Accommodation in emmetropic and myopic young adults wearing bifocal soft contact lenses

Abstract

Purpose: To assess the effect of bifocal soft contact lenses on the accommodative errors (lags) of young adults. Recent studies suggest that bifocal soft contact lenses are an effective myopia control treatment although the underlying mechanism is not understood.

Methods: Accommodation responses were measured for four target distances: 100, 50, 33 and 25 cm in 35 young adult subjects (10 emmetropes and 25 myopes; mean age, 22.8 +/- 2.5 years). Measurements were made under both monocular and binocular conditions with three types of lenses: single vision distance soft contact lenses (SVD), single vision near soft contact lenses (SVN; +1.50 D added to the distance prescription) and bifocal soft contact lenses (BF; +1.50 D add).

Results: For the SVD lenses, all subjects exhibited lags of accommodation, with myopes accommodating significantly less than emmetropes for the 100 and 50 cm target distances (p < 0.05). With the SVN lenses, there was no significant difference in accommodative responses between emmetropes and myopes. With the BF lenses, both emmetropic and myopic groups exhibited leads in accommodation for all target distances, with emmetropes showing significantly greater leads for all distances (p < 0.005).

Conclusions: Overall, myopes tended to accommodate less than emmetropes, irrespective of the contact lens type, which significantly affected accommodation for both groups. The apparent over-accommodation of myopes when wearing the BF contact lenses may explain the reported efficacy as a myopia control treatment, although further studies are required to elucidate the mechanism underlying this accommodative effect.

Figures

Figure 1

Refractometer readings (left panel) and accommodative errors (right panel; lags or leads; mean ± S.E.) measured through either (a) single vision distance contact lenses (SVD), (b) single vision near contact lenses (SVN), or (c) bifocal contact lenses (BF). Emmetropes consistently exhibited less accommodative lag (negative values) and/or more lead (positive values) than myopes. † significant intergroup differences (p < 0.05).

Figure 2

Data from Figure 1 replotted to contrast the effect of lens type on accommodation for (a) emmetropes and (b) myopes. Single vision near (SVN) and bifocal (BF) lenses reduced accommodative lags and/or resulted in accommodative leads; differences significant for single vision distance (SVD) vs BF († p < 0.017), SVD vs SVN (‡ p < 0.017); SVN vs BF (* p < 0.017).

Figure 3

Ocular wave aberrations calculated over a 5-mm pupil for 0.4 µm of Zernike defocus, 0.15 µm of Zernike spherical aberration and the two combined, and simulated retinal images of a 20/50 Snellen E, for each of the three conditions.

Figure 4

Ocular wave aberrations (µm) calculated over a 5-mm pupil for an accommodating eye with −0.15 µm of Zernike spherical aberration (Cheng et al., 2004a), and a range of negative Zernike defocus. Simulated retinal images of a 20/50 Snellen E shown for the same conditions.

Figure 5

Ocular wave aberrations (µm) calculated over a 5-mm pupil for an accommodating eye with 0.3 µm of Zernike spherical aberration, to simulate the effect of a bifocal contact lens, and a range of positive Zernike defocus. Simulated retinal images of a 20/50 Snellen E shown for the same conditions.