Human optical axial length and defocus.

PURPOSE To investigate the short-term influence of imposed monocular defocus on human optical axial length (the distance from anterior cornea to retinal pigment epithelium) and ocular biometrics. METHODS Twenty-eight young adult subjects (14 myopes, 14 emmetropes) had eye biometrics measured before and 30 and 60 minutes after exposure to monocular (right eye) defocus. Four different monocular defocus conditions were tested, each on a separate day: control (no defocus), myopic (+3 D defocus), hyperopic (-3 D defocus), and diffuse (0.2 density Bangerter filter) defocus. The fellow eye was optimally corrected (no defocus). RESULTS Imposed defocus caused small but significant changes in optical axial length (P < 0.0001). A significant increase in optical axial length (mean change, +8 ± 14 μm; P = 0.03) occurred after hyperopic defocus, and a significant reduction in optical axial length (mean change, -13 ± 14 μm; P = 0.0001) was found after myopic defocus. A small increase in optical axial length was observed after diffuse defocus (mean change, +6 ± 13 μm; P = 0.053). Choroidal thickness also exhibited some significant changes with certain defocus conditions. No significant difference was found between myopes and emmetropes in the changes in optical axial length or choroidal thickness with defocus. CONCLUSIONS Significant changes in optical axial length occurred in human subjects after 60 minutes of monocular defocus. The bidirectional optical axial length changes observed in response to defocus implied the human visual system is capable of detecting the presence and sign of defocus and altering optical axial length to move the retina toward the image plane.