Ⅰ. Introduction
Pupil is an important optical element in visual system, which controls the amount of light entering the eye and blocks light getting through the cornea, and allowing the object to be seen clearly. The center of pupil and an anatomical corneal vertex are separate from each other, not being overlapped. A pupil center is located at temporal side of the corneal reflex in general.1) An offset of pupil center is defined as a distance from the corneal vertex to the pupil center (Fig. 2). A visual axis is defined as an axis that connects the entrance of pupil center and the fovea of the retina, and an optical axis as one that connects the corneal vertex and the posterior pole. They do not overlap each other and form an optically constant angle called a Kappa angle (about 5°). A Lambda angle, which is a corresponding value of a kappa angle formed by the pupillary axis and line of sight, is used clinically (Fig. 1). The lambda angle and the offset of pupil center are highly correlated.2,3)
The offset of pupil center can be measured as a distance between the pupil center and the corneal vertex (Fig. 2), which can be changed according to several factors such as age, illumination, accommodation, refractive corrections, and drug.4-12) The changes in pupil size could affecting pupil offset by moving the pupil center, and affecting the lambda angle and the ocular higher-order aberration (HOAs), causing glare, halos, or ghost images that could affecting the quality of vision.13,14)
The ocular aberration can be assessed by measuring the line of sight connecting the fixation point and the entrance of pupil center. The value is minimized when the line of sight and the optical axis overlap each other,15) therefore, HOAs also changes according to pupil size. Wang et al.16) reported that the larger the pupil size, the more the HOAs. In some studies, SA (spherical aberration) and coma are found to be increased with larger pupil size,17) while other studies have reported that the HOAs tend to increase with more shifts of pupil center and coma aberration increased as the distance between the corneal vertex and the geometric pupil center increased.6,18)
The HOAs can vary with pupil center position as well as pupil size. It is reported that the pupil center offset in 3 mm pupil changed HOAs but the amount was not clinically significant, however, with the pupil size of 4 mm, changes in the pupil center offset led to changes in the HOAs.19) The coma increases as pupil center moves away from corneal vertex, which can reduce visual quality.13,20,21) Several studies on changes in pupil center according to drugs, accommodation and illumination have been conducted, and results of HOAs changes have also been reported.4-9)
In refractive surgery, a pupil center can be anatomical landmark. In a study of Tarbernero et al.8) reported pupil center offset of 0.07 mm with 7.00 mm pupil size could reduce visual quality. The pupil center position after corneal refractive surgery could be the causes of glare, and correlated with the best corrected visual acuity, and contrast sensitivity.22) When pupil center offset is larger, the HOAs can increase by up to 20%.6)
The pupil size and the pupil center offset may affect the HOAs and the quality of visual acuity. The age-dependent decrement in the pupil size is known that in this study we investigate the effect of pupil size and pupil center offset on ocular HOAs with different illuminance conditions in myopic eyes of 20s.
Ⅱ. Subject and Method
1. Subject
Thirty three young myopes (23.03±2.89 years, -3.28±1.67 D, range -0.75~-7.00 D) with no history of ocular surgery, systemic disease and have corrected monocular visual acuity less than 0.8 and binocular VA less than 1.0 were participated. All measuring procedures and protocols were approved by the Institutional Review Board (EU17-43), and subjects were informed about the purpose of the study and the measuring method by oral and written explanation.
2. Measurements
1) Subjective refraction
The refractive error was measured with Wavefront Analyzer (KR-1W, Topcon, Japan) and Phoropter (VT-SE, Topcon, Japan).
2) Pupil size and pupil center offset
Pupil size and pupil center offset was measured three times using Wavefront Analyzer (KR-1W, Topcon, Japan) at a pupil size of 4 mm under scotopic condition (0 lx) and photopic condition (20 lx) adjusted by the instrument in the examination room with 100 lx illuminance condition after adaptation. Pupil size and pupil center locations from the corneal vertex (pupil center shift) were measured and pupil center offset was calculated.
