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J.M. Tarrant, H. Severson, C. Wildsoet; How Accurate is Accommodation Through Bifocal Soft Contact Lenses? . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5596.
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Purpose: To assess the effect on the accommodation of young adult emmetropes and myopes of bifocal soft contact lenses, a potential alternative to progressive addition spectacles as a myopia control treatment. Methods: A Grand–Seiko optometer was used to measure accommodation responses for 4 target distances: 100 cm, 50 cm, 33 cm and 25 cm in 36 subjects (11 emmetropes and 25 myopes). Measurements were made with each of 3 different types of corrections: single vision distance contact lenses (SVD), bifocal contact lenses (BF; +1.50 D add) and SV near contact lenses (SVN; +1.50 D added to distance prescription). The SVN lenses were included for comparison as it is likely that the effective add of the BF varied between subjects. Accommodation responses were compared with accommodation demands, taking into account the compensation offered by the lenses. Derived accommodative errors were compared across lens types and refractive groups. Results: Overall, myopes tended to accommodate less than emmetropes, irrespective of the lens type worn and both the BF and SVN lenses elicited reduced accommodation responses compared to the SVD lenses. For the SVD lenses, all subjects exhibited lags of accommodation although the myopes tended to accommodate less than emmetropes for all target distances. Lags ranged from 0.36 to 0.65 D for myopes and 0.23 to 0.50 D for emmetropes. With the SVN lenses, myopes also showed lags of accommodation for all distances while the emmetropes exhibited lags only for the two shortest target distances (33 and 25 cm) and leads for the two greatest distances (100 and 50 cm). Errors ranged from 0.03 to 0.42 D (lag) for myopes and –0.15 D (lead) to 0.15 D (lag) for emmetropes. However, these differences for the SVD and SVN lenses in accommodation responses between emmetropes and myopes were not statistically significant. With the BF lenses, both emmetropes and myopes exhibited leads in accommodation at all target distances. Leads ranged from 0.38 to 0.46 D for myopes and 0.16 to 0.49 D for emmetropes. The responses of the myopes and emmetropes were also significantly different for the 33 cm target distance with the BF lenses (ANOVA, p<0.05). Conclusions: There were three main outcomes: 1. myopes tend to accommodate less than emmetropes, 2. BF contact lenses, like SVN lenses, reduce accommodative demand, 2. all subjects appear to overaccommodate with BF contact lenses. The latter result may represent an artifact of the lens design because the SVN lenses resulted in leads of accommodation for 2 conditions only. While BF contact lenses are an effective method of reducing accommodative effort, whether they also improve the accuracy of accommodation remains to be resolved.
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