Exploring the changes of perceptual eye position after SMILE based on the virtual reality platform

Abstract

OBJECTIVE: To investigate the pattern of changes in perceptual eye position in patients after Femtosecond Laser Small Incision Lenticule Extraction(SMILE), which was examined using the visual perception detection system developed by the National Engineering and Technology Research Center for Healthcare Appliances.

METHODS: A total of 141 patients participated in this prospective study. The patients were divided into three groups according to the degree of myopia: low myopic group

( equivalent spherical refraction ≤ 3.00D), moderate myopic group(3.00D < equivalent spherical refraction ≤ 6.00D), and high myopic group (equivalent spherical refraction > 6.00D). The relationship of perceptual eye position was measured by the computer-controlled visual perception detection system before, 1 day, 1 week and 1 month after surgery to investigate the changes of perceptual eye position after SMILE.

RESULTS: The differences in horizontal perceptual eye position were statistically significant at all time periods (X2=82.004, P<0.001), and the horizontal perceptual eye position improved at 1 day and 1 week postoperatively compared with that before surgery, but rebounded in January postoperatively. The differences in vertical perceptual eye position were statistically significant at all time periods (X2=38.59, P<0.001), and the vertical PEP improved at 1 day, 1 week, and 1 month postoperatively compared with the preoperative level, but the improvement was small. The difference between horizontal perceptual eye position and vertical perceptual eye position in all three groups was not statistically significant, and it can be concluded that the deviation of perceptual eye position after SMILE is not related to the degree of myopia.

CONCLUSION: Most myopic patients have perceptual eye position shift before refractive correction, and SMILE surgery can improve the perceptual eye position shifts significantly in the early postoperative period, but the long-term results need to be further observed.

1. Background

Myopia, the most common eye disease in the world, has become a serious public health problem [1-3]. Holden et al. showed that there are currently 1.406 billion people with myopia worldwide, accounting for 22.9% of the global population, and it is expected that by 2050 there will be 47.58 billions people with myopia, accounting for about half of the global population [4]. Femtosecond Laser Small Incision Lenticule Extraction (SMILE), as one of the refractive correction methods, is considered as a fast and painless vision restoration and less SMILE is an intra-stromal keratomileusis procedure that uses a femtosecond laser to create a microlens by scanning between the corneal stroma and extracting the lenticular corneal tissue through a tiny corneal incision [12]. The cornea is the most densely distributed surface tissue for nerves in vivo, and incomplete nerve regeneration in the cornea after SMILE may lead to reduced corneal sensitivity and may transiently or chronically alter the functional integrity of the ocular surface [13-16]. Based on this study, we speculate that the visual discomfort experienced by patients after SMILE may result from the need for the neural pathways of the brain to receive visual information from the eye to readapt. Vision is a product of the entire visual pathway, where external light is projected onto the retina through the intraocular refractive system, where light signals are converted into electrical signals, and then transmitted by the optic nerve to the visual cortex, where they are integrated and processed into visual perceptual images in the visual centers of the brain [17-19]. Therefore, to understand the source of discomfort in patients after SMILE, the entire visual pathway, including the visual system of the brain, should also be considered.

Perceptual eye positions (PEP), a visual perceptual function used to describe sensory binocular alignment, is a standard psychophysical method that reflects the state of eye position deviation under binocular separation conditions, that is, the ability of the brain center to control eye position, a concept first proposed by Zhao Guohong et al. in 2014 [20]. Traditional masking and corneal reflection methods do not detect bilateral PEP deficits, for example, in patients with non-amblyopia and non-strabismus who may not have significant strabismus but whose perceptual eye position may have deviated, so the use of perceptual eye position as an assessment index in this study may have greater clinical significance [21, 22]. The computer-controlled visual perception detection and evaluation system reveal subtle changes in eye position deviation by showing the results of perceptual eye position deviation through an easy-to-understand crossed-loop test based on brain vision studies. In contrast to traditional eye position detection methods, the computer-controlled visual perception detection and evaluation system uses red and blue glasses to perform binocular segmentation and uses pixels as units to respond to the degree of perceptual eye position shift [23, 24]. The smallest unit of binocular misalignment examined by the visual perception detection and evaluation system is 1 pixel, which is approximately equal to 0.04 prism degrees, and can objectively quantify the perceptual eye position shift more accurately. Previous studies have found varying degrees of perceptual eye position shift in patients with strabismus and amblyopia, but there is a lack of research on the changes of perceptual eye position in patients after SMILE. Therefore, this study compared the changes of perceptual eye position in patients after SMILE using the perceptual examination and evaluation system to investigate the effect of SMILE surgery on perceptual eye position [21, 25].

