Key words
dominant eye - pupil center - angle kappa - refractive surgery
Schlüsselwörter
dominantes Auge - Pupillenmitte - Kappa-Winkel - refraktive Chirurgie
Introduction
A dominant eye is the eye from which a person prefers to receive visual input [1], [2]. The dominant eye plays an important role in binocular visual function and clinical
diagnosis and treatment of ophthalmologic disorders, such as LASIK and multifocal
intraocular lens implantation and other refractive surgeries using monocular vision
correction for the treatment of presbyopia, strabismus, amblyopia, nystagmus, and
other clinical applications [3], [4], [5].
The angle kappa is defined as the angle between the visual axis and the pupillary
axis [6], [7]. Pupil size and center position are important factors in personalized corneal refractive
surgery [8], [9]. Now, eye-tracking technology can track the pupil center of the operative eye through
the noninterference pupillary corneal reflex method, thereby estimating the direction
of the visual axis. The ideal visual axis entry point, according to Pande and Hillman
[10], is the coaxially sighted corneal light reflex. Therefore, angle kappa can be understood
as the distance between the pupil center and the coaxially sighted corneal light reflex
[11], [12]. It has become a consensus among refractive surgeons to adjust the excimer laser
ablation center from the pupil center to the visual axis to
compensate for the offset of the angle kappa [13]. However, whether there is a difference in the angle kappa, pupil size, and center
position between the dominant eye and nondominant eye in personalized LASIK with the
adjusted angle kappa is unknown.
This study explored the dynamic changes of pupil size and center position of the dominant
and nondominant eye in myopic patients who were suitable for excimer laser surgery,
with young college students as the main research subjects. By analyzing the offset
between the pupil center and the coaxially sighted corneal light reflex (P-Dist) and
relative parameters, and comparing the safety, efficacy, predictability, and visual
quality of the dominant and nondominant eyes after surgery, this study clarified the
dynamic changes of the pupil and angle kappa in the dominant and nondominant eye and
their guiding significance for laser myopia surgery. It provides a reference for the
design of LASIK in line with the optical characteristics of individual human eyes.
Patients and Methods
We studied 252 eyes of 126 young subjects (male: 65, female: 61), with a mean age
of 23.2 ± 3.5 years (range: 19 to 35 years). In all of the investigated myopic eyes,
the best-corrected visual acuity (BCVA) was equal to or better than 20/20. The mean
spherical equivalent (SE) was − 5.21 ± 1.27 D (range: − 0.50 D to − 10.00 D), astigmatism
was less than − 1.50 D, and anisometropia ≤ 2.50 D. Ocular dominance was determined
to be present in 126 (100%) subjects. These eyes exhibited normal binocular function.
Subjects wearing their spectacles held a card with a 3-cm diameter hole in the center
with both hands and viewed a target 6 meters away through the hole. One eye was then
randomly covered. When the subject could not see the target, the covered eye was identified
as the dominant eye. Ocular dominance was assigned to a specific eye when the subject
had the same result for this series of two tests. Otherwise, another series of two
tests was administered. If the second series also gave discordant results, then ocular
dominance was classified as alternating. Otherwise, the eye shown to be dominant in
this series was recorded as the dominant eye.
All patients underwent a preoperative ophthalmic evaluation that included autorefraction,
uncorrected distance visual acuity (UDVA), BCVA, slit lamp biomicroscopy, intraocular
pressure (IOP) measurement, keratometry measurements, axial length measurements, and
fundus examination. Exclusion criteria: (1) suspicious keratoconus and other corneal
ectatic diseases; (2) active ocular inflammation or infection; (3) cataracts, glaucoma,
and significant retinal diseases affecting vision; (4) severe ocular adnexal lesions;
(5) severe dry eye; (6) systemic diseases affecting the eyes. All subjects signed
an informed consent form approved by the Ethics Committee of the Affiliated Hospital
of Yanbian University and adhered to the principles of the Helsinki Declaration. The
Wavelight Allegro Topolyzer corneal topographer (WaveLight Laser Technologies AG,
Erlangen, Germany) was used to measure the position of the pupil center and pupil
size. Changes in pupil diameter were measured in all
subjects and recorded by a single experienced surgeon under mesopic conditions for
60 seconds; all acquisitions were performed without pupil dilation, and ambient lighting
conditions were exactly the same during all measurements. The calculated pupil centroid
shift was provided in the horizontal x-direction and vertical y-direction. The distance
to the apex was computed by the radial distance corresponding to the x and y shifts.
