Myopia has reached epidemic levels among teenagers and young adults.1,2 It has no cure, and traditional optical interventions do not treat the core problem of anomalous eye growth. Myopia progression has been associated with extended near work,3,4 and children typically exhibit large accommodative lags when viewing near targets placing the image plane behind the retina.5,6 Also, experimental studies with infant monkeys showed that eyes elongated axially (myopia developed) in response to experimentally induced hyperopia.7,8 Therefore, there is evidence from human and animal studies that habitual exposure to hyperopic defocus (image plane behind the retina) can trigger the anomalous eye growth that is the core characteristic of myopic eyes.7–10
Removing chronic hyperopic defocus has been attempted by fitting young progressing myopes with presbyopic spectacle lens designs (e.g., bifocals) such that during near work the accommodative demand will be reduced, which in turn will reduce the accommodative lag and its associated hyperopic defocus.11 Such near adds have shown varied success when prescribed for young myopes ranging from zero impact up to a 50% reduction in progression.12–17 Reduced efficacy may reflect failure of children to use the bifocal add18 and thus the continued presence of hyperopic defocus.
Zonal bifocal and multifocal contact lenses, which simultaneously image through a distance and near correction, have also been used as myopia control tools.19,20 However, their impact on the sign and magnitude of defocus is complicated by the multiple optical powers involved in creating the retinal image, which can vary significantly with pupil size,21,22 and retinal eccentricity.23 If proximal and binocular convergence cues dominate accommodation,24 young patients fit with bifocals may accommodate to focus the retinal image generated by the distance optic when viewing near targets with multifocal contact lenses, creating a myopically defocused foveal image (in front of the retina) with the near optic. However, if a young patient fit with bifocal contact lenses accommodates less and focuses the image generated by the near optic, the second image generated by the distance optic will now be focused in a plane behind the retina (exaggerated hyperopic defocus). Studies of monkeys have shown that anomalous eye growth can be attenuated even in the presence of hyperopic defocus by simultaneously including a second focused or myopically defocused image.25 Consistent with these monkey studies, myopic human children fit with bifocal and multifocal contact lenses reported reductions of myopia progression between 0.25 and 0.60 diopters (D) over 10-month to 2-year periods.10,26–28 However, little is known of the accommodative behavior of young myopes fit with these multifocal lenses designed originally to treat presbyopia.
Previous studies of young adult eyes viewing monocularly with center-near multifocal contact lenses reported either no effect of the near add on accommodation29,30 or reduced accommodation response with center-distance multizone bifocal contact lenses.31 These studies fail to establish the presence or absence of hyperopic defocus in the clinically relevant binocular viewing conditions. The current study examined accommodative behavior of young adults fit bilaterally with two different designs of multifocal contact lenses to assess the effectiveness of these lenses at removing foveal hyperopic defocus during binocular near viewing.
Eight healthy young adult subjects aged between 23 and 29 years (mean ± SD, 27.25 ± 2.05 years) with right and left eye spherical equivalents between 0.00 and −4.50 D (mean ± SD, OD −1.78 ± 1.6 and OS −1.75 ± 1.80 D, respectively) were recruited. None had any history of binocular vision anomalies, ocular pathology, or refractive surgery, and all had distance corrected visual acuity of 20/20 or better. Subjects were fit bilaterally with three different contact lens designs: (1) CooperVision Biofinity aspheric single-vision contact lenses, with nominal power of −0.25 D; (2) CooperVision Biofinity center-distance multifocal contact lenses, with nominal distance power of −0.25 D, add power of +2.00 D; (3) CooperVision Biofinity center-near multifocal contact lenses, with nominal distance power of −0.25 D and add power of +2.00 D. All three contact lenses had a base curve of 8.6 mm and diameter of 14.0 mm and were manufactured from 52% comfilcon and 48% water. The central zones for center-distance and center-near multifocal contact lenses equal to 3 and 2.5 mm, respectively.32–34 A fixed sphere distance power (−0.25 D) was chosen to avoid variability in the levels of introduced spherical aberration associated with different sphere powers.35 Prior to fitting each subject with these contact lens designs, subjective refraction was performed to determine the distance refractive errors, which were corrected (if needed) during accommodation measurements using trial lenses placed at the spectacle plane after subtracting the distance power of these contact lens designs (−0.25 D). The single-vision contact lens data are considered control data for the multifocal case,30,31 and differences in accommodative behavior between single-vision and multifocal contact lenses can be attributed to the difference in optical design.