2. Materials and methods

2.1 Patients.

One hundred and forty-one myopic patients, 88 males and 53 females, with an average age of 21.22 ± 4.48 years (17 to 45 years), who underwent SMILE treatment in the ophthalmology department of the Affiliated Hospital of Southwest Medical University from June 2022 to September 2022 were selected. The inclusion criteria were age ≥ 17 years, refractive error not exceeding 0.50 D in the last two years, and exclusion of patients with amblyopia, dry eye and other major ocular diseases. The patients were divided into low myopic group (equivalent spherical refraction ≤ 3.00D) group, moderate group (3.00D < equivalent spherical refraction ≤ 6.00D), and high myopic group(equivalent spherical refraction > 6.00D) group according to low, moderate, and high refractive power. Finally, 20 patients were included in the mild myopia group, 83 patients were included in the moderate myopia group, and 38 patients were included in the high myopia group (see Table for summary demographics ). All patients completed written informed consent before inclusion in the study. All study protocols were approved by the Medical Ethics Committee of the Affiliated Hospital of Southwest Medical University and were performed following the Helsinki Declaration on Ethical Principles for Human Research.

2.2 Surgical procedure

All patients underwent a standard SMILE procedure by the same surgeon, using a VisuMax (Carl Zeiss Meditec, Germany) 500 kHz femtosecond laser system for each of the patient's eyes. Postoperative medications: tobramycin dexamethasone eye drops, levofloxacin eye drops, flumethasone eye drops, and non-preservative artificial tears.

2.3 Measurement of perceptual eye position

The equipment used to measure PEP included: Android system tablet PC, a resolution of 1920 * 1200, a refresh frequency of 120HZ ,and red and blue split-vision glasses. The visual perception detection and evaluation system developed by the National Healthcare Apparatus Engineering Technology Research Center was used, and the stimulus templates in the examination model were generated by MATLAB. The examinations were performed at constant room luminance, and all patients wore red and blue glasses for the perceptual eye position examination. The patients' PEP was measured by a cross-in-circle test in which the patient was allowed to see a cross in the left eye and a circle in the right eye (Figure 1,2). Before the test began ensure that the patient was 80 cm away from the screen and adjust the seat height to ensure that each patient's eyes were equal to the exact center of the monitor at the time of measurement. Patients use the mouse to position it at what they believe to be the center of the circle and subsequently click the mouse one by one. The system will record the patient's vertical and horizontal deviations in all directions in order to observe the perceptual control, direction of the deviation and size of deviation of both eyes under different temporal and spatial conditions.

2.4 Statistical analysis

Statistical analyses were performed using SPSS software (version 23.0; SPSS, Inc, Chicago,IL), with normally distributed data expressed as mean ± standard deviation and abnormally distributed data expressed as median (P25, P75). Two-way comparisons were performed using the Friedman test, and the Bonferroni method was used to correct for significance, with P values <0.05 considered statistically significant.

3. Results

3.1 Study population

A total of 282 eyes of 141 patients aged between 17 and 45 years were included in this study, with a mean patient age of 21.12±4.84 years, including 20 cases in the mild myopia group, 83 cases in the moderate myopia group, and 38 cases in the high myopia group, with no statistically significant differences between the three groups in terms of gender (P=0.344) and age (P=0.3) (see Table 1 for a summary of demographics ).

Table 1  Summary Demographics of Patients in the Study

^a Numbers of subjects 

^b Average years of age at first diagnosis

3.2 Comparison of horizontal PEP between different groups

The horizontal perceptual eye quartiles of the 3 groups were as follows (Table 2), and the differences in horizontal PEP between the 3 groups were not statistically significant on preoperative, 1 day postoperative, 1 week postoperative, and 1 month postoperative (P=0.189; P=0.173; P=0.213; P=0.246), and it can be concluded that the changes in horizontal PEP were not related to the degree of myopia. Using the Friedman test performed as follows (Table 3), the difference in horizontal PEP was statistically significant in patients at all periods (X2=82.004, P<0.001), and the horizontal PEP improved at 1 day and 1 week postoperatively compared with the preoperative period, but rebounded in January postoperatively.

Table 2   Comparison of horizontal perceptual eye position in 3 groups

P value < 0.05 was considered statistically significant

Table 3  Two-by-two comparison of horizontal perceptual eye position at different periods

3.3 Comparison of vertical PEP between different groups

The vertical perceptual eye quartiles of the 3 groups were as follows (Table 4), and the differences in vertical PEP between the 3 groups were not statistically significant on preoperative, 1 day postoperative, 1 week postoperative, and 1 month postoperative (P=0.131; P=0.85; P=0.367; P=0.349), and the two-way comparison using the Friedman test was as follows (Table 5), and the differences in horizontal PEP of patients at each time period were statistically significant (X2=38.59, P<0.001), and the vertical PEP at 1 day, 1 week, and 1 month postoperatively were all elevated compared to preoperatively, but the elevation was smaller.

