Effects of novel multifocal soft contact lens on peripheral refraction of myopic eyes when looking at distant and near targets

Background It is generally accepted the association between hyperopic peripheral defocus and myopia progression. To search for a good optical method to slow the myopia progression for the children who need long-time near work, we compared the effects of novel multifocal soft contact lenses (MFSCLs) with single vision soft contact lenses (SVSCLs) on peripheral refraction when looking at both distant and near targets. Methods The refraction of 25 young myopic subjects’ right eye were measured at horizontal retina eccentricities in 10º steps from 30º temporal to 30º nasal, with no correction (baseline), novel MFSCLs and SVSCLs when looking at distant (5 m) and near (0.4 m) targets. Results Subjects wearing MFSCLs presented signicantly more myopic relative peripheral refractive error (RPRE) prole than SVSCLs at all horizontal retina eccentricities when looking at distant targets (all p<0.01). Compared with looking at distant targets, subjects wearing SVSCLs or MFSCLs showed a hyperopic shift of peripheral defocus when looking at near targets, owning to the effects of accommodative lag and hyperopic RPRE change during accommodation (except T20º and T30º wearing SVSCLs and N30º wearing MFSCLs, p=0.822, p=0.950, p=0.390, respectively, all other eccentricities p<0.05). But subjects wearing MFSCLs could still maintain a certain magnitude of myopic peripheral defocus at horizontal retina eccentricities when looking at near targets (T20° and T30°, p=0.023 and p<0.001, respectively). Conclusions The novel MFSCLs imposed strong myopic peripheral defocus when looking at distant targets. They also maintain a certain magnitude of myopic peripheral defocus when looking at near targets, regardless of the hyperopic effect of accommodation lag and hyperopic shift in RPRE during accommodation.


Abstract
Background It is generally accepted the association between hyperopic peripheral defocus and myopia progression. To search for a good optical method to slow the myopia progression for the children who need long-time near work, we compared the effects of novel multifocal soft contact lenses (MFSCLs) with single vision soft contact lenses (SVSCLs) on peripheral refraction when looking at both distant and near targets.
Methods The refraction of 25 young myopic subjects' right eye were measured at horizontal retina eccentricities in 10º steps from 30º temporal to 30º nasal, with no correction (baseline), novel MFSCLs and SVSCLs when looking at distant (5 m) and near (0.4 m) targets.
Results Subjects wearing MFSCLs presented signi cantly more myopic relative peripheral refractive error (RPRE) pro le than SVSCLs at all horizontal retina eccentricities when looking at distant targets (all p<0.01). Compared with looking at distant targets, subjects wearing SVSCLs or MFSCLs showed a hyperopic shift of peripheral defocus when looking at near targets, owning to the effects of accommodative lag and hyperopic RPRE change during accommodation (except T20º and T30º wearing SVSCLs and N30º wearing MFSCLs, p=0.822, p=0.950, p=0.390, respectively, all other eccentricities p<0.05). But subjects wearing MFSCLs could still maintain a certain magnitude of myopic peripheral defocus at horizontal retina eccentricities when looking at near targets (T20° and T30°, p=0.023 and p<0.001, respectively).
Conclusions The novel MFSCLs imposed strong myopic peripheral defocus when looking at distant targets. They also maintain a certain magnitude of myopic peripheral defocus when looking at near targets, regardless of the hyperopic effect of accommodation lag and hyperopic shift in RPRE during accommodation.

Background
It is generally accepted that the worldwide prevalence of myopia has increased rapidly in the past few decades, especially in Asia [1][2][3]. It is estimated that by 2050, myopia will affect nearly 5 billion people globally, about 50% of the world population, and high myopia will affect 1 billion people, about 10% of the world population [4]. These projections represent a 2-fold increase in myopia, from 22% in 2000, and a 5-fold increase in high myopia, from 2% in 2000.
Many animal and human studies support the association between the relative hyperopic peripheral state of defocus and myopia progression, whereas inducing myopic defocus at the peripheral retina of progressive myopes might slow the progression of central myopia [5][6][7][8]. Thus, several optical means have attempted to reduce the magnitude of hyperopic peripheral defocus or peripherally impose myopic defocus while maintaining good correction of foveal refractive errors. Orthokeratology, multifocal soft contact lenses, custom-designed rigid gas permeable lenses, and some novel contact lens designs can effectively regulate peripheral refraction in myopic eyes [9][10][11]. If the hypothesis is correct, it is important to quantify the effect of these lenses on imposing peripheral myopic defocus.
Second, the relative peripheral refractive error (RPRE) pro le might change during the process of accommodation. Earlier studies were not consistent with regard to how the RPRE pro le was affected by accommodation. Walker and Mutti [14] showed that near work at the 3-diopter stimulus level immediately produced a signi cant hyperopic shift in the RPRE about 0.4 diopters from baseline. Calver et al. [15] also found a small hyperopic shift in the RPRE about 0.5 diopters that was restricted to the temporal retina during accommodation. However, most studies showed that increasing accommodative demand did not alter the RPRE pro le [16,17] or even made a slightly peripheral myopic shift [10,18,19]. The variability might due to diverse measurement techniques (e.g., open-eld autorefractors, scanning photorefractor, and Hartmann-Shack aberrometers), subjects with different refractive status, various lenses were used, different measure time during the process of accommodation, and so on.
Third, how much is the effect of contact lenses' design on changing the defocus pro le? In the study, we compared the effects of novel multifocal soft contact lenses (MFSCLs) with single vision soft contact lenses (SVSCLs) on peripheral refraction when looking at both distant and near targets.