3) Higher order aberrations
The high-order aberrations of the eye are expressed by Zernike polynomial, among them, the higher-order aberrations that affect the quality of vision are known as SA and coma aberration. In this study the ocular total HOAs, SA (Z40) and coma (Z31) were measured under the scotopic and photopic condition using Wavefront Analyzer (KR-1W, Topcon, Japan) respectively and correlation between the changes of total HOAs, SA and coma and the pupil center offset were analyzed.
4) Data analysis
Statistic analysis was performed using SPSS version 21.0 (SPSS Inc., Chicago, IL, USA). Correlations between pupil size, pupil center shift, and HOAs according to the refractive error were analyzed using Pearson correlation analysis. The empirical analysis of this study was validated at significance level p<0.050.
Ⅲ. Results
1. Pupil size and refractive error
The pupil diameter was 6.94±0.61 mm in the scotopic condition and 4.94±0.68 mm in the photopic condition and those were significantly different (p<0.001). And the pupil size was correlated with refractive errors in scotopic (r=-0.356, p=0.003) and photopic condition (r=-0.433, p<0.001) (Fig. 3).
2. Pupil center offset
The pupil centers mainly located toward superotemporally and inferotemporally from the cornea vertex in scotopic and photopic conditions (Fig. 4). The directions of pupil center shift were superotemporally, superonasally, inferotemporally, inferonasally 45.4%, 9.2%, 34.8%, and 10.6%, respectively in scotopic condition and 45.4%, 22.7%, 24.2%, and 7.5%, respectively in photopic conditions (unpublished data).
The pupil center offset was 0.29±0.18 and 0.36±0.19 mm in scotopic and photopic condition from the cornea vertex that the amount of pupil center shift was increased by 0.17±0.13 mm in photopic condition compared with scotopic condition (p<0.003). The directions of pupil center shift were superotemporally, superonasally, inferotemporally, inferonasally (31%, 34%, 15%, and 18%, respectively) from scotopic to photopic conditions and the amount and moving directions of shift were different depending on the individual (range: 0.005 ~ 0.519) (Table 1, Fig. 4). The amount of pupil center offset was not correlated with refractive errors in scotopic and photopic conditions (p=0.165, p=0.703).
3. Correlation between HOAs with pupil size and pupil center offset
Total HOAs was 0.108±0.038 and 0.104±0.034 mm respectively in the scotopic and photopic condition and those were not significantly different (p= 0.330) that means total HOAs was not influenced by pupil size. The coma was 0.045±0.019 and 0.550±0.022 ㎛ and ocular SA was 0.029±0.013 and 0.013±0.012 ㎛ in the scotopic and photopic condition, respectively, and those were significantly different (p=0.002, p=0.003). The coma was smaller in the scotopic condition, whereas the SA was smaller in the photopic condition significantly. The coma was not correlated with pupil size in the scotopic (r=0.008, p =0.947) and photopic condition (r=0.008, p=0.947) and SA was positively correlated with pupil size in the scotopic (r=0.338, p=0.006) and photopic condition (r=0.328, p=0.007) (Fig. 5).
The total HOAs and SA were not correlated with pupil center offset in scotopic an photopic condition and significant interactions were found for the coma in scotopic and photopic condition (r=0.273, p=0.027) (r=0.322, p=0.008) respectively (Fig. 6).
4. Pupil center shift and ocular HOAs
The total HOAs and SA had no correlation with the amount of pupil center shift (p=0.303, p=0.996), and the coma showed a correlation with the amount of pupil center shift in photopic condition (r=0.456, p=0.001) (Fig. 7). Therefore in the photopic condition, as the pupil size decreases the SA decreases and coma increases because of pupil center shift increase, whereas in the scotopic condition, SA increases and coma decreases.
Ⅳ. Discussion
In this study we investigate the pupil size, changes of pupil center shift and HOAs in the different illuminance conditions using Hartmann Shark type KR-1W aberration analyzer.
