Differential refractive susceptibility to sustained nearwork.
OBJECTIVE
A possible role for nearwork-induced transient myopia (NITM) (i.e. transient myopia at distance following sustained nearwork) in the development and/or progression of permanent myopia has been suggested. If certain refractive groups are more susceptible to NITM than others, then measures may be implemented to prevent or retard the occurrence of myopia in these individuals. In a recent study (Ciuffreda and Wallis, 1998), it was found that young-adult late-onset myopes (LOM) and early-onset myopes (EOM) were highly susceptible to this NITM accommodative after-effect, while emmetropes (EMMs) were only moderately susceptible, and hyperopes (HYP) were particularly insusceptible, following short periods (10 min) of sustained nearwork at a very close distance (20 cm, 5 D). In the current study, this was extended to encompass a much longer period of nearwork under more naturalistic conditions.


METHODS
Young-adult subjects (n = 16) performed a reading task for a continuous period of 4 h at their habitual working distance under naturalistic conditions, i.e. binocular viewing with blur-related visual feedback present. Distance refractive state (binocular viewing) was assessed objectively using a Canon R-1 autorefractor immediately before (pre-task), every hour during the task, and immediately after (post-task) to assess NITM and its subsequent decay to pre-task refractive levels. Subjects were divided into the four refractive subgroups as above, which were each of equal size (n = 4).


RESULTS
The NITM was induced in all subjects, except for the hyperopes in which there was a hyperopic refractive shift. The LOMs (0.12 D) and EOMs (0.13 D) were highly susceptible, while the HYPs were especially insusceptible (0.44 D hyperopia), and the EMMs were only moderately susceptible (0.09 D).


CONCLUSIONS
The present study extended our earlier investigation by demonstrating the differential refractive susceptibility over a longer period of sustained nearwork, which may be the result of reduced sympathetic activation. It has been speculated that NITM may be a factor in myopigenesis, with a possible mechanism being time-integrated increased retinal defocus from NITM at both distance and near, in conjunction with the normal lag of accommodation during sustained nearwork, thereby causing axial elongation. Hence, a future longitudinal investigation specifically targeting this retinal defocus hypothesis is warranted.
D000065:Accommodation, Ocular; D000293:Adolescent; D000328:Adult; D005260:Female; D006801:Humans; D006956:Hyperopia / Q000503:physiopathology; D008297:Male; D009216:Myopia / Q000503:physiopathology*; D011932:Reading; D012029:Refraction, Ocular; D013997:Time Factors; D015348:Vision, Binocular