FormalPara Key Summary Points

Why carry out this study?

Accommodative lag may lead to hyperopic defocus and contributes to myopia progression, and accommodative/vergence therapy can improve accommodative function.

This study aimed to evaluate novel automatic dual rotational Risley prisms (ADRRPs) as a vergence exercise tool for patients with myopia to improve accommodative lag and accommodative facility.

What was learned from the study?

ADRRPs, a newly designed tool for vision therapy, improve accommodative function with a notable improvement in accommodative lag of 0.30 D after 10 min of training.

This study highlights the potential benefits of incorporating ADRRPs into vision therapy for improving accommodative lag, promoting accommodative relaxation, and offering a convenient alternative to intermittent near-work breaks.

Introduction

Vision therapy is effective for binocular vision dysfunctions, with convergence insufficiency most investigated [1,2,3]. However, there is a lack of studies on the effect of vision therapy on accommodative dysfunctions [4], such as accommodative lag, reduced accommodative facility and amplitude [5, 6], and increased accommodative adaptation [7]. Moreover, accommodative lag is widely discussed and considered a risk factor for myopia progression. Myopic children show insufficient accommodative response to blur [8] and myopes had larger accommodative lags than emmetropes [9, 10].

Recent studies revealed that when the accommodative response does not meet the accommodative demand, the accommodative lag may lead to hyperopic defocus and cause further eyeball axis elongation, resulting in myopia progression. The progression of myopia and inaccurate accommodation was correlated [11,12,13], whereas a significant positive link between improved accommodative accuracy and slowed axial elongation was observed in children wearing orthokeratology [14]. Based on this theory, some myopia treatment modalities were developed, such as progressive addition lenses [15, 16] and the design of prismatic bifocal spectacles incorporating near base-in prisms along with near-addition lenses [17]. Improvement of accommodative lag and activity can be achieved through accommodative/vergence therapy [1]. Accordingly, this study aimed to evaluate the accommodative response after vergence exercises using a novel app-operated automatic dual rotational Risley prisms (ADRRPs) device.

Methods

Participants

A total of 66 adults (29 male and 37 female) with a mean age of 21.43 ± 1.13 years were recruited from the Department of Optometry, Chung Shan Medical University in Taichung City, Taiwan. Those with ophthalmic or systemic diseases, a history of intraocular surgery, strabismus, anisometropia of > 2.0 D, and amblyopia were excluded from this study. Eligible participants underwent manifest refraction, and spherical equivalent soft contact lenses (SECL; Ticon, St. Shine Optical Co., Ltd., Taiwan.) were used to correct visual acuity in both eyes to ≤ 0.1 logMAR. The participants were randomly divided into the treatment group, who wore prism lenses, and the control group, who wore plano lenses during the vergence exercises. In the control group, the prisms were replaced with plano lenses in the same vergence exercise device (ADRRPs).

Informed consent was obtained from all participants, and the experiment was performed according to the Declaration of Helsinki. Ethical approval was granted by the Institutional Review Board of Chung Shan Medical University Hospital in Taichung, Taiwan, ROC (approval number CS2-22104) and National Taiwan University Hospital in Taipei, Taiwan, ROC (approval number 202207058RINB).

Instruments and Procedures

An open-field autorefractor (Grand Seiko WAM-5500) was used to measure the refractive errors of the participants’ eyes. ADRRPs were used for vergence exercises. The participants wore SECL and viewed videos at a 0.4 m distance through the ADRRPs, in a way that they fused the images in both eyes periodically in divergence and convergence.

The experimental procedures were as follows: (1) screening; (2) pre-test baseline ophthalmic examination; (3) vergence exercises using prism or plano lenses with ADRRPs, followed by 15 min of rest; (4) post-test ophthalmic examination; and (5) statistical analysis using SPSS. The experimental procedures are shown in Fig. 1.