Some researchers were reported the pupil size changes according to the refractive error,23,24) while Orr et al.25) reported that the refractive error and the pupil size are irrelevant. In this study, we found in the young myopic eyes, there was a positive correlation between pupil size and the refractive errors that the higher myopic refractive error, the larger the pupil size. The pupil size is a virtually observed through the cornea and is influenced by the corneal refractive power that as the myopic refractive error increases the pupil size may also increase. Pupil size can also change with the illuminance. The SA changes intensively with the pupil size and when the visual axis of the optical elements does not pass through the geometric center of the pupil and coma also changes with the pupil size that those could affect the visual quality.6,8,19)
In everyday life, the amount of pupil center shift is important from the viewpoint of visual function. The pupil center shift have been reported with a different results depending on drug, accommodation and illuminance. When the pupil size changes according to drugs, accommodation and illumination, the pupil center shift seems to show various directions. Porter et al.6) reported the pupil center shifted 0.29 mm to inferonasally and 0.162 mm and Yang et al.14) to superotemporally due to a pharmacologically dilated condition. The direction of movement is very important in refractive surgery. In corneal refractive surgery, corneal resection is performed at an average pupil of 5 to 6.5 mm. Beyond this optical zone is a transition zone to the peripheral cornea that has not been ablated. The relationship between the size and location of the optical zone and the pupil is critical for a successful surgical outcome. For instance, if the corneal ablation is well centered over the pupil of small diameter but becomes decentered with pupils of large diameter, the pupil center off-set may occur and glare, halos, ghost image or double vision symptoms may occur.14)
In this study, the pupil centers mainly located toward superotemporally and inferotemporally from the cornea vertex and pupil center shift were 0.29±0.18 and 0.36±0.19 mm in scotopic and photopic condition with 4 mm pupil size. Depending on the illuminance, the pupil center shift was increased by 0.17±0.13 mm in the photopic condition compared with scotopic condition with the direction of superotemporally, superonasally, inferotemporally, inferonasally (31%, 34%, 15%, and 18%, respectively) from scotopic to photopic conditions and those results were consistent with the study of Lu et al.13) Wyatt et al.4) reported that when the illuminance was changed from mesopic to photopic, the pupil center moved 0.13 mm to superonasally. Tarbernero et al.8) reported pupil center shift of 0.07 mm or more based on a 7 mm pupil and a pupil center shift of 0.20 mm or more based on a 3 mm pupil.
As the pupil center shifted, the pupil center offset can be measured. In this study, the pupil center offset significantly increased as the illuminance increased to from 0.29±0.18 mm in the scotopic condition to 0.36±0.19 mm in the photopic condition. The pupil center offset was correlated with the ocular coma aberration, as the pupil center offset increased, the coma increased with a positive correlation, which was consistent with the results of Lu et al.13) Therefore, the pupil center offset could deteriorate the quality of vision, and could cause of glare, decreased visual acuity, and decreased contrast sensitivity.22)
The coma aberration occurs when the visual axis of the optical elements does not pass through the geometric ceter of the pupil. The coma aberraion increase as the distance between the cornea vertex and the geometric pupil center becomes longer. It has also been reported to have correlation with pupil size.18) However, in this study, no correlation was found between pupil size, and coma.
In conclusion, in the photopic illuminance condition the pupil size decreased compared with the scotopic condition that the SA are decreased accordingly and the pupil center shift and the offset are increased, It is considered that the coma aberration increases as the pupil size decreases as the illuminance increases and the pupil center offset increases. Therefore, aberrations may vary depending on the illuminance, which may affect vision.
Ⅴ. Conclusion
In young myopic eyes, the pupil size was 4.94±0.68 mm in the photopic condition and 6.94±0.68 mm in the scotopic condition. The pupil size showed correlation with refractive error that it was larger in the high myopes than that of low myopes. In the photopic condition, the pupil size was decreased and as with the pupil size the pupil center mainly moved toward superotemporally or superonasally that pupil center offset was increased by 0.17±0.13 mm compared with that of scotopic condition. Since there was correlation between the offset and SA, the smaller the pupil size, the smaller the SA, and the larger the offset, the higher the coma.
With the illuminance changing, the pupil size and pupil center offset are changed, that those effect on the SA and coma, which could affect the quality of vision.
