We next assessed the effects of increased IOP on the thickness of whole retina and individual retinal layers. Using GLM with repeated measurements, we found a significant difference between the thicknesses of all OCT retinal layers over time (
P ≤ 0.001). Pairwise comparison between the whole retina and individual layers showed significant differences between the whole retina and OPL (
P ≤ 0.001), IPL (
P = 0.002), IS/OS (
P = 0.002), and INL (
P = 0.004) over time but no significant differences between the whole retina and ONL (
P = 0.085) and RNFL (
P = 0.058). We then looked at the whole retina and individual layers at each specific time point using ANOVA and Bonferroni's modification. Statistical analysis revealed that OHT caused a significant reduction in whole retinal thickness at both 3 (158.03 ± 4.95 μm;
P = 0.017) and 8 (156.80 ± 7.34 μm;
P = 0.011) weeks compared with baseline (172.19 ± 5.17 μm;
Fig. 5A). Examination of the individual layers of the retina showed that RNFL thickness was significantly reduced at 3 weeks (31.47 ± 2.08 μm;
P = 0.031) but not at 8 weeks (33.94 ± 4.29 μm;
P = 0.383), compared with baseline (36.65 ± 1.21 μm;
Fig. 5B). Although there were no noteworthy changes in other inner retinal layers (IPL and INL), ONL thickness showed significant reduction at both 3 (40.87 ± 2.01 μm;
P = 0.035) and 8 (37.74 ± 2.54 μm;
P ≤ 0.001) weeks after IOP elevation (
Fig. 5B).
Given that GLM analysis suggested that there was a correlation among whole retina, ONL, and RNFL thickness over time, we next investigated this specifically. We found a strong correlation between whole retinal thinning and reduced thickness in the ONL (r = 0.748; P ≤ 0.001) and the RNFL (r = 0.500; P = 0.035). This confirmed that whole retina, RNFL, and ONL followed a similar trend over time.
To further analyze the effects of IOP exposure on the thinning of the retinal layers, we normalized the reduction in retinal layer thickness to cumulative IOP exposure. Mean changes in retinal layer thickness from baseline were calculated and then divided by the integral IOP exposure at 3 and 8 weeks after OHT induction (
Fig. 5C). This analysis suggested that IOP exposure caused more damage in the RNFL and ONL retinal layers at the earlier (3 weeks) than later (8 weeks) time points. In the RNFL, reduction in its thickness was 0.012 ± 0.002 μm per mm Hg day at 3 weeks compared with 0.004 ± 0.001 μm per mm Hg day at 8 weeks after IOP elevation (
P = 0.049). Reduction in ONL thickness was 0.010 ± 0.001 μm per mm Hg day at 3 weeks compared with 0.008 ± 0.001 μm per mm Hg day at 8 weeks (
P = 0.051). This suggests that RNFL and ONL were more susceptible and sensitive to IOP exposure in the early time points.