Fig. 1
figure 1

Experimental procedures. ADRRPs automatic dual rotational Risley prisms

Full size image

Design of the Automatic Dual Rotational Risley Prisms (ADRRPs)

Figure 2a–c shows schematic diagrams of the ADRRPs, two counter-rotating Risley prism pairs, and a participant wearing ADRRPs for the vergence exercises, respectively. The ADRRPs consisted of two counter-rotating Risley prism pairs, one for each eye. Each component of the Risley prism is an 8Δ prism; therefore, the equivalent prism power of double Risley prism pairs can be continuously changed between 32Δ (base out; BO) and 32Δ (base in; BI) in operation. In this experiment, the prism power operating range was set from 30Δ (BO) to 10Δ (BI). The simulated layouts of virtual image distance were obtained using light tools software, as shown in Fig. 2d. When a target is placed 400 mm in front of a participant’s eyes, the target point makes an angle (α = 4.289°, with an interpupillary distance of 60 mm) with the straight-ahead line, which is also the convergence angle of each eye to view the target. When the double Risley prism pairs provide 30Δ (BO) for the eyes, the rotation angle of each eye increases to 12.93° to view the centrally located virtual image at a 130.5 mm distance. In contrast, when the double Risley prism pairs are adjusted to 10Δ (BI) for the eyes, the rotation angle of each eye decreases to 0° to view the centrally located virtual image at a 16,000 mm distance in front of the eyes.

Fig. 2
figure 2

Schematic diagram of a ADRRPs, b two counter-rotating Risley prism pairs, c a participant wearing ADRRPs for the vergence exercises, and d simulations of virtual image distance. ADRRPs automatic dual rotational Risley prisms, BI base in, BO base out, OD right eye, OS left eye

Full size image

Simulated relationships among the rotation angle of each prism, prismatic powers of the double Risley prism pairs, and corresponding virtual image distance are shown in Fig. 3. The paraxial prism diopters (D) is a function of the rotation angle θ and is expressed as follows:

Fig. 3
figure 3

Simulated plots of the rotation angle of each prism vs. the prismatic powers of the double Risley prism pairs and the corresponding virtual image distance

Full size image
$$\mathrm{D}=32\times \mathrm{sin}(\theta )$$
(1)

The rotation angle of − 90° corresponds to − 32Δ, with the minus sign indicating the base-out status of the double Risley prism pairs. When each prism’s rotation angle (θ) reaches 0°, the overall prismatic power is 0, indicating no deviation between the incident and exit rays.

However, when θ reaches 18°, the virtual image is presented at a 16,000 mm distance in front of the eyes, which can be seen as infinite and used to relax the eyes.

Accommodative Response

Accommodative response is the ability of the eyes to adjust their focus to maintain a clear image as the viewing distance changes. Participants with full refractive correction viewed the target 0.3 logMAR (Snellen E-Chart) at 6.0 and 0.4 m before and after the vergence exercises, respectively. The binocular open-field autorefractor (Grand Seiko WAM-5500) was used to measure the refractive errors over SECL. At a 0.4 m viewing distance, the stimuli required for accommodation was − 2.50 D. Participants with full refractive correction needed an accommodation response of 2.50 D to see the target clearly. The accommodative response was specified as the difference between the refractive status of the two-stage measurements and was determined using the following formula, with accommodative response in diopters [18],

$${\text{Accommodative response}} = {\text{RE}} - {\text{RS}},$$
(2)

where RE is the refractive error at 6.0 m and RS is the refractive status at 0.4 m.

Accommodative Facility

The binocular accommodative facility was measured in both eyes with a ± 2.00 D flipper using an accommodative rock card. The participants were instructed to read one of the 0.3 logMAR letters aloud sequentially as soon as it was recognized clearly. The rock card was placed at 0.4 m in front of the participants, and the lenses were flipped to the opposite power after each letter was called out. The cycles per minute (cpm) of flips were recorded, with one cycle being equivalent to the number of letters read aloud while viewing through the + 2.00 D lens and then the − 2.00 D lens.

