Abstract
Abstract: :
Purpose: Although retinal defocus is a presumed myopiogenic factor, few studies have assessed the depth–of–focus (DOF) in different refractive groups. Furthermore, most studies used a subjective means of assessment, and they have been conducted using a Badal optical system providing blur only information. The aim of the present study was to measure the objective DOF of myopes in free space which has never been done before, with both blur and proximal information present, and compare to a matched emmetropic cohort. Methods: 12 young adult myopes (ages 21 to 33 years; –1.5 to –5D) and 11 emmetropes (ages 20 to 30 years; + 0.5D to – 0.5D) served as subjects. A new technique was devised to measure the DOF objectively in free space monocularly by monitoring accommodation dynamically using the Power Refractor (resolution at least 0.25D). A high contrast target (Maltese cross) subtending 2.3 deg (with a range from 1.8 to 2.8 deg) was placed on an optical bench 25cm (4D) from the subject. Following baseline level assessment, it was displaced slowly (∼ 0.1D/s) distally and proximally, until the first consistent steady–state change in accommodative level was noted. The mean dioptric difference between the distal and proximal endpoints was averaged, which represented the total DOF. Results: The mean DOF in the 12 myopes was ± 0.55 ± 0.06D, with a range of ±0.40 to ±0.75D. The mean DOF in the 11 emmetropes was ± 0.48 ± 0.16D, with a range of ±0.27 to ±0.68D. The mean myopic and emmetropic DOF values were not significantly different (t= –1.60, p= 0.12). Conclusions: These objective DOF results in free space were similar for both the young adult myopes and emmetropes. The present findings are consistent with Gwiazda et al.’s (1993) study of accommodative accuracy in myopic versus emmetropic children under similar conditions, in which both blur and proximal information were also present. The slightly larger DOF found in the myopes is in accord with the notion that small amounts of retinal defocus integrated over a long time interval may be myopiogenic. Dynamic changes in retinal defocus magnitude for sustained near viewing periods are unknown, but remains relevant to the question of nearwork as a possible myopiogenic factor.
Keywords: ocular motor control • myopia • adaptation: blur