The pupil diameter recording mode also recorded the axis of the pupil center (the
center of the cornea is the origin).
A corneal flap with a diameter of 8.5 mm and thickness of 110 mm was created using
a WaveLight FS200 Hz femtosecond laser (WaveLight, GmbH, Erlangen, Germany). The patient
was asked to lie flat and focus on the green indicator light. The performer could
see the reflective point of the corneal vertex and the red reflection in the center
of the pupil under the microscope and adjusted the lighting to keep the pupil size
consistent. If the actual pupil diameter differed from the diagnostic image by more
than 20%, it was possible to modify the actual pupil size and diameter by changing
the light conditions. P-Dist was recorded using the x- and y-axis eye-tracking adjustment
program of the WaveLight EX500 excimer laser system (Wavelight GmbH). The 100% P-Dist
adjustments were manually entered into the excimer laser device, with the excimer
laser ablation center from the pupil center to the direction of the visual axis (coaxially
sighted corneal light reflex).
The 50 dominant eyes and 50 nondominant eyes were randomly selected to compare safety,
efficacy, predictability, and visual quality 12 weeks after surgery. Safety, efficacy,
and predictability can be evaluated by changes in the safety index, efficacy index,
and residual SE. A quality of vision (QoV) questionnaire was used to evaluate visual
quality. The questionnaire included nine visual symptoms such as glare, halos, starbursts,
and visual haze. Each symptom included three items: frequency of occurrence, severity,
and degree of disturbance. Each item could be divided into four levels from light
to heavy according to the degree. To avoid misunderstandings by patients, relevant
visual symptom pictures were provided during the study to help them choose.
Statistical analysis
All analyses were performed with SPSS software (version 17, SPSS, Inc., Chicago, United
States). Continuous data were reported as the mean ± SD, categorical data were reported
as frequencies and percentages, and the chi-square test was used for linear trends.
Changes in pupil diameter and the P-Dist between the dominant and nondominant eye
were statistically evaluated with the paired t-test, and correlation with relative
parameters was evaluated using the Pearson correlation test. For data that did not
follow a normal distribution, the median (interquartile range) [M(P25, P75)] is used.
A p value less than 0.05 was considered statistically significant.
Results
Ocular dominance occurred predominantly in the right eye (right vs. left: [178] 70.63%
vs. [74] 29.37%; p < 0.001). The dominant eye had consistent preoperative and postoperative
measurements. The mean corneal thickness was not statistically significantly different
between the dominant and nondominant eyes (542.6 ± 26.5 µm vs. 544.0 ± 26.9 µm; p = 0.347).
The mean anterior chamber depth was 3.25 ± 0.29 mm in the dominant eye and 3.21 ± 0.26 mm
in the nondominant eyes (p = 0.126).
[Table 1] shows the pupil size changes of the dominant and nondominant eye. There were no
statistical differences in pupil diameter between the dominant and nondominant eye
under photopic and mesopic conditions (p = 0.797 and p = 0.092, respectively). The
pupil size change (mesopic – photopic pupil diameter) was 3.35 ± 0.50 mm in the dominant
eye and 3.28 ± 0.42 mm in the nondominant eye. The pupil size change in the dominant
eye was larger than that in the nondominant eye (p = 0.045). [Fig. 1] shows these results in the form of boxplots.
Table 1 Photopic and mesopic pupil size changes in the dominant and nondominant eye.
|
Category
|
Dominant eye
|
Nondominant eye
|
Difference
|
P value
|
|
*P < 0.05, paired t-tests
|
|
Photopic pupil (mm)
|
3.06 ± 0.49
|
3.05 ± 0.52
|
0.01 ± 0.22
|
0.797
|
|
Mesopic pupil (mm)
|
6.41 ± 0.74
|
6.34 ± 0.80
|
0.04 ± 0.31
|
0.092
|
|
Change (mm)
|
3.35 ± 0.50
|
3.28 ± 0.42
|
0.07 ± 0.25
|
0.045
|
Fig. 1 Photopic and mesopic pupil diameter for the dominant and nondominant eye. The 95%
median confidence interval (external) and the interquartile intervals are shown.