A high-resolution single-pass Shack-Hartmann aberrometer (ClearWave; Lumetrics, Inc., Rochester, NY) confirmed that the trial lenses used in this study contributed a mean of less than 0.001 μm of spherical aberration for the pupil sizes observed during the experiment. Power maps of each lens were measured ex vivo using the ClearWave single-pass Shack-Hartmann aberrometer.36
Each subject was given 10 to 15 minutes to adapt their ocular surface to the new contact lenses. However, subjects were prevented from viewing near targets through the new contact lenses with their distance corrections in place, thus preventing visual and/or accommodative adaptation to the novel optical scenarios encountered during the accommodation measurements.
A high-resolution (210-μm-diameter lenslets) Shack-Hartmann aberrometer (Complete Ophthalmic Analysis System; AMO WaveFront Sciences, LLC, Albuquerque, NM) was used to measure the accommodation and pupil behavior while subjects viewed a high-contrast 20/40 character on a 400-cd/m2 white background sequentially moved from 2 m to 20 cm (0.50 to 5.00 D) in 0.25-D steps. Target size was adjusted on a microdisplay (852 × 600, 15-μm2 pixels) to remain 20/40 for each viewing distance, which generated character stroke widths of 80 and 8 pixels at the farthest and closest test distances, or always below one eighth of the display Nyquist limit. The aberrometer used a near-infrared light source (840 nm) and reported aberrations scaled to 555 nm. The aberrometer was modified by rotating the instrument 90 degrees temporal to the primary line of sight of the right eye and reflecting the instrument optical axis into the eye via a 45-degree “hot mirror,” through which the right eye viewed the stimulus. A diagram of the apparatus and additional technical details are given in a previous article.37 This modification makes the spectacle plane and not the pupil plane conjugate with the aberrometer lens-let array, and thus all optical measurements are made at the spectacle plane.
The microdisplay was moved along a rail directly toward the right eye requiring no eye rotation to maintain fixation, but during binocular viewing, the left eye must rotate to maintain fixation as the target approaches. During data collection, subjects were instructed to fixate the letter and keep it as focused as possible (single and focused in binocular experiments). In order to ensure any pupil constriction observed was purely due to accommodation/convergence and not due to increasing amounts of light entering the eye as the display moved closer to the eye, a white board placed at 3 m was illuminated by a projector at approximately the same 400 cd/m2 generated by the microdisplay. This board covered the full field of view of the microdisplay when at 20 cm, and thus, as the microdisplay was moved away from the eye, the corneal plane illuminance remained constant at approximately 10 lux. All measurements were taken with room lights off. The sequence of monocular/binocular testing with the three test lenses was randomized.
Wavefront data were fit with Zernike polynomials up to the eighth order, and five repeat measurements were made for each condition. The accommodative response was quantified by monitoring changes in refractive state associated with changing target vergence. Three criteria were used to define refractive state of the eye:
- (1) Minimum root mean square refractive state: the sphere lens that corrects the overall spherical wavefront curvature across the entire pupil and minimizes the total wavefront root mean square error. It is pupil size dependent and reflects the refractive characteristics of the peripheral optics (70% from pupil center).38 Therefore, this criterion reflects the refractive state in the midperiphery, hence the surround near and distance optics for center-distance and center-near multifocal contact lenses, respectively. It can be calculated from wavefront data by Equation 1:
- where minRMS is minimum root mean square,
- is the Zernike defocus coefficient, and R is the pupil radius.
