Background
Corneal refractive surgery is performed worldwide [
1]. Satisfactory vision correction is achieved in most patients; however, certain patients may experience poor night vision even when their general visual acuity is 20/20 or better [
2]. The mechanism underlying this phenomenon has not been fully elucidated. Previous studies have revealed that higher-order aberrations (HOAs) may increase after refractive surgery, reporting that they are a potential source of poor night vision [
3,
4]. Varying illumination also induces changes in the accommodation of the lens [
2,
5], which reduces night vision; therefore, investigation of the effects of varying illumination on optical quality will enhance ophthalmologists’ understanding of postoperative night vision.
Changes in refraction (lower-order aberrations) and HOAs cause degradation in retinal image quality [
6‐
8]. There have been reports of changes in refraction with varying illumination in natural eyes. Leibowitz and Owens [
2] discovered that poor night vision may cause a night myopic shift, wherein a person appears to become more nearsighted at low illumination. However, only a few studies have been conducted with the aim of investigating whether night myopia is retained after refractive surgery, or whether varying illumination causes other postoperative changes in refraction. Such studies have generally focused on HOAs. For example, Villa et al. [
3] reported that secondary astigmatism, coma, and spherical aberrations increased under night vision conditions after laser in situ keratomileusis (LASIK), and that they were statistically significantly correlated with the halo disturbance index. However, changes in HOAs do not comprehensively explain changes in optical quality.
Furthermore, a compensation effect between the anterior corneal surface and internal ocular optics assists in maintaining stability for optimal optical quality. There is ample evidence for a compensation effect for HOAs in natural eyes [
9,
10]. However, corneal refractive surgery may disrupt this effect, leading to higher postoperative aberration values and poorer night vision. Lee et al. [
11] reported that, compared with preoperative aberrations, postoperative anterior corneal aberrations exceeded the compensation effect of the internal ocular optics, resulting in a statistically significant increase in overall ocular aberrations. Nevertheless, there are few reports describing the influence of varying illumination on the compensation effect before and after corneal refractive surgery. Such studies may aid in elucidating the mechanism of changes in optical quality under different levels of illumination.
The objective of our current study was to investigate the differences in refraction, HOAs, and compensation for aberrations between mesopic and photopic illumination before and after small incision lenticule extraction (SMILE). To the best of our knowledge, no similar study has been reported to date, and this study may contribute to a better understanding of postoperative night vision after SMILE.
Methods
Aim
The objective of our current study was to investigate the differences in refraction, HOAs, and compensation for aberrations between mesopic and photopic illumination before and after SMILE.
Participants
This prospective study involved 40 consecutive participants (the right eye of each patient was selected) who underwent SMILE at Tianjin Eye Hospital, Tianjin Medical University, China, for the correction of myopia and myopic astigmatism between June and November 2016. The inclusion criteria were as follows: age ≥ 18 years, stable refraction (a change ≤0.50 diopters [D]) in the past 2 years, discontinuation of soft contact lens use ≥2 weeks and of rigid gas permeable lens use ≥4 weeks, a clear cornea without opacities, central corneal thickness > 500 μm, residual stromal bed thickness > 250 μm, intraocular pressure < 21 mmHg, and no other ocular conditions. The exclusion criteria were keratoconus (verified or suspected) and systemic diseases such as diabetes or connective tissue disease.
The study protocol adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee of Tianjin Eye Hospital, Tianjin, China (No. 202056). Informed consent was obtained from all participants after thorough explanation of the nature and possible consequences of the procedures were provided.
All participants underwent a thorough preoperative eye examination. Routine examinations included uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) examinations, slit lamp microscopy, manifest and cycloplegic refraction, indirect fundoscopy, and tonometry. In addition, wavefront refraction and HOAs were measured both preoperatively and 3 months postoperatively using a wavefront analyzer (KR-1 W, Topcon Corp., Tokyo, Japan). UDVA examination, tonometry, and slit lamp microscopy were also examined postoperatively.
Regarding surgical planning, as the refractive accuracy achieved using manifest refraction is better than that predicted using wavefront refraction [
12], the former was used for surgical planning. However, wavefront analyzers can be used to measure refraction at both mesopic and photopic illumination. Additionally, Lebow and Campbell [
13] confirmed that spherical and cylindrical refraction measured with a wavefront refractor are suitable for refractive analysis. Therefore, wavefront optometry data were used to analyze differences in refraction between the two illuminations in the current study.
Measurement of higher-order aberrations
Ocular aberrations included corneal and internal aberrations. Corneal topography was performed, and ocular aberrations measured, using the wavefront analyzer. The device was first used to measure corneal and ocular wavefront aberrations along the same axis; using these measurements as reference points, internal aberrations could be calculated accurately in a relatively short amount of time.
