CT in foveal and parafoveal areas showed high correlation coefficients and ICC values (ρ > 0.8, ICC > 0.9).24–26 Similarly, in the present study, CT measurements showed relatively high agreement (ρ > 0.8, ICC > 0.85) at the subfoveal, nasal 5, nasal 6, temporal 1, and temporal 2 points, which correspond to the center and inner ring (3-mm diameter area centered on the fovea) of the ETDRS map. However, CT measurements showed relatively low agreement at the nasal point 4 and temporal point 3 (ρ = 0.577 and 0.667, respectively), corresponding to the outer ring of the ETDRS map. It is difficult to determine the reason for the low interocular symmetry of CT at these points. However, there have been several studies related to this issue. In the study by Chen et al. [mean (min, max) subfoveal CT = 334 (172, 568) µm in the right eye and 333 (133, 555) µm in the left eye],24 the correlation coefficient for CT in the temporal area 3 mm from the fovea was 0.490, which was the lowest value among all areas measured in their study. In addition, other reports27, 28 posited that peripapillary CT variability could result from the presence of watershed zones, primarily near the optic disc.29 The very low CT correlation coefficient for nasal point 4 compared to nasal point 5 in this study could be attributable to watershed zones.
To measure CT in the nasal peripapillary and peripheral areas in subjects with uncomplicated pachychoroid, wide-field imaging is required. Several studies measured peripheral choroidal CT using wide-field imaging.27, 28, 30–32 However, to our knowledge, this is the first study to investigate interocular differences in CT in nasal peripapillary and peripheral areas in subjects with uncomplicated pachychoroid. In this study, the correlation coefficients of CT in the nasal peripapillary and peripheral areas were lower than in the macular area (except for nasal point 4 and temporal point 3), thus demonstrating that CT symmetry in the nasal peripapillary and peripheral areas is generally lower than in the macular area. However, the reason for these differences is not clear. In our previous study,32 CT measurements were compared between pachychoroid and “normochoroid eyes” using wide-field SS-OCT. Even in pachychoroid eyes, pachyvessels were sometimes absent from the nasal peripapillary and peripheral areas, leading to smaller than expected CT values. This may partly explain the greater interocular CT variation in nasal peripapillary and peripheral areas compared to the macular area.
CT is known to be affected by multiple factors. In this study, we also analyzed the relationships of interocular differences of CT with other clinical factors. In the macular area, interocular CT and AL differences had a significant negative correlation, i.e., the choroid in the macular area becomes thinner as the AL increases, and vice versa. However, in the nasal peripapillary and peripheral areas, no such correlations were seen. Thus, AL may have a greater effect on CT in the macular area than in the nasal peripapillary and peripheral areas, as suggested by previous studies.28, 32 Other factors, such as age, sex, IOP, and SE, also showed no relationships with interocular CT differences. However, only young adults were included in our study, and the age range was narrow (24–35 years; mean age = 27.55 years), which limited the generalizability of the findings. Age is known to be related to CT, but therefore further research including detailed subgroup analyses is needed on this.
This study had some other limitations. CTs were measured manually, which can lead to measurement inaccuracy. However, the ICC and CV values showed good reproducibility, suggesting that any inaccuracy was minimal. Nevertheless, obtaining automatic CT measurements via software may be useful. We did not analyze other factors that may affect CT, such as mean arterial pressure or diurnal variation. Despite these limitations, this was the first study to compare CT among macular, nasal peripapillary, and peripheral areas in subjects with uncomplicated pachychoroid. In addition, the wide-field (16-mm) SS-OCT modality captured images without time lag, thus minimizing errors caused by time differences. Moreover, the image quality was higher, and the CT measurements were more precise, in this study compared to other studies using SD-OCT or EDI-OCT. This study clearly demonstrated that interocular CT variation can occur in nasal peripapillary and peripheral areas.
In conclusion, interocular CT generally showed bilateral symmetry in our patients with uncomplicated pachychoroid, although this differed among areas. In addition, only interocular CT and AL differences were significantly correlated in the macular area; there were no significant associations for any other clinical factor. This suggests that the interocular CT difference in nasal peripapillary and peripheral areas is due to anatomical variation alone, rather than other clinical factors. Physicians should be aware of the possibility of interocular CT differences; when an uncomplicated pachychoroid patient exhibits an abnormal CT difference, it is important to perform detailed examinations to identify the factors, including other ophthalmic diseases, which may be responsible.