- (2) Paraxial refractive state is the lens that flattens the wavefront at the center of the pupil. This should not be pupil size dependent (although Montés-Micó et al.39 found small changes in central optics as pupil size changed) and should be unaffected by the peripheral optics. This refractive state defines the correcting lens or target vergence that cancels the r 2 terms within
- (Equation 2).
- are coefficients for defocus, and primary, secondary, and tertiary spherical aberrations, respectively. This criterion therefore will reflect the refractive state of the central distance and near optics for center-distance and center-near multifocal contact lenses, respectively. For example, for an eye wearing center-near multifocal contact lens and the lens is well centered, the expected paraxial refractive state (eye + contact lens) when eye's accommodation is relaxed would be 2.00 D of myopia and 0.00 D when wearing center-distance multifocal contact lens. Previous studies have defined paraxial refractive state of eyes using r 2 values embedded in second- and fourth-order polynomials,40,41 and others have also included higher-order terms in this calculation (up to the sixth order).42 When fit with a single-vision contact lens, canceling the r 2 terms in
- provided a pupil size independent refractive state measure. When fit with center-distance and center-near multifocal contact lenses, pupil independency of paraxial estimates of refractive state required r 2 terms from
- with the center-distance multifocal and r 2 values from
- with the center-near multifocal.
- (3) Peak image quality. The target vergence that maximized image quality was quantified using the visual Strehl ratio calculated from the optical transfer function metric,43 using a through-focus analysis in which a series of sphere lenses are introduced to identify which lens produced peak image quality for each contact lens and stimulus condition.
We also determined the local maximum/minimum refractive state, which was used to determine the pupil location containing the multifocal contact lens optical center (see examples in Fig. 1). For a well-centered multifocal contact lens, the most myopic or hyperopic refractive state will appear in the pupil center for a center-near and center-distance multifocal designs, respectively. However, because soft contact lenses usually tend to decenter temporally and inferiorly,44–46 eyes fit with center-near and center-distance multifocals exhibited a peak and minima in power that was generally decentered in the pupil.
Clinical subjective refractions were obtained and corrected in the spectacle plane of the instrument, and therefore residual uncorrected refractive errors were typically small (mean, 0.15 D). Refractive state and target vergence were quantified at the spectacle plane relative to the distance corrected far point.47 A myopic shift in the measured refractive state can therefore reflect either added positive power of a contact lens near optics or increased positive power of the eye (accommodation).47
The refractive state and target vergences relative to the spectacle corrected far points were transferred to the pupil plane using Equations 3 and 4.
where TVSP is the target vergence specified at the spectacle plane, DL is the eye's distance spectacle correction (lens power in trial frame), VD is the back-vertex distance of the spectacle lens measured in mm for each eye tested, and TVpp is the target vergence at the pupil plane relative to the eye's far point, which is equivalent to minus the eye's accommodative stimulus.
where RSSP is the measured refractive state at the spectacle plane as reported by aberrometer, and RSPP is the refractive state at the pupil plane relative to the eye's distance refractive error (change in refractive state introduced by multifocal lenses or accommodation).
The measured wavefront size served as an estimate of the eye's entrance pupil (Equation 5), in which marginal pupil refraction was used because it determined the rays that formed the exiting beam size and thus the effective pupil size at the measurement plane.
In this equation, P pp is the pupil size (mm) at the eye's entrance pupil plane, P SP is the pupil (mm) measured by the aberrometer at the spectacle plane, measured RS is the marginal refractive state measured at spectacle plane, and VD is the lens to entrance pupil plane vertex distance. The aberrometer measures pupil size by the size of the beam exiting from the eye, which in our setup is captured at the spectacle plane. For a myopic eye, the exiting beam from the eye will be converging such that the beam size at the spectacle plane is smaller. Equation 5 uses the measured defocus values plus knowledge of the distance refractive correction to compute beam size as it exits the eye or equivalently the eye's entrance pupil.
