We read with interest the article by Sanchez-Cano and associates, which evaluated choroidal thickness in young adults using enhanced-depth imaging spectral-domain optical coherence tomography (OCT), and we agree that a normative database is important to facilitate comparisons of choroidal thicknesses among patients with ocular disease. It is increasingly evident that the choroid plays important roles both in normal physiology and in ocular diseases, the study of which will be greatly facilitated by enhanced-depth imaging and the new swept-source OCT devices.
There are several other studies that have similarly reported choroidal thickness in healthy young adults, and it will be interesting to compare the findings of the various studies. In Sanchez-Cano’s study, the mean central subfield choroidal thickness (in the 1-mm zone centered on the fovea) was 381.94 μm for emmetropic subjects and 307.04 μm for myopic subjects with a mean spherical equivalent of −1.50 diopters (D). In contrast, in a study of 150 healthy young Chinese adults of similar age, the central subfield choroidal thickness was 438.5 μm for emmetropic subjects (spherical equivalent +0.50 D to −0.49 D) and ranged from 381.1 μm for subjects with low myopia (−0.50 D to −1.99 D) to 238.9 μm for myopic subjects of spherical equivalent −8.00 D or higher. Since both of these studies performed manual segmentation of OCT scans obtained using the Heidelberg Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany), the images should be comparable and it is likely that the observed differences may be attributable to ethnicity or other population-specific factors.
The authors reported correlations between subfoveal choroidal thickness and axial length (−43.84 μm/mm) and refractive error (−10.45 μm/D). These results fall within the range reported by similar studies, although values of up to −58.2 μm/mm and 25.4 μm/D have been described. It is important, however, to consider the rate of change of choroidal thickness, which may differ at various regions of the macula.
We agree that “measurement of a small number of points can be influenced by local changes in choroidal thickness or irregularities in the choroidoscleral border.” Studies have reported significant topographic variation of choroidal thickness in different macular regions. Although earlier studies have reported that the choroid is thickest centrally using either mean thickness or point thickness measurements, Sanchez-Cano and associates reported that choroidal thickness was thickest in the superior and temporal areas. This is similar to results described by Tan and associates. However, in addition, important differences in the location of the thickest sector based on the spherical equivalent and axial length were described. In myopic subjects with spherical equivalent of −2.00 D or worse, the choroid was thickest in the superior and temporal sectors relative to the central subfield. In contrast, the central and temporal subfields were of similar thickness in those with spherical equivalent −1.99 D to +0.50 D. A similar pattern was observed based on the effects of axial length, suggesting that these ocular parameters influence the region where the choroid is thickest.
In summary, we congratulate the authors on their study, which establishes baseline choroidal thickness among young adults and allows comparison of choroidal thicknesses among different populations.