Ⅰ. Introduction
Recent development of electronic devices, such as computers, smartphones, and/or tablets, is associated with increased number of symptoms of visual stress at near.1) These symptoms include fatigue, headaches, diplopia, asthenopia, and blurred vision, and these can be caused by accommodative and non-strabismic binocular dysfunction.2-4) This binocular dysfunction and vergence disorder can interfere visual system and person can become symptomatic after prolonged near work.5)
To alleviate the symptoms of binocular vision problems various methods, such as vision therapy, plus lenses, and prisms, have been used. Plus lens has been used to decrease both the lag of accommodation and esophoria at near.6) Also, plus lenses reduce the demand on negative fusional vergence. In addition, several studies were conducted using prescribed single vision lenses with undercorrection and rationale for these experiments is that undercorrection reduces the accommodative stimulus and demand at near.7-8) And undercorrection can be applied to balance accommodation and vergence systems.6) Positive lens can decrease the lag of accommodation and the measured esophoria. Therefore, plus lens help to reduce demand on negative fusional vergence and improve the equilibrium between accommodation and vergence system.9)
The ocular parameters which normally used for characterizing the accommodative and vergence system are accommodative amplitude, accommodative facility, accommodative response, vergence facility and near and distance horizontal phoria, and so on.10) Recently, multifocal contact lens has been applied for many occasions, especially to release the binocular problems and vergence disorder. In addition, there has been various studies performed to evaluate the effect of multifocal contact lens on accommodation and vergence disorder, yet the conclusions were mixed.11-15 Therefore, the aim of this study is to investigate the effect of undercorrection using multifocal soft contact lens and +1.00 D spectacle lens on binocular visual functions in non-presbyopic population.
Ⅱ. Methods
1. Participants
Seventeen individuals (male = 10, female = 7, mean age of 23.58±2.12 years) who have VA of more than 0.00 logMAR without any vision correction were recruited. Their spherical equivalent for right eye was –0.35±0.45 D (range of +0.25 D~-0.50 D), and for left eye was – 0.31±0.56 D (range of +0.50 D~-0.75 D). Since there is no correction for the astigmatism in multifocal contact lenses, all the recruited participants have less than –0.75 D astigmatism to minimize the effect of uncorrected astigmatism. Thus, spherical equivalent was used and uncorrected spherical equivalent amount is within –0.50 D. All participants were screened for their suitability to participate in the study by clinical examination and informed consent was obtained. Inclusion criteria were (1) being able to wear contact lenses, (2) no ocular pathology such as cataract, glaucoma or age-related maculopathy which might influence visual function, (3) best sphere refractive error within the range of ±1.00 D with less than –0.75 D of astigmatism, (4) aided binocular VA better than 6/6 (0.0 logMAR), (5) having normal binocular visual function. Informed consent was obtained from all participants, and the research protocol was approved by the Daegu Catholic University, Institutional Review Board (CU-IRB 2017-0060).
2. Vision Correction
In this study, repeated measures were performed using three different correction options; No correction (NC), undercorrection using +1.00 D lenses (UL), correction using multifocal contact lenses (CM). The reason for choosing +1.00 D for undercorrection was that it may have least effect on vision and can be used to decrease the symptoms of binocular vision for non-presbyopic subject. Trial frame with +1.00 D lenses were fitted for UL condition. The multifocal contact lens used for the study was a daily disposable (simultaneous design, centrenear) with plano power for distance and addition up to +1.00 D for near which is the spherical equivalent of the near spectacle prescription. The reason for choosing this design was that it is the most commonly used multifocal contact lens in current market and +1.00 D in the center of the lens was to match with undercorrection using +1.00 D lenses.
3. Binocular visual function measurements
The battery of binocular visual function measures includes amplitude of accommodation, accommodative response, accommodative facility, amplitude of divergence, convergence facility and near/distance phoria. Amplitude of accommodation was measured using OPTO ruler which has sliding target. The subject was asked to response until a target is sustained blur. The minimum distance to see clear is converted into diopter. Accommodative responses were measured at 40 cm distance target, giving accommodative stimulus of +2.50 D. Auto refractor (Shin-Nippon NVision-K 5001, Rexxam Co. Ltd., Osaka, Japan) was used and the difference between looking far and near target at 40 cm was calculated for accommodative response. Accommodative facility was measured using +2.00 D/-2.00 D flippers to assess the dynamics of accommodative system. The number of flips to see letter clearly was counted in cycles per minute (cpm). For amplitude of divergence, maximum divergence was measured using a phoropter while target is divided to up and down. For convergence facility, 12 Base-out prisms / 3 Base-in prisms flipper was used to measure its function. Lastly, Howell chart for distance and near phoria, maddox rod for near phoria, were used to measure phoria.
4. Statistical analysis
The normality of the collected data distributions was confirmed. The visual performance measures were analysed using repeated measures ANOVAs with correction type (FC, UL and CM) as the within-subjects variable. Where Mauchly’s test was significant, and sphericity could not be assumed, the Greenhouse Geisser correction was used. Also post-hoc multiple comparisons (Tukey or Tamhane T2 tests) could be used for assessment of significant differences across visual conditions. For gender difference, paired-t test was performed for all measurement, and there was no significant difference (p>0.050), therefore data of male and female was combined together for statistical analysis.