Because of the small sample size in this study, nonparametric Wilcoxon signed rank test was used to determine statistical difference between the gains (refractive state/target vergence) when viewing monocularly and binocularly for each contact lens design.
Determining Refractive State with Multifocal Contact Lenses
The complexities of determining refractive state in eyes fit with multifocal lenses can be visualized by examining the power distributions across the multifocal contact lenses (Fig. 1, left column) and refractive state maps across the pupil when these lenses are fit on an eye that is accommodating to different target vergences (Fig. 1, right columns). Ex vivo maps reveal that the single-vision lens is aspheric with a power of −1.00 D at the edge of the measured 8-mm pupil with the central power of −0.05 D. Ex vivo power maps for the center-distance or center-near multifocals reveal central power of −0.50 and +1.50 D, respectively, with power in the outer zones of +1.50 and −0.50 D, respectively, consistent with an approximately +2.00 D add in both the center-distance and center-near lenses. The small central zones and rapid changes in power associated with transitions between center and surround optical zones mirror those previously described for these lenses.32–34
The refractive state maps of distance corrected eyes (RS = −(dWFE/dr)/r)48 over the measured pupil diameters (between 3.5 and 5 mm) reveal the consistent myopic drift as the accommodation levels increased from the left-to-right panels. When fit with either the center-distance and center-near multifocals, refractive state varies dramatically across the pupil (increased contour density in the maps). The aspheric single-vision lens introduces negative spherical aberration into this eye + contact lens, leaving the optics over the pupil center slightly more myopic than the surround (in this example, the contact lens center is located inferior to the pupil center). A similar but exaggerated trend can be seen in the eye fit with center-near multifocal (bottom row), which introduced high levels of negative spherical aberration. The converse pattern is seen when fit with the center-distance multifocal, where the pupil surround was generally greater than 2.00 D more myopic than the pupil center. In both of the multifocal contact lens examples, the lens center appears slightly inferior and temporal to the pupil center, a typical decentration seen with soft contact lenses.44–46
With the single-vision lens, the pupil center is well focused for all viewing distances (paraxial refractive state = target vergence, black dotted ring in pupil center), whereas with the multifocal lens examples, a region of the pupil between those that define the minimum root mean square error and peak image quality refractive states is generally focused (black dotted contours are generally observed between the blue and black dashed lines). If a midperipheral region of the pupil is well focused, the minimum root mean square refractive state will also be reasonably well focused, and the pupil center and margins will be defocused with opposite signs. In the case of the center-distance and center-near multifocal, therefore, the expectation is for the paraxial refractive states to be hyperopically and myopically defocused, respectively, and the converse will be true for the marginal refractive states. These predictions are made more complicated by contact lens decentration; for example, in the sample center-distance maps in Fig. 1, the pupil center (red circle) is aligned with the transition zone of the multifocal contact lens.
All three of the tested contact lenses introduced changes in the measured spherical aberration of the eye + contact lens over the natural pupil sizes (Fig. 2). The negative primary spherical aberration of the aspheric single-vision lens (
= −0.31 μm for an 8-mm pupil) was sufficient to cancel the generally positive spherical aberration levels of the unaccommodated tested eyes (eye + contact lens
at target vergence of −0.50 D was −0.002 ± 0.05 μm). When fit with center-distance and center-near multifocals, primary spherical aberration in the unaccommodated eyes was positive (
= +0.17 ± 0.11 μm) and negative (
= −0.03 ± 0.07 μm), respectively. However, because of the significant secondary and tertiary spherical aberration introduced by the multifocal lenses, most of the r 4 wavefront error changes are included in the
polynomials.49 We capture the total r 4 wavefront error by plotting the Seidel primary spherical aberration coefficient in Fig. 2B (
), which reveals the elevated levels of positive and negative spherical aberration introduced by the center-distance and center-near lenses, respectively, when on the eye. For all three contact lens designs, there was a negative shift in primary spherical aberration as the eyes accommodated (−0.03 μm of
per diopter of target vergence for natural pupils). As predicted, therefore, the negative spherical aberration observed at distance in eyes fit with the center-near multifocal become even larger at near (
increased from −7.60 μm to −9.65 μm) despite the associated accommodative pupil miosis. At a target vergence of −5.00 D, the ocular negative spherical aberration introduced by accommodation was almost sufficient to cancel the positive spherical aberration of the center-distance multifocal, reducing its effective multifocality at near.