[Table 2] shows the measured centroid shift, defined as the difference in distance to the
apex between the photopic pupil and the mesopic pupil. There were no significant changes
in horizontal (x) and vertical (y) pupil center shift of the photopic dominant and
nondominant eyes (p > 0.05). Under mesopic conditions, the horizontal pupil shift
was − 0.07 ± 0.14 mm in the dominant eye and 0.01 ± 0.13 mm in the nondominant eye
(p = 0.001) (the temporal displacement of the dominant eye under mesopic conditions),
and there were no significant differences in the vertical (y) pupil shift (p = 0.164;
[Fig. 2]). The centroid shift of the pupil (photopic – mesopic) was 0.304 ± 0.107 mm in the
dominant eye and 0.276 ± 0.169 mm in the nondominant eye, and the dominant and nondominant
eyes were significantly different (p = 0.034).
Table 2 Horizontal (x) and vertical (y) pupil shift and distance to the apex for the photopic
and mesopic pupil and the corresponding measured centroid shift for the dominant and
nondominant eye.
|
Category
|
Dominant eye
|
Nondominant eye
|
P value
|
|
Photopic pupil
|
|
x Shift (mm)
|
− 0.05 ± 0.13
|
− 0.02 ± 0.14
|
0.092
|
|
y Shift (mm)
|
0.02 ± 0.14
|
0.01 ± 0.15
|
0.177
|
|
Distance to apex (mm)
|
0.18 ± 0.10
|
0.19 ± 0.10
|
0.764
|
|
Mesopic pupil
|
|
x Shift (mm)
|
− 0.07 ± 0.14
|
0.01 ± 0.13
|
0.001
|
|
y Shift (mm)
|
− 0.02 ± 0.13
|
− 0.04 ± 0.15
|
0.164
|
|
Distance to apex (mm)
|
0.19 ± 0.09
|
0.18 ± 0.11
|
0.701
|
|
Pupil center shift
|
0.304 ± 0.107
|
0.276 ± 0.169
|
0.034
|
Fig. 2 Changes in pupil center position in the dominant and nondominant eye under photopic
and mesopic conditions.
The P-Dist histograms were bell shaped and centered on 0.20 mm with a longer right
tail ([Fig. 3]). The average offset distribution of P-Dist was 0.208 ± 0.098 mm (range: 0.005 – 0.492 mm).
The P-Dist was 0.202 ± 0.095 mm in the dominant eye and 0.215 ± 0.103 mm in the nondominant
eye (p = 0.021). For the dominant eye, the P-Dist for 35% of eyes was ≤ 0.15 mm and
for 64% of eyes, it was ≤ 0.20 mm. For the nondominant eye, the P-Dist for 37% of
eyes was ≤ 0.15 mm and for 62% of eyes, it was ≤ 0.20 mm. The coaxially sighted corneal
light reflex tended to the temporal side of the corneal center; it was superior temporal
for 34% of the dominant eyes and inferior temporal for 29% of nondominant eyes. The
SE was negatively correlated with P-Dist for the dominant eye (r = − 0.223, p = 0.012),
the nondominant eye (r = − 0.199, p = 0.025), and both groups combined (r = − 0.210,
p < 0.001; [Fig. 4]).
Fig. 3 Histogram of P-Dist (distance from pupil center to coaxially sighted corneal light
reflex).
Fig. 4 Relationship between the SE and the P-Dist for the dominant and nondominant eye groups
(r = − 0.223, p = 0.012 for the dominant eye; r = − 0.199, p = 0.025 for the nondominant
eye). The P-Dist (distance from pupil center to coaxially sighted corneal light reflex).