Ⅲ. Results
1. Amplitude of accommodation
Types of vision correction had no significant effect on amplitude of accommodation (F(2, 32)= 0.56, p=0.570) (Table 1, Fig. 1).
2. Accommodative response
There was a statistically significant difference in accommodative response with near target located at 40 cm (F(1.53, 50.46)=39.39, p<0.050). Multiple comparisons test indicated that acco mmodative response with no correction was statistically significantly bigger than other two correction options, induced +1.70 D with a target located at 40 cm (+2.50 D accommodative stimulus). In addition, CM also induced statistically significant accommodative response compared to UL (Table 2, Fig. 2).
3. Accommodative facility
There was a statistically significant difference in accommodative facility measured with +2.00 D/-2.00 D flipper at near target located at 40 cm (F(2, 32)=6.235, p<0.050). Multiple comparisons test indicated that accommodative facility with no correction and CM were statistically significantly better than accommodative facility of UL (Table 3, Fig. 3).
4. Amplitude of divergence
There was no statistically significant difference found among the three groups (F(2, 32)=0.83, p=0.446) (Table 4, Fig. 4).
5. Vergence facility (convergence facility)
There was a statistically significant difference in vergence facility measured with 12 Base-out prims / 3 Base-in prims flipper at near target located at 40 cm (F(2, 32)=6.96, p<0.050). Multiple comparisons test indicated that convergence facility with UL and CM were statistically significantly better than NC. However, there was no difference found between the group of UL and CM (Table 5, Fig. 5).
6. Distance phoria
Distance phoria was measured using Howell chart at 3m. Type of vision correction had no significant effect on distance phoria (F(2, 32)=1.03, p=0.369) (Table 6, Fig. 6).
7. Near phoria
Two different tests, howell chart and maddox rod, were performed to measure near phoria. There was a statistically significant difference in near phoria measured with Howell chart ((F(2, 33)=9.07, p=0.001). Multiple comparisons test indicated that Howell chart with UL and CM were statistically significant than NC (p=0.004 for UL and p=0.039 for CM). However, there was no difference found between the group of UL and CM (p=0.909) (Table 7, Fig. 7).
There was a statistically significant difference in near phoria measured with Maddox rod ((F(2, 33)=24.81, p=0.001). Multiple comparisons test indicated that near phoria measured with Maddox rod with UL was statistically bigger than NC. However, there was no difference found between the group of UL and CM (p=0.078) (Table 8, Fig. 8).
Ⅳ. Discussion
Accommodative amplitude did not present any statistically significant difference among three correction options. In Gong et al.’s study, the results fell into the normal range, yet binocular amplitude of accommodation was greater in subjects with multifocal contact lens.15) However, there was no significant difference between multifocal contact lens wearers and single vision lenses wearers.
Accommodative response with no correction presented statistically significant difference than other two groups. Multifocal contact lens presented higher accommodative response than undercorrected with +1.00 D lenses. Accommodative response means the combination of correcting lens and eyes,16) which means when the contribution of any lens worn is constant, any increase in response represents an actual ocular accommodation. Therefore, any increase in response represents true ocular accommodation and positive addition in spectacles or center-near multifocal lens relaxes accommodation, thus undercorrected with +1.00 D presented the lowest accommodative response. In accommodative facility, multifocal contact lens group did not show any statistical significance. Especially with young subjects, our result matches with other study that simultaneous image multifocal do not alter accommodative functions.16) For vergence facility, this study found both undercorrected with +1.00 D and multifocal contact lens group present the easiness of the test than the no correction group, which means plus addition may help with the patients with symptoms of vergence disorder.
For distance phoria, there was no significant difference among the three groups. However, near phoria with full correction presented statistically significantly lower result than other two correction options. For near phoria, two different tests (howell chart and maddox rod) were performed and the results agreed that undercorrection with +1.00 D lens and multifocal contact lens produced exophoric shift, more in undercorrection with +1.00 D group. This result agreed with previous study.14-15) When viewing through multifocal lenses accommodative response was reduced, eyes are more exophoric15) and it is supported by the theory that increased depth of focus provided by the multifocal design to relax the accommodation.17)
Overall, multifocal contact lens and undercorrection with +1.00 D lens were effective in relieve accommodative burdens and vergence problems comparing to no correction group. The existence of +1.00 D would relax the accommodation of the lens wearers, which will help to decrease the symptoms of prolonged near work. In addition, if the wearers present the near esophoria, this study would prove that it is helpful to shift the esophoria to divergent direction and symptoms of esophoria would be decreased while using this multifocal contact lens. However, this study has limited study sample and in the future study we consider the larger sample size to support our findings in this study.
Ⅴ. Conclusion
This study demonstrated that the performance of aspheric, center near design multifocal contact lens and undercorrection with +1.00 D lens related to the binocular visual functions, such as accommodation and vergence, and suggest that the multifocal contact lens do not provide important changes in accommodative system yet provide better accommodative stimulus-response and vergence system. The subjects with multifocal contact lens exhibited decreased accommodative response and more exophoric shift. Our findings of reduced accommodative stimulus-response and greater exophoria at near are consistent with using the positive addition at near. Thus, simultaneous image multifocal contact lenses may be useful for myopic children and young adults who presented accommodative problems (i.e. accommodative insufficiency) or vergence disorder.