Subjects
Twenty-ve myopic young adults (male 13, female 12) were recruited at The Eye Hospital of Wenzhou Medical University. The inclusion criteria were participants having prescription ranged between −0.50 and −6.00 DS and no more than -1.50 DC of astigmatism (spherical equivalent of refraction's mean ± SD: -3.74 ± 1.32 D), best corrected binocular vision acuity of 1.0 (Decrimal Equivalent) or better. Age between 18-30 years (mean ± SD: 24.9±1.9 years). Participants with manifest strabismus, amblyopia, any ocular diseases, history of ocular surgery, recently contact lenses wearing (rigid gas-permeable lens within 4 weeks; soft contact lens within 2 weeks) were excluded from participation in this study.

Contact Lenses
In this study, we compared two different types of soft contact lenses (novel MFSCLs and common SVSCLs). (Table 1) The MFSCLs (SOFTOK SMR, ArtMost Vision, Inc., Australia) is a center-distance multifocal contact lens. It has ve different zones (Fig. 1). The center optical zone (0.5 mm from the center, corresponding to approximately 2.5° retinal eccentricity each side) ( Study design This is a cross-sectional, self-controlled study. Every subject would go through the peripheral refraction measurement with uncorrection (UC) and SVSCLs during the rst visit, with MFSCLs during the second visit after a two-week washout period. The centration, coverage movement and rotation were assessed by the same optometrist after wearing each contact lens for 30 minutes to con rm clinically acceptable ts. The horizontal peripheral refraction between 30º temporal (T) to 30º nasal retina (N) (in 10º steps) were measured by an open-eld autorefractor (Grand Seiko WAM-5500, Grand Seiko Co., Ltd., Hiroshima, Japan). Subjects were instructed to occlude their left eyes by an eye patch and use the right eyes to xate on the seven distant targets and the seven near targets. The seven distant targets (arranged 5m away, letter size 5M) and the seven near (1) M = sph + (cyl/2),

Comparisons between looking at distant and near targets
Wearing SVSCLs or MFSCLs showed more hyperopic defocus pro le when looking at near targets compared to looking at distant targets at most horizontal retina eccentricities (except wearing SVSCLs at T30 º, p=0.08, all other eccentricities p<0.05) (Fig. 5A, 6A). Wearing SVSCLs or MFSCLs also showed more hyperopic RPRE change when looking at near targets compared to looking at distant targets at most horizontal retina eccentricities (except T20º and T30º wearing SVSCLs, p=0.822 and p=0.950, respectively, all other eccentricities p<0.05; except N30º wearing MFSCLs, p=0.390, all other eccentricities p<0.05) (Fig. 5B, 6B) Looking at distance or near targets did not result in signi cant changes in J45º astigmatic components pro le wearing either SVSCLs or MFSCLs at all horizontal retina eccentricities (all P>0.1) (Fig.5C, 6C). For subjects wearing SVSCLs, the J180º astigmatic component pro le didn't signi cantly change between looking at distant or near target (except N30º p=0.024, all other eccentricities p>0.0.5) (Fig. 5D). However, for subjects wearing MFSCLs, the J180º astigmatic component pro le showed less negative when looking at near targets compared to looking at distant targets (T10 º to T30 º , P<0.001, all other eccentricities p>0.05) (Fig. 6D).

Accommodation
When looking at near targets (accommodative demand is 2.5D), subjects' accommodation response were  [23] reported that SVSCLs (Acuvue 2, Vistakon, USA ) reduced the degree of relative peripheral hyperopia in half compared to UC. However, Kang et al. [24] and De la Jara et al. [25] reported that SVSCLs (Proclear Sphere SCLs, CooperVision, USA; Acuvue 2, Vistakon, USA) increased relative peripheral hyperopia compared to UC in both low and moderate myopes. In our study, we did not nd signi cant differences in the RPRE between SVSCLs (Biotrue ONEday SCL) and UC when looking at distant targets. Both of them exhibited hyperopia RPRE pro les in both the temporal and nasal retina, which was consistent with other single vision soft contact lenses (PureVision2 [22], Bausch & Lomb, USA). The various outcome of SVSCLs are likely to be in uenced by many factors, including differences in lens design (manufacturers or lens parameters), lens t, and individual variations of the study subjects. Given the lack of consistency among the studies, adopting SVSCLs to adjust peripheral defocus does not seem to be the ideal approach.
Studies generally agreed that many commercially available MFSCLs are incapable of reducing the amount of the relative peripheral hyperopia or inducing relative peripheral myopia [9,10] [26][27][28]. In our study, the novel MFSCLs could imposed large relative myopic defocus on the periphery, especially in the temporal retina, up to -5.50 diopters at 30° eccentricity. If the myopic relative peripheral defocus does help slow myopic progression, we expect that the novel MFSCLs would be an e cient method.

