Effect of accommodative adaptation on static and dynamic accommodation in emmetropia and late-onset myopia

Prolonged near work is considered to be an environmental factor leading to the development of late-onset myopia. Accommodation may be the specific mechanism underlying the association between near work and late-onset myopia. To determine whether late-onset myopes have abnormal accommodation, we compared accommodative static responses and dynamic facilities before and after a near task in two groups of subjects, emmetropes and late-onset myopes.

In experiment 1, the accommodative stimulus/response function with monocular viewing and the dark focus (the accommodative response in the dark) were objectively measured with a Canon R-1 infrared optometer before (preadaptation) and after (postadaptation) a 20-min near task. In experiment 2, monocular accommodative facility (AF) and dark focus were measured before and after the near task. Facility was measured as the subjective time needed to clear an accommodative target (20/40 letters) at 40 cm through +/-2.00 D lenses. The time between when the subject flipped the lenses from viewing through the +2.00 D to the -2.00 D lenses was recorded by a computer.

In both experiments, inward shifts of the dark focus were observed after the near task. In experiment 1, after the near task, static accommodative responses also showed a small but statistically significant inward shift. Neither postadaptation effect differed between refractive groups. The only difference between groups was that late-onset myopes had a lower slope of the accommodative stimulus/response function, both pre- and postadaptation. In experiment 2, both refractive groups showed the same results. After the near task, the duration for accommodation from near to far (relaxation) increased but the duration for accommodation from far to near (stimulation) did not change.

Late-onset myopes have shallower accommodative stimulus/response functions. As suggested in a previous study, this may be due to their reduced sensitivity to defocus. In both emmetropes and late-onset myopes, the near task causes an increase in static accommodative responses. Although our results show it to be a small increase, it is consistent with predictions of Hung and Semmlow's model of accommodation. In both emmetropes and late-onset myopes, the near task also increases the duration for relaxing accommodation, but not for stimulating accommodation. This suggests there are two subsystems which may adapt to prolonged accommodation differently.