[Table 3] shows that 12 weeks after surgery, the safety index (postoperative BDVA/preoperative
BDVA) of the dominant eye group and the nondominant eye group was 1.20 (1.00, 1.22)
and 1.20 (1.00, 1.20), respectively, and the efficacy index (postoperative UDVA/preoperative
BDVA) was 1.00 (1.00, 1.20) and 1.00 (1.00, 1.20), respectively. There was no statistically
significant difference between the two groups in terms of safety index and efficacy
index (p = 0.921, 0.769), see [Table 3]. Twelve weeks after surgery, the proportion of patients with UDVA ≤ 0 (LogMAR) in
the dominant eye group and the nondominant eye group was 100 (50 eyes) and 98% (49
eyes), respectively, the proportion of patients with postoperative UDVA equal to or
better than preoperative BDVA was 82 (41 eyes) and 84% (42 eyes), respectively, and
the proportion of patients with postoperative UDVA improved by one line compared to
preoperative BDVA was 34 (17
eyes) and 36% (18 eyes), respectively; neither group had a decrease in BDVA by one
line or more. During the follow-up period, all patients completed the QoV questionnaire
survey. The most common visual symptom after surgery in both groups was visual haze,
with a frequency occurrence of 80, 63, and 31% at 1 week, 4 weeks, and 12 weeks after
surgery), respectively, in the dominant eye group, and a frequency occurrence of 78,
62, and 29% at 1 week, 4 weeks, and 12 weeks, respectively, after surgery in the nondominant
eye group (see [Fig. 5]). The severity of visual symptoms and their degree of disturbance to patients were
mostly mild or less. Patient satisfaction was high after surgery, with 98 and 100%
of patients in the dominant eye group and nondominant eye group, respectively, reporting
significant or great improvement in visual quality after surgery.
Table 3 Comparison of visual acuity and refractive status between the two groups of patients
at 12 weeks postoperatively.
|
Index
|
Dominant eyes
|
Nondominant eyes
|
p
|
|
Sex
|
50
|
50
|
|
|
UDVA [M (P25, P75), LogMAR]
|
− 0.05 (− 0.10, 0.00)
|
− 0.05 (− 0.10, 0.04)
|
0.896
|
|
BDVA [M (P25, P75), LogMAR]
|
− 0.10 (− 0.10, − 0.10)
|
− 0.10 (− 0.10, − 0.10)
|
0.905
|
|
Safety index [M (P25, P75)]
|
1.20 (1.00, 1.22)
|
1.20 (1.00, 1.20)
|
0.921
|
|
Efficacy index [M (P25, P75)]
|
1.00 (1.00, 1.20)
|
1.00 (1.00, 1.20)
|
0.769
|
|
Residual spherical equivalent
|
0.03 ± 0.32
|
0.02 ± 0.36
|
0.857
|
Fig. 5 Postoperative visual symptoms and frequency of occurrence in the two groups of patients.
Discussion
Currently, angle kappa adjustment is a vector percentage compensation between the
pupil center and the coaxially sighted corneal light reflex. However, it does not
account for the dynamic changes of the pupil, dominant eye, and other factors. Studies
have shown that ideal angle kappa compensation should change with the dynamic changes
of the pupil [14]. The size and central position of the pupil can be affected by various factors such
as light intensity, emotional tension, surgical stimulation, and close gaze at indicator
lights [15]. To determine if the individualized angle kappa adjustment can be obtained according
to the dynamic change data of the angle kappa of the dominant and nondominant eyes,
and if the ablation center point closest to the visual axis can be found to ensure
that each excimer laser spot is hit at the correct position, it is necessary to deeply
study the dynamic changes of the pupil and angle kappa
of the dominant and nondominant eyes in patients undergoing excimer laser myopic surgery.
This has significant implications for the design of personalized refractive surgery.
In our study, we found that ocular dominance occurred predominantly in the right eye
(p < 0.001). The pupil size change in the dominant eye was larger than that in the
nondominant eye (p = 0.045). Under mesopic conditions, the horizontal pupil shift
was significantly different between the dominant and nondominant eyes (p = 0.001).
The centroid shift of the pupil was also significantly different between the dominant
and nondominant eyes (p = 0.034). The P-Dist was significantly different between the
dominant and nondominant eyes (p = 0.021). The SE was negatively correlated with P-Dist
for the dominant eye (p = 0.012), the nondominant eye (p = 0.025), and both groups
combined (p < 0.001).
