Abstract
Purpose
Digital eye strain encompasses a range of ocular and visual symptoms across all age groups. Recently, symptoms associated with accommodative or binocular vision stress have become a major problem, especially in young individuals. The purpose of this prospective, single-blinded study was to objectively quantify the accommodative response and visual performance of low-add soft contact lenses (CLs) in young non-presbyopic individuals.
Methods
A daily disposable low-add bifocal design lens (low-add CL) was tested. It employs a centre-distance optical zone and peripheral zone with the added power of +0.50 D to support near vision. Sixteen subjects aged 20–39 years were enrolled in the study. Refractive state and accommodation were measured using an open-field autorefractor with three target vergences, namely, −0.20 D, −2.5 D, and −4.0 D. Binocular visual acuity at high (100%) and low (40%, 20%) contrast and reading ability were assessed. Monofocal soft CLs were used as controls.
Results
Accommodative response with low-add CLs was significantly smaller than those with two monofocal CL wearing conditions, i.e., at 40 cm (2.5 D of stimulus) and 25 cm (4.0 D of stimulus) (all p < 0.05). The 20% contrast visual acuity at distance was significantly better with low-add CLs and second-time monofocal CLs compared to first-time monofocal CLs (all p < 0.05). The reading ability was not significantly different.
Conclusions
Quantification of accommodative response and visual performance demonstrated that using low-add CLs alleviated the accommodation under the near-vision condition, without sacrificing distance vision, in non-presbyopes.
1
Introduction
In recent years, a remarkable increase in digital device usage has been observed across all age groups for both social and professional purposes. Digital eye strain encompasses a range of ocular and visual symptoms, and its prevalence is reported to be 50% or more among computer users. [ ] Digital eye strain symptoms can be classified into those associated with dry eye or those related to accommodation [ ]. Devices such as smartphones, tablets, and small computers are commonly held at closer viewing distances than conventional printed materials. The small size of some portable screens may necessitate the use of reduced font sizes. Staring at the tiny screen of a smartphone for long hours can also create a need for close-distance viewing, which makes high demands upon ocular accommodation. Socially, it has been a very common problem that an increasing number of people in their 20 s and 30 s are reporting symptoms associated with accommodative or binocular vision stress similar to presbyopia. Individuals who wear contact lenses (CLs) in their 20 s and 30 s are generally wearing monofocal CLs; therefore, accommodation is needed for near vision. Prolonged use of digital devices at closer viewing distances with the CLs corrected for distance vision may cause symptoms due to accommodative stress in aged non-presbyopic individuals.
For aged presbyopic CL users, purely CL correction, including monovision and bifocal or multifocal CLs, is available. Use of multifocal CLs or monofocal CLs with reduced correction can reduce accommodative fatigue in some cases for non-presbyopes. However, the main disadvantage is the unwarranted decrease in distant vision. In recent years, a wide variety of add powers have been commercially available for multifocal CLs including the lower-add power. Regarding lenses with an additional power less than +0.75 dioptre (D), CLs with a very low-add power such as +0.50 D have been introduced to reduce the accommodative or binocular vision stress without sacrificing distance vision in non-presbyopic populations who engage in prolonged use of digital devices. The purpose of this study was to objectively quantify the accommodative response and visual performance of low-add soft CLs in young non-presbyopic individuals.
2
Methods
This prospective, single-blinded study was reviewed and approved by the institutional review board of Osaka University Hospital. The study adhered to the tenets of the Declaration of Helsinki. All subjects provided written informed consent after receiving an explanation of the nature and possible consequences of the study.
2.1
Subjects
Experienced soft CL wearers, who had been wearing spherical disposable CLs in both eyes, between 20–39 years of age were eligible to participate if they met the following criteria: (1) refractive error between +0.00 D and −6.00 D; (2) astigmatism less than 1.50 D; (3) best corrected visual acuity of 20/20 or better in each eye; (4) able to wear CLs more than 8 h per day and more than 5 days per week; (5) use of digital devices such as smartphones, tablets, or small computers more than 3 h per day.
Exclusion criteria were extended CL usage; toric CLs, multifocal CLs, or hard CLs; manifest strabismus or dry eye; clinically significant anterior segment abnormalities or ocular and systemic diseases that would preclude CL wear; and pregnancy or lactation.
2.2
Tested lenses
A daily disposable low-add bifocal design lens (SEED 1dayPure moisture Flex; SEED CO., LTD., Tokyo, Japan) (low-add CL) was tested in this study. This lens is made from a copolymer of 2-hydroxyethylmethacrylate (2-HEMA) and methacrylate compounds containing ammonium, carboxyl, and methyl groups cross-linked by ethylene glycol dimethylacrylate (EDGMA). The material is zwitterionic, comprising both cations and anions with a water content of 58%. This lens has a diameter of 14.2 mm. It is classified by the U.S. Food and Drug Administration as a group IV lens. It employs a centre-distance optical zone and peripheral zone with an added power of +0.50 D to support near vision. ( Fig. 1 ) According to the manufacturer, the widely designed centre-distance zone together with the gentle and progressive transition of lens refractive power between lens zones can provide natural vision of monofocal lens. As a control lens, a daily disposable monofocal lens (SEED 1dayPure moisture; SEED CO., LTD.) (monofocal CL) was used. Both lenses were identical in materials and the only difference was in their designs.
