Study design
In this single-center, prospective, longitudinal trial, patients with convergence insufficiency or accommodative dysfunction were randomly allocated to either the virtual reality or OBVAT group in a 1:1 ratio using a random allocation sequence generated with the Clinical Trial Management Public Platform. The examiner who measured the outcome measures before and after treatment will be masked to treatment assignment. Figure 1 presents the study flowchart.
Patients
Table 1
Inclusion criteria, exclusion criteria and diagnostic criteria
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Inclusion criteria
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Age 18-35 years
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Best correct visual acuity of 20/25 or better in each eye at distance (5 m) and near (40 cm)
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Random-dot stereopsis no less than 480 seconds of arc (40 cm)
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No previous prism or near add before study enrollment
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Willing to wear appropriate refractive correction for at least 2 weeks before vision therapy (spectacles are required for diopters that meet the following criteria).
Myopia ≤-0.75 D spherical equivalent in either eye
Hyperopia≥+2.00 D spherical equivalent in either eye
Anisometropia≥0.75 D spherical equivalent
Astigmatism≥1.00 D in either eye
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Exclusion criteria
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≥2△ esophoria at distance
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History of strabismus, amblyopia, manifest or latent nystagmus
History of vision therapy
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Organic lesions of the eye
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History of any ocular surgery
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History of head trauma or known disease of the brain
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Systemic or mental diseases, such as diabetes or anxiety, affecting accommodation, vergence, and ocular motility
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Use of ocular or systemic medications containing atropine, pirenzepine, or antiepileptic in the past 3 months
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Diagnostic criteria
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Convergence Insufficiency
We enrolled participants with CI who had a near exophoria at least 4△ greater than distance exophoria and met at least two of the following three criteria:
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1) Near point of convergence break point≥6cm
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2) Reduced near positive fusional vergence(break point≤15△ or failed Sheard’s criterion)
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3) Convergence Insufficiency Survey Score ≥21 points
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Accommodative Dysfunctions
We enrolled participants with AD who met at least one of the following two criteria:
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1) The monocular amplitude of accommodation≥2 diopters below the minimum prediction(15-age/4)
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2) Monocular accommodative infacility≤6 cycles per minute with±2.00 D lenses
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The patients were enrolled between May 2019 and October 2020 at the Department of Ophthalmology, West China Hospital of Sichuan University. The study was approved by the West China Hospital of Sichuan University Biomedical Research Ethics Committee. The study conformed to the tenets of the Declaration of Helsinki. The patients signed informed consent before any study procedures were performed. The inclusion, exclusion, and diagnostic criteria are listed in Table 1.
Ocular health examination
Slit-lamp microscopy, scanning laser ophthalmoscopy, and tonometry examinations were used to exclude patients with eye disease. Sodium fluorescein paper was used to test the tear break-up time and exclude patients with dry eye.
Clinical outcome measures
The patients received binocular vision examinations and completed subjective questionnaires (Convergence Insufficiency Symptom Survey, CISS) at baseline and after 6 and 12 weeks of therapy. The examination could not be performed during the same day as the training, but it was completed within 1 week after the last training. Binocular vision functions were tested under best-corrected refraction. The primary outcome measure of convergence insufficiency was the CISS score, and the secondary outcome measures were near point of convergence (NPC), positive fusional vergence (PFV), and near horizontal phoria. The primary outcome measures of accommodative dysfunction were monocular accommodative amplitude and facility. In this study, only the amplitude and facility of the right eye were reported because the left eye and binocular amplitude and facility had a similar tendency to those of the right eye.
Treatment
All therapies were conducted under best-corrected refraction.
Virtual reality-based vision therapy group
Virtual reality-based vision therapy was based on the principle of dividing two similar images (targets) into both eyes. This was accomplished by dissociating the eyes with two screens (Figure 2). The screen resolution was 1920×1080 pixels, and the display refresh frequency was 120 Hz.