In order to achieve the desired expanded depth of field, multifocal contact lenses inevitably reduce peak image quality.21 Also, the image quality achievable with multifocal lenses is affected by ocular high-order aberrations, pupil size, and contact lens decentration.21,50 Pupil diameters were smaller during binocular than monocular viewing (mean ± SD, 3.62 ± 0.48 and 4.80 ± 0.75 mm and decreased by −0.16 ± 0.13 and −0.25 ± 0.16 mm/D of accommodation for all three lens types, respectively). Using the experimentally observed pupil sizes, through-focus image quality calculations were performed to identify the target vergence that maximized image quality. Because the presbyopic contact lenses used in this study are multifocals,32,34 through-focus plots of image quality mostly exhibited a dominant image quality peak (Fig. 3), which is consistent with reports of a single peak in through-focus visual acuity and contrast sensitivity plots for eyes fit with Biofinity multifocal lenses51 or other multifocal presbyopic corrections.52,53 With single-vision lenses, these plots reveal that young adult eyes typically achieve peak image quality for a distance slightly farther than the target vergence (−0.26 D of accommodative lag). Similar behavior is seen in the center-distance corrected eyes, with peak image achieved for target vergence −0.46 D beyond the stimulus plane (for target vergence ≥1.00 D). The converse was found with center-near lenses where subjects achieved optimum image quality +0.88 D closer than the target vergence. The resulting mean image quality values as the eyes accommodated were similar to those previously reported for eyes fitted with single-vision and center-distance contact lens (peak visual Strehl ratio calculated from the optical transfer function = 0.21 and 0.19, respectively)54 but were lower when fitted with center-near multifocal contact lenses (peak visual Strehl ratio calculated from the optical transfer function = 0.14).
Figures 4A to C plot the mean change in refractive state as target vergence varied for single-vision, center-distance, and center-near multifocal lenses. In addition to the paraxial (blue symbols) and minimum root mean square (red symbols) refractive state measures, the refractive states that maximize image quality (peak visual Strehl ratio calculated from the optical transfer function, black symbols) are also shown. With single-vision lenses, there was no significant difference between binocular (filled symbols) and monocular (open symbols) accommodative gains (P = .89 for both refractive state criteria), but gain was higher for paraxial than for minimum root mean square refractive criteria (1.06 and 0.97, respectively). Eyes fit with single-vision lens (Fig. 4A) exhibit no evidence of an accommodative lead (refractive state is not more myopic than required to focus the target) for target vergences lower than −1.00 D and no evidence of an accommodative lag paraxially (refractive state is not less myopic than required to focus the target) at near, but a small and consistent lag (−0.35 ± 0.06 D) in the minimum root mean square refractive states for target vergences greater than −1.50 D. As predicted for eyes with relatively low levels of spherical aberration,38 peak image quality was achieved with correcting lenses similar to those that minimize wavefront root mean square (compare red and black symbols in Fig. 4A). The paraxial gains of more than 1 seem counterintuitive (more accommodation than needed), but with the drift to greater levels of negative spherical aberration as the eyes accommodated (Fig. 2), the pupil center became more and more myopic relative to the refractive state at the pupil's midperipheral region. Therefore, if the subjects were accommodating by focusing the pupil's midperiphery with a high gain (0.97 D/D in Fig. 4A), a paraxial gain of more than 1 is anticipated.55
Multifocal contact lenses such as those used in the present study are specifically designed to create different refractive states in different zones of the pupil, for example, the center zone and annular surround are designed to differ by 2.00 D in the lenses tested. Measuring refractive states in eyes fit with these lenses is complicated further by the significant area of the lens over which optical power gradually transitions from distance to near (Fig. 1). Also, because of ocular spherical aberrations, which vary in sign and magnitude with accommodation,41,56,57 the resultant changes in refractive state across the pupil depend on both contact lens and ocular aberrations.21 This interaction is especially significant in the pupil margins, making a determination of distance refractive error especially challenging for eyes fit with center-near lens designs. Experimental results show that some zonal bifocal/multifocal lens designs retain a clear peak in subjective image quality with target vergence equal to the single-vision determined distance refractive error, whereas others do not.58 In order to disentangle these multiple factors influencing refractive state, we refrained from performing a distance overrefraction of eyes fit with the multifocal lenses because it might have been corrupted by the lens add powers. Instead, the aberrometer-measured refractive states and target vergences are plotted relative to the far point measured in the instrument with the single-vision lenses. Target vergences and refractive states therefore represent stimuli and retinal conjugate planes closer to the eye than the single-vision measured far point.