Vergence Exercises

Participants viewed a mobile phone screen through ADRRPs to perform the vergence exercises. The phone was placed at approximately 0.4 m in front of the participant, who watched videos of their choice for 10 min. During the video-watching period, the Risley prisms rotated continuously, producing horizontal prism diopters (Δ) from 10Δ (BI) to 30Δ (BO). The participants were asked to focus and see the mobile phone screen clearly. The ADRRPs were set to alternate between 30Δ (BO) and 10Δ (BI). It took 1 s to rotate from 30Δ (BO) to 10Δ (BI), persisting for 3 s, and then another 1 s to rotate back to 30Δ (BO), persisting for 5 s; one round took approximately 10 s and was repeated 60 times.

Data Analysis

Statistical analysis was performed using SPSS Statistics 22.0, and a paired samples t test was used. Statistical significance was set at p < 0.05.

Results

A total of 56 participants aged 20–24 years completed this study. The treatment group included 22 women and 17 men, with a mean age of 21.82 ± 1.10 years, whereas the control group consisted of 8 women and 9 men, with a mean age of 20.53 ± 0.51 years. In the treatment group, the average myopia and astigmatism measurements in the right eye (OD) and left eye (OS) were − 4.35 ± 2.85 D and − 0.68 ± 0.53 D and − 4.27 ± 2.90 D and − 0.65 ± 0.45 D, respectively (stereoacuity < 40 seconds of arc in all participants). In the control group, the average myopia and astigmatism measurements in the OD and OS were − 3.97 ± 2.79 D and − 0.81 ± 0.46 D and − 3.77 ± 2.86 D and − 0.92 ± 0.54 D, respectively (stereoacuity < 40 seconds of arc in all participants).

Accommodative Responses

Figure 4a shows the participants’ accommodative responses before and after the vergence exercises at a 0.4 m distance. Before the vergence exercises, the treatment group had accommodative responses of 1.93 ± 0.27 D (OD) and 1.97 ± 0.33 D (OS), whereas the control group had accommodative responses of 1.94 ± 0.29 D (OD) and 1.96 ± 0.33 D (OS). The accommodative lag values were 0.57 D (OD) and 0.53 D (OS) and 0.56 D (OD) and 0.54 D (OS) in the treatment and control groups, respectively, at a 0.4 m distance. After the vergence exercises, the accommodative responses for the treatment and control groups were 2.23 ± 0.22 D (OD) and 2.29 ± 0.24 D (OS) and 1.99 ± 0.29 D (OD) and 1.90 ± 0.38 D (OS), respectively. The accommodative lag values were 0.27 D (OD) and 0.21 D (OS) and 0.51 D (OD) and 0.60 D (OS) for the treatment and control groups, respectively. In the treatment group, the accommodative responses showed a significant difference before and after exercise in each eye (p < 0.001); however, there was no significant difference in accommodative responses in the control group (p = 0.403 (OD), p = 0.519 (OS)). After performing the vergence exercises, the accommodative lag changed by 0.30 D (OD) and 0.32 D (OS). The effect sizes, as measured by Cohen’s d, were 0.932 (OD) and 1.227 (OS), indicating a significant effect size for both eyes. Figure 4b shows the participants’ refractive errors over SECL before and after the vergence exercises at a 6.0 m distance, which were − 0.01 ± 0.30 D (OD) and 0.03 ± 0.34 D (OS) and 0.15 ± 0.32 D (OD) and 0.19 ± 0.28 D (OS), respectively, in the treatment group. Before and after the vergence exercises in the control group, the refractive errors over SECL were − 0.05 ± 0.42 D (OD) and 0.07 ± 0.88 D (OS) and − 0.05 ± 0.43 D (OD) and 0.02 ± 0.83 D (OS), respectively. In the treatment group, there were significant differences between the pre- and post-test values for both OD and OS, with approximately a 0.16-D difference (p < 0.001), whereas no significant differences in refractive errors were observed in the control group (p = 1.000 (OD), p = 0.358 (OS)). This also indicated an accommodative relaxation of 0.16 D when observing at 6.0 m after performing convergence exercises.