All measurements were performed in a dark room after 10 min of rest, immediately after the participant blinked to reduce tear film-related HOA deterioration. Measurements were conducted for the right eye only, with an undilated pupil, under quiet conditions. Pupil diameter (mm) was used as a reference for defining mesopic and photopic illumination, as there is an empirical relationship between pupil diameter and illuminance (lx) [
14,
15]. Mesopic (0.017 lx) and photopic (10.411 lx) illumination were generated using the wavefront analyzer. Consecutive, automatic measurements of aberrations were performed in triplicate by an experienced technician at each illumination. Real-time aberrations and pupil diameters were recorded for analysis. Readings were considered valid if they adhered to the manufacturer’s guidelines.
Surgical technique
All procedures were performed by the same experienced surgeon (Y.W.). Three drops of oxybuprocaine hydrochloride (Benoxil) were applied 3 min before surgery for topical anesthesia. Thereafter, participants were instructed to fixate on a target light to allow the initiation of suction. The surgeon confirmed the alignment of the center of the ablation zone with the center of the pupil. Thereafter, surgery was performed using a 500 kHz VisuMax femtosecond laser (Carl Zeiss Meditec AG, Jena, Germany), with a laser energy of approximately 170 nJ. The details of the surgical procedure have been previously described [
16]. Briefly, the laser spots were spaced 1.5 μm apart, creating photodisruption in the stroma. Four cleavage planes were created on the anterior and posterior surfaces of the refractive lenticule on its vertical edge, as well as a single side-cutting incision with a circumference of 2.0–5.0 mm at the 12 o’clock position. Once the femtosecond laser-cutting procedure was completed, the suction was switched off and the refractive lenticule was extracted from the small incision. The diameter of the optical zone was 6.0–6.6 mm, with a transition zone of 0.1 mm. The cap thickness was 110 μm. Nomogram adjustments were implemented for all 40 eyes, set by the same experienced surgeon.
Postoperatively, 0.3% ofloxacin (Tarivid) eye drops were instilled four times daily for 3 days, and 0.1% fluorometholone (Flumetholon) eye drops were instilled four times daily for 2 weeks; the dosages were tapered over 2 months (one drop less every 2 weeks).
Calculation of the compensation factor
The compensation factor (CF), as defined by Artal and Guirao [
17], was calculated as the relative efficiency of the compensation for aberration. In this study, the CF between the anterior corneal surface and the internal ocular optics was calculated as 1 − (
w/
c), where
w was the aberration of the whole eye, and
c the aberration of the anterior corneal surface. The aberration of the whole eye was equal to the aberration of the anterior corneal surface when CF = 0, i.e., when there was no compensation effect by the internal optics. A compensation effect was present when CF > 0. Typically, the aberration of the anterior corneal surface is partially compensated by that of the internal ocular optics (CF ranging from 0 to 1). Additionally, a negative value (CF < 0) indicates augmentation, indicating that the aberration of the whole eye is larger than that of the anterior corneal surface.
In the current study, which included 40 eyes, the relationship of a certain HOA (K) between the internal ocular optics and the anterior corneal surface appeared as a compensation effect in x eyes and as an augmentation effect in y eyes (x + y = 40). The proportion of eyes that demonstrated a compensation effect for K was calculated as x/40 × 100 (ranging from 0 to 100%). A higher value indicates that K of the internal ocular optics tends to compensate for K of the anterior corneal surface.
Statistical analysis
All data were collated and calculated using Microsoft Excel 2007 (Microsoft Corp., New Mexico, United States), and statistical analysis was performed using IBM SPSS Statistics for Windows (version 23.0, IBM Corp., New York, United States). Data normality was examined using the Kolmogorov–Smirnov test. Normally distributed data were described as means ± standard deviations, while non-normally distributed data were described as medians [interquartile ranges]. A two-sample paired t-test was used for the comparison of wavefront refraction and HOAs between mesopic and photopic illumination. CFs for HOAs were compared between mesopic and photopic illumination using the Chi-squared test. Statistical significance was set as a two-tailed p-value < .05.
Discussion
In the current study, the effect of varying illumination on refraction, HOAs, and compensation for aberrations was investigated before and after SMILE.
In previous studies on poor postoperative night vision [
18,
19], mathematical conversion was used to compare the differences in HOAs under different pupil diameters. In the current study, the pupil diameter was scaled with varying illumination. Moreover, with the exception of pupil diameter, Leibowitz and Owens [
2,
5] reported that accommodation of the lens was different under night vision conditions; therefore, in the current study, we also investigated these changes. Hence, the current study was more representative of real-world conditions than previous studies.
