Time course of axial length changes in response to competing episodes of myopic and hyperopic defocus

Purpose To investigate the short-term changes in human axial length (AxL) in response to continuous and competing episodes of myopic and hyperopic defocus. Methods The right eye of 16 young adults was exposed to 60 min episodes of either continuous or competing myopic and hyperopic defocus (+3 D & -3 D), with the left eye optimally corrected to maintain far accommodation. During competing defocus, the eye was exposed to either 30 min or 15 min of alternating cycles of myopic and hyperopic defocus, with the order of lens wear reversed in separate sessions to assess the effects of defocus order. During each 60 min trial, the subjects watched a movie at 6 m in low photopic conditions. The right eye’s AxL was measured at baseline and then at 15 min intervals using Lenstar optical biometry. A binocular beam splitter periscope system was used to maintain distance fixation (left eye) and defocus exposure (right eye) during measurements. Results AxL underwent a greater magnitude of change during continuous defocus than during competing defocus. The maximum AxL change observed during continuous hyperopic defocus was +7 ± 2 µm of elongation (p = 0.015) and during continuous myopic defocus was -8 ± 2 µm of reduction in AxL (p = 0.07). When the eye was exposed to 30 min cycles of competing myopic and hyperopic defocus of equal duration, the opposing blur sessions cancelled each other and the eye was at near baseline levels following the final defocus session (final change from baseline, both p > 0.05). When the frequency of alternating cycles was 15 min, there was a slight AxL reduction after 60 min of myopic then hyperopic defocus, and hyperopic then myopic defocus, and were -3 ± 2 µm and -4 ± 2 µm, respectively (final change from baseline, both p > 0.05) (Figure 1). Conclusions In the human eye, the AxL changes within minutes in response to short-term imposed myopic and hyperopic defocus. With competing defocus, at 30 min exposure frequencies, the effects of myopic and hyperopic blur largely cancel each other. At higher frequencies (15 min of alternating exposure), the effect on AxL of myopic defocus appears to be slightly more potent than hyperopic defocus