Table 4 Comparison of horizontal perceptual eye position in 3 groups at different periods

P value < 0.05 was considered statistically significant

Table 5  Two-by-two comparison of vertical perceptual eye position at different periods

4. Discussion

Binocular vision is not exactly the same as the visual function that we think of as the visual function that humans value when performing object recognition and processing in traditional ophthalmology; it includes both the visual function part of traditional ophthalmology and part of the functional vision aspects. Not only visual acuity, but also binocular functional vision, such as stereo vision, perceptual eye position and binocular suppression system, which is integrated by information from the brain [24]. Most studies have investigated the discomfort of patients after SMILE based on traditional visual function tests, visual quality instruments, and dry eye examinations, but few studies have investigated postoperative binocular visual function [26-30]. Previous studies have found changes in stereoscopic vision after SMILE, but there are no studies on changes in perceptual eye position after SMILE [31]. Therefore, in the present study, we compared perceptual eye position in patients with different degrees of myopia after SMILE to investigate whether deviations in perceptual eye position are related to SMILE surgery and the occurrence of their degree of myopia, complementing the existing studies related to binocular vision after SMILE. This study found an improvement in horizontal PEP and no significant change in vertical PEP changes in patients after SMILE. The improvement in the horizontal PEP at both one day and one week postoperatively may indicate that the visual center of the brain has enhanced control of eye position postoperatively and that the brain perception level has increased to control the position of the eye. In contrast, the horizontal PEP did not improve at one month postoperatively, and we speculate that the newly established neural reflex pathways may require longer and repetitive training. This suggests that we should also strengthen PEP training in the postoperative period to help patients regain good eye position as soon as possible.

PEP differs from conventional eye position in that this eye position deviation may better reflect the ability of the visual center of the brain to control eye position in the divided vision state, and abnormalities in PEP are more indicative of the state and degree of impairment of central perceptual function [20]. In children with normal corrected visual acuity and normal eye position, the mean vertical PEP was 1 to 3 pixels, while the mean horizontal PEP was 4 to 8 pixels; however, in amblyopic children, the deviation of PEP pixels was much higher than this and depended on the severity of amblyopia; for this reason, our study excluded patients with monocular amblyopia to maintain homogeneity of the patients' PEP tests [32]. The results of our experiment found statistically significant horizontal PEP comparisons and no statistically significant vertical PEP comparisons in patients after SMILE, contrary to the results of Yang et al [23]. By comparing myopic patients with severe refractive error with those without refractive error, Yang's team found that the two groups had statistically significant comparisons of results between vertical PEP, while there was no statistically significant comparison in horizontal PEP. Their results suggest that patients with a severe refractive error have unstable vertical PEP, and thus speculate that instability in vertical perceptual eye position may be related to the development of severe refractive error. In our study, on the other hand, we found an improvement in horizontal PEP after surgery, and therefore ventured to speculate that the instability of horizontal PEP might be related to refractive correction. Similarly, this inspires us to pay more attention to the changes in vertical perceptual eye position in patients with concomitant refractive aberrations, to consider whether to make relevant interventions before surgery, and to follow up well after surgery.

The limitation of this study is that the sample size is too small and the observation time is only one month after surgery, which can only reflect the changes of PEP in the early postoperative period after SMILE. In the future, this study should expand the sample size and continue to extend the observation period to further investigate the changes of PEP in patients after SMILE, and provide PEP training to patients when necessary to help restore good eye position as soon as possible.

In conclusion, the results of the small sample suggest that the PEP shift is not only limited to patients with strabismic amblyopia, but also exists in patients with refractive error, and the SMILE procedure can improve the PEP shift, and the long-term results remain to be further observed.

Conclusion

In conclusion, by using virtual reality technology, We found Most myopic patients have perceptual eye position shift before refractive correction, and SMILE surgery can improve the perceptual eye position shifts significantly in the early postoperative period, but the long-term results need to be further observed.

Declarations

Acknowledgements

Not applicable.

Authors’ contributions

LLB and KXM analyzed the data and write the manuscript.LLB performed the

statistical analysis. LLB and KXM collected the data. HJT and GL designed

part of the study, as well as revised and polish the manuscript. LHB designed the study and analyzed the data. All authors read and approved the final manuscript.

Funding.

This study is supported by Science and Technology Program of SiChuan of China(N0.2020JDKP0072).

Availability of data and materials

The datasets generated during and/or analyzed during the current study are 

available from the corresponding author on reasonable request.

Ethics approval and consent to participate

The study protocol was approved by the Affiliated Hospital of Southwest Medical University. A written informed consent  was obtained from each patient before surgery. All methods were carried out in accordance with relevant guidelines and regulations.

Consent for publication

Not applicable.

Competing interests

All authors declare that they have no competing interests.

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