The human axial length and choroidal thickness responses to continuous and alternating episodes of myopic and hyperopic blur

Abstract

Purpose To investigate the change in axial length (AxL) and choroidal thickness (ChT) in response to continuous and alternating episodes of monocular myopic and hyperopic defocus. Methods The right eye of sixteen young adults was exposed to 60 minute episodes of either continuous or alternating myopic and hyperopic defocus (+3 DS & -3 DS) over six separate days, with the left eye optimally corrected for distance. During alternating defocus conditions, the eye was exposed to either 30 or 15 minute cycles of myopic and hyperopic defocus, with the order of defocus reversed in separate sessions. The AxL and ChT of the right eye were measured before, during and after each defocus condition. Results Significant changes in AxL were observed over time, dependent upon the defocus condition (p < 0.0001). In general, AxL exhibited a greater magnitude of change during continuous than alternating defocus conditions. The maximum AxL elongation was +7 ± 7 μm (p = 0.010) in response to continuous hyperopic defocus and the maximum AxL reduction was -8 ± 10 μm of (p = 0.046) in response to continuous myopic defocus. During both 30 and 15 minute cycles of alternating myopic and hyperopic defocus of equal duration, the effect of opposing blur sessions cancelled each other and the AxL was near baseline levels following the final defocus session (mean change from baseline across all alternating defocus conditions was +2 ± 10 μm, p > 0.05). Similar, but smaller magnitude, changes were observed for ChT. Conclusions The human eye appears capable of temporal averaging of visual cues from alternating myopic and hyperopic defocus. In the short term, this integration appears to be a cancellation of the effects of the preceding defocus condition of opposite sign.

Effects of brief periods of clear vision on the defocus‐mediated changes in axial length and choroidal thickness of human eyes

“…Exposure to 60 min of continuous hyperopic defocus resulted in a small, but statistically significant axial elongation and subfoveal choroidal thinning. These findings are consistent with previous studies in humans [12][13][14][15][16][17] and other animals. [9][10][11] However, when 1 h of hyperopic defocus was interrupted periodically with 2-min episodes of clear vision, the magnitude of change in axial length and subfoveal choroidal thickness reduced significantly relative to the uninterrupted hyperopic defocus condition.…”

“…In human eyes, however, the findings have been mixed. Whilst most studies have reported comparable magnitudes of change in choroidal thickness and/or axial length in response to myopic and hyperopic defocus, 12–16 a recent investigation found a greater response to hyperopic than to myopic defocus 17 . The reasons underlying these differences in the magnitude of response to defocus are not obvious, but could be related to the refractive error of the cohort tested 14 …”

“…Further details of this experimental set-up have been provided elsewhere. 12,13 To improve the accuracy of the axial length measurements, only five consecutive measurements with a cumulative standard deviation (as displayed on the Lenstar) of ≤7 µm were included. If any of the five consecutive measurements yielded a cumulative S.D.…”

“…The left eye had clear vision (optimal distance refractive correction) and controlled the state of accommodation. Further details of this experimental set‐up have been provided elsewhere 12,13 . To improve the accuracy of the axial length measurements, only five consecutive measurements with a cumulative standard deviation (as displayed on the Lenstar) of ≤7 µm were included.…”

Macular Thickness Variations with Axial Length in Healthy Individuals: Review Article

“…After a few months of rapid axial length elongation, which occur in the first 3 to 6 months after birth, growth slows down for the next two years, and by the time a three-year-old is born, he or she has reached adult size (11) .…”