Purpose: The purpose of the present investigation was to determine objectively the initial magnitude and decay of nearwork-induced transient myopia (NITM), and its potential additivity, following successive but interrupted periods of marked, sustained accommodation at near in asymptomatic young-adult myopic subjects.
Methods: Fifteen visually-normal, asymptomatic young adults (ages 18–28 years) were tested with full distance refractive correction. They included nine early-onset (EOM) and six late-onset (LOM) myopic subjects. Accommodation was assessed objectively with a Canon R-1, open-field, infrared auto-refractor under monocular viewing conditions (RE). The distance refractive state was measured immediately before and after a 10 min period of focusing upon a moderate contrast (50%), very near target (12 cm; 8 D) subtending a visual angle of 1°. The task was repeated twice with a 5-min inter-trial rest period of imposed far viewing. NITM was defined as the post-task minus pre-task change in distance refractive state and was determined for each of the three trials.
Results: Significant amounts of NITM were generated following nearly each trial in each subject. These ranged from 0.11 to 0.71 D, with a mean of 0.31 D. The group mean NITM was 0.32, 0.29, and 0.31 D for trials 1, 2, and 3, respectively. For the EOMs subgroup, NITM was 0.28, 0.30, and 0.34 D, while for the LOMs subgroup, it was 0.38, 0.29, and 0.26 D, for trials 1, 2, and 3, respectively. Decay of NITM was prolonged in many of the subjects (67%). However, additivity of NITM was not found following the sequences of interrupted near tasks.
Conclusions: There was no evidence of NITM additivity following a marked and sustained, but interrupted, near task. Although NITM has been reported to be additive following long periods of uninterrupted and sustained reading at lower dioptric levels, providing rest periods between each near task trial appears to prevent a cumulative effect (i.e. additivity effect).