2.3
Experimental procedure
Fig. 2 details the outline of the study visits. Once trial suitability was established at the baseline visit, subjects were fitted bilaterally with the monofocal CLs for 2 weeks. The lens power was selected on the basis of the spherical equivalent of the subjective refraction. At the end of 2 weeks, the on-eye visual performance of the lens and accommodative response were evaluated. Subsequently, subjects were asked to wear low-add CLs for another 2 weeks, after which the same measurement procedure was repeated. Each subject then wore the monofocal CLs for another 2 weeks, after which the same measurement procedure was repeated. Subjects were masked to the type of each lens. Both lenses were worn bilaterally in a daily wear with daily disposable modality.
At each visit, subjects were assessed in the following order: 1) CL fitting examination using the slit-lamp, 2) binocular visual acuity evaluation at high and low contrast, 3) reading ability evaluation, 4) accommodative response evaluation, 5) slit-lamp examination of the ocular surface.
2.4
Measurements
Binocular visual performance test results, including distance visual acuity measurements at high (100%) and low (20%, 40%) contrast levels at 5 m, were obtained using the Landolt C chart. Near visual acuity measurements were also obtained for high (100%) and low (20%, 40%) contrast levels at 40 cm. These tests were performed using Binoptometer 4 P (Oculus Optikgeräte GmbH, Wetzlar, Germany).
Binocular reading ability was measured using the electronic version of the MNREAD-J, which is the Japanese version of the MNREAD acuity charts [ ]. One sentence comprised 30 characters (3 printed lines of 10 characters per line), and the character contrast was approximately 85%. The patients were asked to read the sentences aloud on the tablet display screen. The distance for the measurements was 40 cm. The time required for reading and the frequency of mistakes were recorded. In accordance with the previous studies [ , ], the following parameters of the MNREAD-J were calculated: (1) maximum reading speed: the mean reading speed of the fixed value (i.e., level at which no further increase in reading speed occurs); (2) critical character size: the smallest character size with which the maximum reading speed is attained; (3) reading acuity: the smallest character size with which the patient can read (regardless of speed).
Objective refractive state and accommodation were measured using a Grand Seiko WAM-5500 open-field autorefractor (Grand Seiko Co., Hiroshima, Japan) under binocular viewing conditions. The principles and procedures involved in the use of the WAM-5500 open-field autorefractor have been described elsewhere. [ , ] While it allows binocular viewing, the measurements of this device are monocular. In this study, the CL of the non-dominant eye was taken off and the consensual response of the non-dominant eye was measured with the CL on the dominant eye. Ocular dominance was assessed by the hole-in-card test. This method was adopted since measuring accommodative response with an open-field auto-refractor through a multifocal contact lens can lead to errors. Moreover, the similarity of the accommodative state between the two eyes has been reported from previous studies that measured it using a binocular optometer under natural viewing conditions in the normal eye. [ ] Dynamic accommodative responses were measured at three stimulus distances: at 5 m (0.20 D of stimulus), 40 cm (2.5 D of stimulus), and 25 cm (4.0 D of stimulus). The accommodation target was a high-contrast Maltese cross positioned on the subject’s midline. Subjects were instructed to keep looking fixedly at the target. A total of 5 measurements were obtained at each of the stimulus levels and averaged.
Following the measurements mentioned above, ocular surface examination with the slit-lamp was performed by removing CLs from eyes and applying fluorescein.
2.5
Statistical analyses
Regarding sample size calculation, a 0.5 D difference in the accommodative response was considered as clinically meaningful when comparing low-add CL wear and monofocal CL wear. It was calculated that with a sample of 10 patients, the study would have 80% power to detect 0.5 D mean difference in the accommodative response, with a type I error of 5%. For power analysis, a standard deviation of 0.5 D in the accommodative response based on the data of a preliminary study was used.
Statistical analyses were performed using MedCalc for Windows, version 18.11 (MedCalc Software, Ostend, Belgium). Friedman repeated-measures ANOVA on ranks was used to compare the parameters of visual performance, reading ability, and accommodative response among the 3 conditions (first-time monofocal CL wear, low-add CL wear, and second-time monofocal CL wear).
3
Results
Sixteen subjects (6 men and 10 women; age 24.0 ± 2.2 years) were enrolled in the present study. The mean spherical equivalent refractive error was −4.02 D ± 1.64 D. All subjects completed the study.
Visual performance for high and low contrast visual acuity under distance- and near-vision conditions are demonstrated in Fig. 3 . The 20% contrast visual acuity in the distance-vision condition was significantly better with low-add CLs and second-time monofocal CLs compared to first-time monofocal CLs (all p < 0.05). No significant differences (all p > 0.05) were found among the three conditions for both high and low near contrast visual acuity.