Nonzero measured refractive states can reflect the added power in the multifocal contact lens, added power in the eyes marginal optics, or accommodation. Most important, with this plotting format, measured refractive states that do not equal the target vergence (the dashed y = x line) indicate relative myopia (above the line) or relative hyperopia (below the line) compared with the stimulus plane.
For eyes fit with center-distance multifocal lenses (Fig. 4B), measured paraxial refractive state was almost zero (−0.05 D) for target vergences between zero and −1.00 D, indicating zero accommodation. As expected from the surround transition and near add zones, the average measured minimum root mean square refractive state was more myopic, −0.83 D when binocularly viewed and −1.25 D when viewing was monocular, both significantly less than the −2.00 D expected from the +2.00 D added to the outer annular zone. All data reveal very little accommodative response over this most distant range of target vergences (gain = 0.30 for target vergences between −0.25 and −1.25 D). Once the target vergence was greater than −1.50 D both paraxial and minimum root mean square refractive states increased to track the target vergence, and the gains approached 1. The minimum root mean square refractive states remained close to the target vergence, but paraxial refractive states were always defocused hyperopically (mean relative hyperopia at the pupil center for target vergences greater than −1.00 D = −0.85 D).
To retain focus of the paraxial optics, therefore, subjects fit with the center-distance lenses would have to increase their accommodation as the target vergence was increased from −0.50 D. However, to focus the more peripheral regions of the pupil, subjects would have to reduce their accommodation. The approximate focus achieved with the minimum root mean square refractive states revealed that subjects fit with the center-distance multifocal were accommodating to achieve focus in the pupil midperiphery, taking advantage of some of the add power provided by the lens to focus near targets, and leaving the pupil center hyperopically defocused. The negative drift in ocular spherical aberration as the eyes accommodated (Fig. 2) resulted in less positive spherical aberration for the eye + contact lens combination at near and thus smaller differences between the paraxial and minimum root mean square measures of refractive state.
Eyes fit with center-near multifocals (Fig. 4C) exhibited approximately stable paraxial and minimum root mean square measures of refractive state (−1.90 and −0.92 D, respectively) for target vergences lower than −1.00 D consistent with the +2.00 D add in the pupil center and little accommodation over this stimulus range. However, as target vergence increased, the minimum root mean square refractive states observed with binocular viewing closely matched the target vergences (gain = 0.96 D/D, for target vergence greater than −1.50 D with a mean error of +0.10 D). With binocular viewing, paraxial refractive state exhibited approximately a −1.25 D of relative myopia over this dioptric range. Unlike with single-vision lenses, monocular paraxial and minimum root mean square accommodative gains with the center-near multifocal were both lower (0.84 and 0.71 D/D, respectively) than binocular accommodative gains (1.08 and 0.97 D/D, respectively) (P = .07 and .01, respectively), resulting in hyperopically defocused monocular minimum root mean square refractive states for target vergences greater than −2.00 D.
