Choroidal thickness (ChT) is not constant but fluctuates with the diurnal cycle
3 and can be altered by different visual stimuli; it was reported initially in chicks that the choroid responded to defocus by spherical lenses,
4,5 and similar responses were detected subsequently in human eyes.
6,7 Therefore a potential role of ChT in dysregulated, myopiagenic eye growth has been intensively studied in humans.
8 Recently, it was reported that the human choroid also responds to astigmatic defocus.
9 Moreover, in human subjects, reading black text on white paper or white text on black paper for a short time period (30 minutes)
10 and narrowband blue or red light exposure
11 induced differential changes in ChT, which were detectable in vivo by monitoring ChT. Animal models are important for understanding the complexity of eye growth and for elucidating signaling pathways in the eye that locally control its functional and physical homeostasis.
12 Based on studies in chicks, it is well accepted that changes in ChT displace the retina, moving the photoreceptors toward (under positive defocus) or away from (under negative defocus) the plane of focus.
4,5,13,14 This bidirectional modulation of axial elongation might allow for eyes to grow toward functional emmetropia
15 because the choroid becomes thinner during faster eye growth and thicker during slower eye growth. Studies on ChT in chickens have found that ChT in untreated eyes was variable,
16 but it was not associated with the susceptibility to form deprivation myopia (FDM).
17 Later it was confirmed by Nickla and Totonelly,
18 who reported more extensive data not only from FDM, but also from plus and minus lens-induced defocus and drug treatments. ChT has also been shown to be variable among individual children
19 and to vary with age. For example, Read and colleagues
20 reported that ChT increased with age from 4 to 12 years, and Wakatsuki et al.
21 reported that ChT decreased with age from 20s to 70s. Similarly, a longitudinal, population-based observational study in humans by Hansen et al.
22 concluded that thinness of the subfoveal choroid at age 11 years did not predict the axial eye elongation and incident myopia from age 11 to 16 years; however, their reported ChT at age 11 years was 361 ± 77 µm—which is thicker than the average ChT reported among emmetropic children at comparable ages among Australian,
20 Chinese,
23 and Turkish children.
24 In addition to the fact that myopia in human children and growing guinea pigs undoubtedly differs on many grounds, given the imperfection of any animal model of disease, it is worth noting that a ChT of 361 ± 77 µm is thicker than that found in emmetropic human eyes. Thus it may not have been in the range typical of myopic children but rather within the range suggested to be protective against myopia development in guinea pigs (GPs), if our previous findings in GPs are directly translatable to human children.
14