Effects of SVSCLs and MFSCLs on defocus pro le when looking at near targets
The near peripheral defocus pro le can be attributed to three factors: (1) the hyperopic RPRE change during accommodation; (2) the hyperopic defocus change across the horizontal retina eccentricities due to accommodative lag; (3) the effect of contact lens on changing the defocus pro le.
First, we found that the RPRE pro le of myopic eyes became slightly hyperopic wearing either SVSCLs or MFSCLs during accommodation. This was consistent with the report by Walker et al. [14] who found the hyperopic change in RPRE immediately followed commencement of accommodation and remained unchanged after 1h of sustained accommodation. Walker et al stated it occurs because of the change of retinal shape. The prolate change in retinal shape is caused by choroidal tension during the initial accommodation, which increases the axial length [30][31][32]. But, some disagreed with it, Smith et al. [29] found an increase in curvature of eld (a decrease in Petzval radius) during accommodation, which indicates a myopic shift in relative peripheral refractive error.
Second, the accommodative lag pushes the near image shell backward. The defocus pro le tends to be hyperopic across the horizontal retina eccentricities. Thus, it likely push the peripheral defocus to hyperopic status. In this study, we found the same subjects accommodated 2.14±0.22 D wearing the SVSCLs and 1.67±0.31D wearing the MFSCLs facing 2.50-diopter accommodation stimulus. Thus, MFSCLs showed more accommodation lag compared to SVSCLs with the purpose that move the center on-axis near image backward with signi cant blurriness in order to relax accommodation and pull the para-axis near image forward for a clearer image.
Third, we found the novel MFSCLs is capable of imposing strong myopic defocus on the periphery owing to the special design. Even though the RPRE became slightly hyperopic during accommodation and the accommodation lag pushed the image shell backward when looking at the near targets, the peripheral defocus could still showed a certain magnitude of myopic. Thus, we expect the MFSCLs could be e cient in myopia control especially for children need long-time near working.
Effects of SVSCLs and MFSCLs on astigmatic component pro le proposed a possible explanation that the increased astigmatism may be due to upsetting the optical balance between the cornea and the internal optics of the eye by contact lens, thereby increasing the oblique astigmatism. However, it is still unknown if off-axis astigmatism in uences myopic development. The J180º astigmatic component pro le changed slightly during accommodation. It showed negative change in the far periphery wearing the SVSCLs, which is consistent with the study of Whatham et al. [19] It didn't change signi cantly wearing MFSCLs, which is in agreement with the study of Liu and Thibos [20]. They found that any effect of accommodation at axial or lateral positions of the pupil has negligible effect on ocular astigmatism.
Temporal-nasal asymmetry in spherical equivalent refraction and astigmatism In our study, we found the magnitude of spherical equivalent refraction and astigmatism exhibited temporal-nasal asymmetry wearing MFSCLs when looking at distant and near targets. Both spherical equivalent refraction and the astigmatism of were greater in the temporal retina. Many other researchers also found that astigmatism across the retina exhibits temporal-nasal asymmetry [34][35][36][37] with the temporal astigmatism being greater. The asymmetry might be attributed to the misalignment of the visual axis with the optical axis. It suggests that the foveal astigmatism is not at the center of that symmetry, which is typically a combination of axial and oblique astigmatisms [33]. This idea was supported by Shen et al. [23] who found that the temporal-nasal asymmetry of J180 could be removed by referencing the optical axis, which is 5° temporal from the foveal line-of-sight. Another possible reason for this asymmetry is the temporal decentration of the contact lens that occurs when the eyelids blink, forcing the lens to move over the temporal-nasal asymmetry of the corneal shape.
There are two concerns regarding the asymmetry of peripheral refraction. First, it remains to be determined if the asymmetry can alter myopic progression. Second, the possibility that the asymmetric peripheral refraction may promote undesirable asymmetric ocular growth [26] will require a future longitudinal study Limitations of the study Our study design has some potential limitations. Firstly, all measurements were made on young adults rather than children. Second, when participants view the near target monocularly, the accommodation response could be underestimated to some degree since it might eliminate accommodation components companying converge. However, the study of Tarrant et al found the difference between monocular and binocular accommodative response measurements is clinically small. The average differences are <0.125 D for the 33 cm target distances [13]. Third, we only measured the change in peripheral refraction immediately after the commencement of accommodation. Thus, the data we collected could only indicate the initial effect of accommodation on the peripheral refraction, it can't represent the change after a long-time near task.

Conclusions
The novel MFSCLs imposed strong myopic peripheral defocus when looking at distant targets. They also maintain a certain magnitude of myopic peripheral defocus when looking at near targets, regardless of the hyperopic effect of accommodation lag and hyperopic shift in RPRE during accommodation. It may serve a good way of slowing the myopia progression.