The failure to obtain optimum image quality or take full advantage of the add power when viewing binocularly may reflect the powerful influence of binocular convergence on the accommodative response.24,26,59,60 This effect is clearly seen in the difference between monocular and binocular refractive states of one subject fit with the center-near lenses (Fig. 5). When viewing binocularly, the measured right eye retained approximate focus with minimum root mean square refractive state for all target vergences and was myopically defocused paraxially. However, when viewing monocularly, their accommodative behavior changed as the target approached the far point of the added near correction (50 cm). At approximately 40 cm (−2.50 D of target vergence), this eye relaxed its accommodation and by doing so shifted from minimizing the root mean square to focusing their paraxial optics. This behavior is consistent with subjects switching from focusing the midperiphery to central (near add) optic. This accommodative switching behavior was never observed with binocular viewing.
The current study aimed to determine the accommodative behavior of young adult eyes fit with multifocal contact lenses. Can multifocal contact lenses remove hyperopic defocused images from young adult eyes viewing near targets or introduce myopically defocused images, both of which have the potential to control myopia development?10,25 The complexity of the refractive state maps across the pupils (Fig. 1) necessitated examining multiple different definitions of accommodation including monitoring the retinal conjugate planes that maximized image quality, focused the pupil center, and minimized overall wavefront error (Figs. 1 and 4). When fit with multifocals, subjects generally refrained from accommodating to target vergences lower than −1.50 D, but as the target vergence increased from −1.50 D to −5.00 D, subjects accommodated with high gain when viewing binocularly. This behavior resulted in the minimum root mean square defined refractive state being quite accurate and consequentially resulted in hyperopic defocus image (accommodative lag) paraxially when fit with center-distance lenses and paraxial myopic defocus image (accommodative lead) when fit with the center-near lenses. Accommodative responses that minimized the wavefront error also produced defocus in the pupil margins that was opposite to that observed paraxially (see examples in Fig. 1).
These results reveal that young accommodating adults fit with multifocal contact lenses do take advantage of the lens add powers to reduce their accommodative responses. Previous results observed with single-vision optics showed that young adults viewing binocularly can focus the retinal image of near targets when viewing through plus lenses by relaxing their accommodation (≥2 D of negative relative accommodation).24,60 However, the influence of binocular convergence will tend to dominate the accommodative response as the blur stimulus is degraded (opening the defocus loop).24,59 Our binocular viewing data are consistent with less than normal levels of negative relative accommodation (subjects did not reduce their accommodative responses by the 2.00 D add power provided by these lenses), perhaps due to the partial opening of the defocus loop by using multifocal optics. Similar reduced accommodation was observed in binocularly viewing children fit unilaterally with a bifocal contact lens.26
Our monocular viewing data observed with the center-near lens (Figs. 4C and 5) support the hypothesis that convergence-driven accommodation prevents young subjects from taking full advantage of the near add. With binocular vergence open loop (monocular viewing), subjects generally relaxed their accommodation as the target approach to focus the center-near optic resulting in retinal conjugate planes that optimize image quality (open diamonds in Fig. 4C) or focus the paraxial optics (circles in Fig. 5B). Therefore, it may be the influence of binocular convergence, and the resulting less than optimum negative relative accommodation that placed optimum image quality planes beyond (lag) or closer (lead) than the actual stimulus when viewing binocularly with center-distance and center-near lenses, respectively.
This bias to focus with the midperipheral pupil may also reflect the choice of multifocal contact lenses used in this study, which had a single central optical zone (3- and 2.5-mm diameter for center-distance and center-near designs, respectively) surrounded by an annular transition zone, which was also surrounded by an aspheric surround optical zone.32–34 Because the tested lenses were concentric multifocal designs, the images generated depend critically on pupil diameter (mean, 3.66 ± 0.49 mm and 3.64 ± 0.58 mm for center-distance and center-near designs, respectively).21,22,50,61 Therefore, for the 3- to 5-mm pupil diameters routinely encountered in our study, the “transition” zone surrounding the central zone often had an area larger than that of the center-near design center and surround optical zones and thus contributed most light to the image (Fig. 6). The transition zone has a rapidly changing power (e.g., ≈2 D/mm34). Accommodative pupil miosis of −0.14 mm/D, which reduced pupil diameter from a mean of 3.84 mm when viewing at 2 m to 3.31 mm when viewing binocularly at 20 cm introduced a small change in the proportion of light being imaged by each zone.