Fig. 4
figure 4

Participants’ a accommodative responses at 0.4 m, b refractive errors over spherical equivalent soft contact lenses at 6.0 m, and c accommodative facility before and after the vergence exercises. OD right eye, OS left eye, OU both eyes

Full size image

Accommodative Facility

Figure 4c shows the participants’ accommodative facility before and after the vergence exercises. In the treatment group, the mean accommodative facility values of the participants before and after the vergence exercises were 14.88 ± 3.36 cpm and 15.59 ± 3.60 cpm, respectively (p < 0.01). In the control group, the mean accommodative facility of the participants before and after the vergence exercises were 14.85 ± 4.15 cpm and 15.21 ± 5.89 cpm, respectively (p = 0.732). The effect sizes, as measured by Cohen’s d, were 0.078, indicating a small effect size in accommodative facility after performing the vergence exercises.

Accommodative Response and Refractive Error Variations in 4.25 h

To further understand the effect of vergence exercises on accommodation, we randomly selected six participants in the treatment group and observed their variations in accommodative responses and refractive errors over SECL at distances of 0.4 and 6.0 m over a 4.25-h period, as shown in Fig. 5a, b, respectively. Fifteen minutes after finishing the vergence exercises, we measured participants’ accommodative responses and refractive errors at distances of 0.4 and 6.0 m, respectively, for the first time and then hourly four times. As shown in Fig. 5a, the participants’ first measurements showed increased accommodative responses after the vergence exercises. However, most participants showed some fluctuations in their accommodative responses from the first to the third hour. Furthermore, the participants showed improved accommodative responses after vergence exercises compared with their baseline accommodative responses. As shown in Fig. 5b, the refractive errors of participants tended to change to positive refractive errors at 15 min, indicating relaxation of accommodation after the vergence exercises. However, measurements of refractive errors after 15 min were inconsistent.

Fig. 5
figure 5

a Accommodative response variations and b refractive error variations in six randomly selected participants in the treatment group over a period of 4.25 h. OD right eye, OS left eye

Full size image

Discussion

We evaluated the effect of a novel vision therapy device, ADRRPs, in accommodative function; 56 participants completed this study. In 39 young adults with myopia in the treatment group that underwent vergence exercise using ADRRPs, improvement of accommodative lag, accommodative facility, and relaxation of accommodation was observed after 10 min of training.

The accommodative stimulus–response varies at different viewing distances. The accommodative lag was quantified as the difference between the accommodative demand and response, and both the accommodative demand and lag increased as the viewing distance decreased [19]. Normal asymptomatic individuals have an accommodative lag of 0.50 ± 0.25 D during binocular accommodation at a 0.4 m distance [20]; however, accommodative lag is higher in people with myopia than those with emmetropia [8, 18, 21]. Previous studies have also suggested that accommodative lag may increase the prevalence of myopia [18, 22] as hyperopia defocused and blurred images on the retina as a result of accommodative lag, accelerating the elongation of the eye axis and leading to the progression of myopia [22]. A study on adults with myopia aged 18–22 years showed that a higher accommodative lag increased myopia progression rates [23, 24]. In addition, an accommodative lag produced a higher degree of visual discomfort [25], and the correlation between accommodative lag and visual discomfort was strongly positive [26]. Whether vision therapy can improve accommodation function remained inconclusive. Allen et al. did not find the effect of vision therapy on accommodative lag with lens flipper exercises using a + 2.00 D/− 2.00 D flipper at 40 cm. In their study, the exercises were performed for 18 min per day for up to 6 weeks [6].