1. Virtual reality-based vision therapy for vergence
A. Stimuli
Visual targets presented on the virtual reality screen were horizontally separated and induced vergence demands. The targets were two similar images for two eyes and included human figures, animal figures, or objects (e.g., gourds) (Figure 3A). The background was blurred to highlight the target at the center. The two images approached or moved away simultaneously to change the vergence demands, which ranged from 20△ base-in to 30△ base-out, and the eyes were trained for both convergence and divergence.
B. Task
The patients were asked whether they saw a figure that appeared to be floating closer than the plane of the screen (Figure 3B). The targets, which were selected by the therapist, varied in their level of complexity, thereby affecting the ability of the observer to fuse the images properly. For instance, visual targets with simple lines, larger areas, and less fine detail were easier to fuse, whereas those with complex shapes and finer detail were more difficult to fuse.
The patients first needed to fuse the two targets into one at the zero prismatic demand position. The instrument then increased the prismatic demand at 2 PD per second. To ensure that the patients received proper training during the therapy, they had to maintain a clear, fused vision. If they saw a double image and/or lost depth perception, they immediately reported it to the therapist, who would restart the vergence demand from zero.
Figure 3A shows the images seen by each eye, with the two images moving simultaneously to produce a converging or diverging stimulus. Figure 3B shows the complete stereo image after fusion. Figure 3C shows balls of different sizes and colors with optotypes appearing on the screen. In training sequences, balls appeared on the screen one by one.
C. Training procedure
There were 15 trials with different targets and different vergence demands. At each visit, the patients performed 4 to 5 procedures. Each training session lasted approximately 8 to 10 min, with a short break between sessions.
2. Virtual reality-based vision therapy for accommodation
A. Stimuli
In the training sequences, balls of different sizes and colors appeared sequentially on the screen (Figure 3C). The optotype on the balls was the letter ‘E’ at different sizes, as is the case in the visual acuity chart. The background was blurred to highlight the targets. The balls were moved from far to near or vice versa by changing the binocular parallax. A ball with an optotype appeared on the screen every 1.5 to 3 s.
B. Task
During ball movement, the patients were required to always keep the optotype clear. Usually, the task began with slower moving balls and larger targets. To ensure that the patients received proper training during the therapy, they had to maintain clear vision. If the patients saw a blurred optotype, they immediately reported it to the therapist and proceeded with the training protocol after a short break.
C. Training procedure
There were 8 trials with different sequences of targets and ball movement speeds. Each session lasted approximately 5 to 8 min, with a short break between sessions.
OBVAT group
The treatment followed the CITT therapy protocol[25]. The OBVAT procedures were divided into three phases. Phases 1 and 2 included the Brock string and Barrel card tests for cross convergence and vectograms, LifeSaver cards, aperture rule, and eccentric circles for fusional vergence. Phase 3 mainly focused on jump vergence and added prism facility. For accommodative therapy, phases 1 and 2 included letter chart, lens sorting, and monocular accommodative facility. Phase 3 added binocular accommodative facility. Personalized training programs were used according to each patient’s situation and training progress.
Statistical analysis
The sample size of CI was calculated based on the outcome measures reported in the previous study of VR and the CITT Manual of Procedures. The mean improvements in the CISS score after 12 weeks were 16.2 and 10, respectively. In addition, the predicted standard deviation was 5.5. Power analysis (power of 80% and significance level of 5%) indicated that 14 participants were needed for each group, and we estimated that approximately 20% of participants might withdraw. Therefore, a total of 18 participants with CI were randomly assigned to each treatment group.
All data analyses were performed using IBM SPSS Statistics for Windows, Version 23.0 (IBM Corp., Armonk, NY). A chi-square test was used to compare sex differences between the groups. An independent t-test was used to compare the baseline findings between the groups. Repeated-measures analysis of variance was used to determine any significant effect of visit and group on binocular functions and symptoms (within-subjects factor: duration of treatment, between-subjects factor: treatment group). The Pearson chi-square test was used to compare the proportion of each indicator reaching normal in the two groups. P values < 0.05 were considered statistically significant.