Regarding refraction, we observed a night myopic shift in both natural and postoperative eyes. In addition, compared with natural eyes, postoperative eyes exhibited an augmentation in astigmatism under mesopic illumination. There are several possible explanations for these findings. First, night myopic shifts caused by aberrations in the lens are not altered by SMILE, as demonstrated in the present study. Additionally, the exaggeration in postoperative positive spherical aberration may affect spherical refractive error [
20,
21]. Finally, aberrations such as coma may cause an amplification in astigmatism. Putnam et al. [
22] reported that a corrective method that considered interactions between HOAs and lower-order aberrations improved night vision through precise correction of the cylinder.
The greater elevation in refraction under mesopic vs. photopic illumination in postoperative eyes may explain the poor night vision in selected patients. Refraction that is not apparent under photopic illumination may become obvious under mesopic illumination. Bamashmus et al. [
23] discovered that uncorrected vision was significantly correlated with the postoperative refraction after LASIK. Thus, the higher refractive error under mesopic illumination may lead to poorer postoperative night vision. Therefore, it is necessary to measure wavefront refraction at different illuminations and correct even small degrees of myopic error—particularly in patients with poor night vision—as these patients may be more sensitive to a myopic shift at low illuminations.
In addition, we observed larger proportions of spherical aberrations and horizontal coma in postoperative eyes, compared with natural eyes, when switching from photopic to mesopic illumination. Similar differences were observed in spherical- and coma-like aberrations between pupils of 3 and 7 mm in diameter after photorefractive keratectomy [
24]. These changes may be explained from the perspective of surgical ablation. First, in natural eyes, peripheral corneal flattening and a radial gradient in the refractive index of the lens offset the increase in spherical aberration under low illumination [
10]; however, in postoperative eyes, the peripheral cornea is steeper than the central cornea, leading to a higher positive spherical aberration under low illumination. Furthermore, the increase in coma under low illumination is thought to be associated with the presence of mild levels of SMILE-induced decentration [
25]. Additionally, there may be a correlation between angle Kappa and coma [
26]. Finally, the correction of astigmatism with SMILE creates a lenticule with an oval posterior surface. Hence, the extraction of the oval lenticule from the stroma may be a source of postoperative coma [
27].
The observed abundance of HOAs under mesopic relative to photopic illumination also influences postoperative night vision. Chalita et al. [
18] evaluated correlations between HOAs and night vision symptoms after LASIK, demonstrating that double vision was correlated with total and horizontal coma, and that starburst was correlated with total coma. In addition, there was a correlation between glare and spherical aberrations. Furthermore, Amigó et al. [
21] discovered that differences in spherical aberration were inversely correlated with differences in subjective refraction, using an adaptive optics system. Thus, the higher occurrence of positive spherical aberration under low illumination may result in a myopic shift in postoperative eyes. Additionally, contrast sensitivity worsens with an increase in spherical aberration [
28]; therefore, HOA measurements are needed for patients with poor night vision, and wavefront-guided retreatment can be used to improve night vision by reducing induced HOAs, if necessary [
29].
There are several reports of compensation effects for aberrations of the anterior corneal surface and the internal optics of natural eyes, which assist in the optimization of optical quality under low illumination [
11,
17,
30]. In the current study, we observed stronger compensation effects for fourth-order (75 and 47.5% of eyes demonstrated a compensation effect under mesopic and photopic illumination, respectively) and primary spherical (80 and 42.5% of eyes demonstrated a compensation effect under mesopic and photopic illumination, respectively) aberrations under mesopic illumination in natural eyes. Conversely, a weaker compensation effect for horizontal trefoil (42.5 and 82.5% of eyes demonstrated a compensation effect under mesopic and photopic illumination, respectively) was found under mesopic illumination in natural eyes. Spherical aberrations are one of the most important factors in optical quality. Consequently, this phenomenon may be a mechanism by which natural eyes maintain adequate night vision; however, postoperative eyes seem to lose their spherical aberration-specific compensation ability under low illumination. Therefore, in addition to differences in aberration values, differences in compensation for aberrations may be one reason for poor night vision after SMILE. As stated by Benito, Redondo, and Artal [
31], customized procedures should be performed to maintain the natural compensation ability and achieve improved night vision outcomes.
There were some limitations to this study; first, the optical zone was not strictly constrained for all 40 eyes, and may have had an indirect confounding effect on lower- and higher-order aberrations under different illuminations. Second, due to the sample size of this study, stratified analysis could not be conducted based on the level of myopia; additionally, the degree of night myopia varies among individuals. In this study, the changes of refraction under mesopic illumination was represented by an average value. It is necessary to conduct an age-stratified study on a larger sample. Finally, subjective night vision parameters were not included. In future studies, correlations between optical quality parameters (refraction, HOAs, and compensation for aberrations) and subjective night vision parameters need to be investigated, which may help surgeons identify the most important factors affecting postoperative visual quality, as well as determine suitable methods to improve surgical outcomes.
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