Additivity of near work-induced transient myopia and its decay characteristics in different refractive groups.

PURPOSE

To determine the additive effect, if any, of NITM after 1 and 2 hours of reading in different refractive groups.

METHODS

Fifteen early-onset myopes (EOMs), 14 late-onset myopes (LOMs), and 15 emmetropes (EMMs), as well as progressive myopes (PMs) and stable myopes (SMs), were tested. Subjects read binocularly for 2 hours at a distance of 35 to 40 cm. Distance refractive state of the right eye was assessed every 2 seconds for 30 seconds after the first hour of reading, and then every 2 seconds for 120 seconds after the second hour of reading. NITM was calculated as the difference in posttask distance refractive state compared with the pretask distance refractive state after each hour.

RESULTS

Initial NITM values (mean +/- SE) recorded at the end of the near work tasks were 0.22 +/- 0.03 D and 0.29 +/- 0.03 D for the EOMs, 0.14 +/- 0.02 D and 0.20 +/- 0.03 D for the LOMs, 0.14 +/- 0.02 D and 0.15 +/- 0.02 D for the EMMs, 0.20 +/- 0.03 D and 0.27 +/- 0.03 D for the PMs, and 0.09 +/- 0.04 D and 0.20 +/- 0.05 D for the SMs, after the first and second hours of reading, respectively. After the second hour, only in the EOMs and LOMs was NITM significantly greater than that found after the first hour. Seventy percent of the myopes (EOMs and LOMs) but only 47% of the EMMs exhibited increased NITM in the second hour compared with the first hour. Only EOMs exhibited longer decay duration after the second hour of reading. NITM was increased in progressive myopes (PMs), but not in stable myopes (SMs), after the first hour of reading only. Within the PMs, NITM was increased after the second hour compared with the first hour. The time constant for decay was greater in the PMs than in the SMs. Lastly, many myopes (up to 46%) did not experience decay to baseline after the near task over the 120-second posttask period.

CONCLUSIONS

EOMs and LOMs demonstrated larger NITM than the EMMs and exhibited NITM additivity, but the EOMs also exhibited prolonged decay of NITM compared with the EMMs and LOMs. PMs, but not SMs, exhibited additivity of NITM. These findings may be attributed to impaired sympathetic function in the subjects with myopia. It is speculated that with repeated cycles of near work, residual NITM may contribute to the progression of permanent myopia.