Abstract
Purpose: :
A recent study has demonstrated cumulative changes in corneal topography for short durations of near–work (10–120 mins) related to upper eyelid forces in a small sample of subjects (n=6). The present study investigated similar short–term effects of near–work on corneal topography in a larger sample of young adults with comparison between two refractive groups (myopes and emmetropes).
Methods: :
10 myopes (–0.75 to –4.75D) and 9 emmetropes (+0.75 to –0.50D) read for one hour at 40cm with a 30 degree downward gaze angle. Using the Orbscan II, 3 corneal topography measurements were taken pre and post–reading (5, 15, 30, and 60 min) from the left eye to assess changes in corneal power and thickness. Differences in corneal power (DCP) between the center and 2.75mm superior were calculated and compared at each time frame. Differences between baseline and 60 minutes of reading were calculated for the following: central corneal thickness (DCT), superior corneal thickness (DST), lower–order aberration (LO), higher–order aberration (HO), and total root mean square (RMS), for corneal aberrations.
Results: :
Group mean DCP after 5, 15, 30, and 60 minutes of near task ranged from 0.33 to 0.48D across all subjects, with it being 0.28 to 0.43D in the emmetropes and 0.32D to 0.53D in the myopes, over the same time interval. Two way repeated measures ANOVA revealed that there was no significant effect of either refractive group or time as a function of DCP. Mean DCT between baseline and 60 minutes in the myopes and emmetropes was 3.40 and 4.88µ, respectively. Mean DST between baseline and 60 minutes in the myopes versus emmetropes was 23.11 and 21.16µ, respectively. No significant differences were found in either measurement. Mean difference in RMS of LO, HO, and total corneal aberrations, between baseline and 60 minutes in the myopes was 0.15(±0.03), 0.06(±0.02), and 0.12(±0.02)µ, respectively. Mean difference in RMS of LO, HO, and total corneal aberrations, between baseline and 60 minutes in the emmetropes was 0.06(±0.02), 0.09(±0.02), and 0.08(±0.02)µ, respectively. A significant difference was only found in the LO aberration RMS computed between baseline and 60 minutes of reading for the myopes versus emmetropes.
Conclusions: :
In contrast to the earlier study, no significant group mean changes in either corneal power or thickness were observed following the short duration near–task in either refractive group. The lower DCP values measured in our study as compared to the earlier investigation may be attributed to the interrupted measurements within the test period versus separate timed sessions in the previous study. The LO RMS differences between refractive groups are consistent with literature findings.
Keywords: cornea: clinical science • myopia • eyelid