In the present study, participants were first fully refractive error-corrected and viewed a target placed 0.4 m in front of them while performing vision training. Before the vergence exercises, the accommodative responses were 1.93 ± 0.27 D (OD) and 1.97 ± 0.33 D (OS). The accommodative lags were 0.57 D (OD) and 0.53 D (OS). After the vergence exercises, the accommodative responses improved to 2.23 ± 0.22 D (OD) and 2.29 ± 0.24 D (OS), and the accommodative lags decreased to 0.27 D (OD) and 0.21 D (OS), which are consistent with those in previous clinical studies [20]. Huang et al. reported a change in accommodative lag of 0.12 D after an 11-min training duration [27], whereas Ma et al. reported a change of 0.46 D after 12 weeks [28]. In our study, the treatment group showed a 0.30 D change in accommodative lag after a training duration of 10 min, which showed a better efficiency in the improvement of accommodative functions. Moreover, performing vergence exercises using ADRRPs made the training course more appealing and less time consuming, as the participants could choose their favorite movies or games as the viewing task while performing the exercise. The control group, who solely watched videos on a mobile phone screen through the plano lens in the ADRRPs for 10 min, did not show significant differences in their accommodative functions.

The effect of accommodative relaxation has rarely been explored in previous vision therapy studies. In the present study, participants in the treatment group showed a positive shift in refractive errors measured using an open-field autorefractor. Before and after the vergence exercises, the refractive errors were − 0.01 ± 0.30 D (OD) and 0.03 ± 0.34 D (OS) and 0.15 ± 0.32 D (OD) and 0.19 ± 0.28 D (OS), respectively. With the same principle of contact lens prescription and refractive status measurement, our study did not find any significant differences in accommodative relaxation for the control group.

The expected values of accommodative facility vary with age, and the mean binocular accommodative facility was 5 and 10 cpm for ages 8–12 years and 13–30 years, respectively [29, 30]. The accommodative facility is also considered one of the factors contributing to myopia progression [23]. The values of the accommodative facility in the present study were consistent with those in previous studies. The mean accommodative facility in the treatment group was 14.88 ± 3.36 cpm before the vergence exercises, which increased to 15.59 ± 3.60 cpm after the exercises. In the control group, the mean accommodative facility increased slightly by 0.35 cpm after the vergence exercises. We speculated this may be due to the participants gaining familiarity with the measurement procedure. Patients with myopia with reduced near accommodative facility may experience difficulties switching their accommodative focus at different distances, resulting in hyperopic defocus and high accommodative lag. Recently, myopia has become an important global public health issue, and near-work was regarded as a significant risk factor for the onset and increase in myopia incidences [31]. Public health studies have reported that some behaviors help accommodation relaxation, such as intermittent near-work breaking and fixating on distant objects, may benefit myopia control [32]. However, whether relaxation of myopia and improved accommodation function are directly associated with myopia improvement requires further long-term research involving direct measurements of the eye axial length and refractive errors. In our study, follow-up measurements were performed on selected participants in the treatment group for 4.25 h. Improvements in accommodative responses were sustained in most cases. Refractive errors over SECL showed variability. Whether or not the effects of vergence exercises can be sustained also requires more research.

The present study was limited by the small number of cases and relatively short duration of follow-up. In the future, we will conduct larger-scale studies on children and adults with myopia using periodic ADRRPs exercises and follow up on their accommodative responses, accommodative lag, and interaction between accommodative convergence/accommodation ratio [12, 33]. A longer study period is also necessary to evaluate the change in axial length and refractive errors and further evaluate the relationship between myopia control and vergence exercise.

Conclusions

The proposed novel ADRRPs design showed potential in improving accommodative lag and facility, which are important factors associated with eye strain and myopia progression. The use of ADRRPs in vision therapy may help in accommodative relaxation and may be a convenient alternative to intermittent near-work breaks.