The amount of light passing through the central, transition, and outer zones will also be affected by lens decentration.62 Fig. 7 plots proportion of pupil area covered by each of the three zones as a function of lens decentration while wearing center-distance multifocal (left column) and center-near multifocal (right column). For most pupil diameters included in the current study (3.0 to 5.0 mm) and typical single-vision contact lens decentrations of 0.75 mm or less,44 a combination of all three zones contributes to the final image. Specifically, in the center-near multifocal case, the highly aberrated transition zone covers the largest area of the pupil for 4.0- and 5.0-mm pupil diameters and small decentrations.
Although calculations in Fig. 7 reveal often significant changes in the contributions of the center, transition, and outer zones as the contact lens decenters, a recent study by Rio et al.50 using a simulated bifocal contact lens and decentration of less than 3/4 mm found little effect of decentration on perceived image quality. However, modeling by Chateau and Baude61 emphasized the impact of lens decentration on the choice of center zone size. Also, large decentrations are not typically experienced with soft contact lenses,44–46 being typically less than 1 mm. However, because of the parallax caused by the axial separation of the contact lens and the iris planes, large effective decentrations may exist for optical beams originating in the peripheral field (see similar calculations for off-axis decentration of zonal bifocal contact lenses and for small pupil corneal inlays).63 Because of this parallax effect (decentration = ACD * tan(θ), where ACD = axial separation between contact lens and entrance pupil, and θ is the peripheral field angle), a more and more peripheral region of the corneal plane, and thus contact lens, will image the beam passing through the pupil from peripheral field locations. For example, for an axial separation of 3 mm between the contact lens and entrance pupil planes, no light will pass through both the 3-mm central zone and the pupil when the parallax reaches 3 mm, which will happen at a field eccentricity of 45 degrees. This dominance of the surround optics in the peripheral field is a core principle behind some myopic control lenses designed to provide myopic defocus in the peripheral retina and have successfully introduced myopic shifts between 1 and 2 D at field eccentricities of 30 to 35 degrees.23,64 Of course, by the same principle, center-near lenses will not introduce myopic defocus in the peripheral retina.
In conclusion, our data reveal that young accommodating subjects viewing binocularly through center-distance and center-near multifocal contact lenses with significant transitions zones generally do not focus either the pupil center or pupil margins. However, because of the limited sample size (eight subjects), it would be inappropriate to assert that the sample means (Figs. 2 and 4) are representative of the broader patient population.
The observed behavior may reflect the fact that the transition zones of the tested lenses dominated the pupil. Also, the reduced quality of the multifocal retinal image may have partially opened the defocus loop for accommodation amplifying the control influence of convergence on the resultant accommodation responses to near objects. Because the transition zones were reasonably well focused at near, the center zone of the center-near design was myopically defocused, as were the pupil margins of eyes fit with center-distance design. Therefore, in both cases, some part of the pupil was myopically defocused, which according to Arumugam et al.25 may slow any myopic eye growth stimulated by hyperopic defocus caused by accommodative lags. Conversely, the degraded image quality observed when wearing multifocal contact lenses52,53 might serve as a stimulus to anomalous eye growth.65 It is clear from this study that zone geometry and pupil size may have an impact on accommodative behavior with these contact lens designs. Before making conclusions about the implications of these data for myopia control, this study needs to be replicated on the target population of progressing young myopes and to supplement the day of fitting to data collected after the myopia control lenses have been used for perhaps a week or more. Finally, the large range of refractive states observed across the pupil in these young eyes fit with multifocal contact lenses raises the obvious concern that subjective “overrefractions” performed in these eyes may be biased by the pupil regions containing the distance, near, or transition zones.
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