Abstract
Purpose::
Nearwork-induced transient myopia (NITM) refers to the short-term, myopic shift in distance refraction following sustained near work. Based on laboratory and modelling studies, it has been proposed to be one of the environmental causes of permanent myopia. Previous studies involving both short and long durations of nearwork have speculated on the possible additive nature of NITM. However, this has not been confirmed. The purpose of the present study was to determine the additive effect of NITM, if any, following 1 and 2 hrs of reading in different refractive groups.
Methods::
15 early-onset myopes (EOMs), 14 late-onset myopes (LOMs), and 14 emmetropes (EMMs) read binocularly for 2 hours at a distance of 35-40 cms with blur-related visual feedback present. Distance refractive state (binocular viewing) of the right eye was assessed objectively using a Canon R-1 autorefractor every 2 seconds for 30 seconds immediately before and after one hr of reading. Following the second hr of reading, these measurements were repeated every 2 seconds over a period of 2 minutes. NITM was calculated as the difference in refractive shift pre- and post-reading following each hour.
Results::
The initial mean NITM following 1 and 2 hrs of nearwork was 0.22D & 0.29D for EOMs, 0.14 D & 0.21D for LOMs, and 0.15 D & 0.16D for EMMs, respectively. Following 2 hrs of nearwork, NITM in EOMs and LOMs was significantly greater than that found after the first hour. Equal increments of NITM (0.07D) were noted for both the EOMs & LOMs between the first and second hr of reading; in contrast, it remained essentially the same for EMMs (0.01D). 70% of the myopes (EOMs and LOMs), but only 47% of the EMMs, exhibited increased NITM in the second hour as compared with the first hour. Furthermore, myopes exhibited a longer time course for decay of NITM to baseline (EOMs: 90 secs, LOMs: 60 secs) as compared with the EMMs (45secs).
Conclusions::
Myopes manifested a greater magnitude of NITM, exhibited an NITM additive effect, and took longer to decay to baseline than did the EMMs, thus suggesting increased nearwork susceptibility. This suggests inherent impaired sympathetic function in the myopes. Over an extended period of time, repeated cycles of nearwork followed by distance viewing with residual undecayed NITM could translate into increased myopiogenic retinal defocus, and thus permanent myopia.
Keywords: refractive error development • myopia • refraction