However, we also found some results that were not significant. The mean corneal thickness
was not statistically significantly different between the dominant and nondominant
eyes (p = 0.347). The mean anterior chamber depth was also not statistically significantly
different between the dominant and nondominant eyes (p = 0.126). There were no statistical
differences in pupil diameter between the dominant and nondominant eyes under photopic
and mesopic conditions (p = 0.797 and p = 0.092, respectively). There were no significant
changes in horizontal (x) and vertical (y) pupil center shift of the photopic dominant
and nondominant eyes (p > 0.05). There were also no significant differences in the
vertical (y) pupil shift under mesopic conditions (p = 0.164).
The present study indicates that ocular dominance occurred predominantly in the right
eye (70.63%), which is consistent with other reports [16], [17], [18]. In this study, the average P-Dist was 0.208 ± 0.098 mm, the minimum was 0.010 mm,
and the maximum was 0.580 mm. The P-Dist for the dominant eye was 0.202 ± 0.095 mm,
and that for the nondominant eye was 0.215 ± 0.103 mm. The dominant eyes had a smaller
angle kappa than the nondominant eyes. The coaxially sighted corneal light reflex
shifted mainly to the temporal side of the corneal center, and it was mainly distributed
in the superior temporal region for dominant eyes and in the inferior temporal region
for nondominant eyes. The pupil size of the nondominant eye was smaller than that
of the dominant eye, but there was no significant difference. In general, the angle
kappa is relatively small in myopic eyes, which means that the visual axis
and the center of the pupil are relatively close. As a result, during myopic ablation,
the laser can accurately target the central part of the cornea, creating a larger
optical zone, resulting in a flatter and more uniform corneal curvature after ablation.
Comparison of pupil diameter indicated no significant differences between dominant
and nondominant eyes. The present study found a significant association of myopia
and the dominant eye in regard to photopic and mesopic pupil size change. We found
that the pupil size change corresponded to 3.35 ± 0.50 mm for dominant eyes and a
3.28 ± 0.42 mm relative reduction for nondominant eyes; the pupil size change of dominant
eyes was greater than that of nondominant eyes. We speculate that the dominant hemisphere
of the brain may affect which eye is held closer to the plane of the near task, especially
when writing. Because of the inconsistent fixation distance, the nondominant eye in
the accommodative response of two eyes in binocular viewing of real targets needs
more accommodation to achieve the same vision status as the dominant eye, resulting
in a larger angle kappa relative to the dominant eye [19], [20], [21]. However, there is currently not enough evidence to determine whether these changes
are congenital or acquired [22], [23]. Cheng et al. [24] found that the dominant eye plays a primary role in accommodation in binocular viewing,
resulting in greater defocus compared with nondominant eyes in myopia. The pupil center
(x-axis) of the dominant eye was − 0.07 mm and that of the nondominant eye was 0.01 mm.
The pupil centers of the nondominant eye were basically distributed around the center
of the cornea, and those of the dominant eye were 0.08 mm more temporal than for the
nondominant eyes. Theoretically, the larger the angle kappa, the greater the distance
between the pupil center and the coaxially sighted corneal light reflex [25], [26].
In this study, the angle kappa of nondominant eyes was greater than that of dominant
eyes. The center position and angle kappa of dominant and nondominant eyes could be
evaluated, and the individual angle kappa adjustment vector percentages could be obtained
to find the ablation centration point closest to the visual axis. The pupil center
difference between the dominant and nondominant eyes guides operations so that the
ablation center of the dominant eye is positioned as far as possible from the center
of the cornea, slightly to the temporal side, within − 0.07 mm, such as in the pupil
center. When the percentage of the angle kappa adjustment vector is individualized,
it is suggested that the proportion of the angle kappa adjustment in dominant eyes
is lower than that in nondominant eyes [27]
[28]
[29].
Both groups had good safety, effectiveness, and predictability in the short term after
eye surgery. Both groups had visual symptoms after surgery, with visual blurring being
the most common, but overall patient satisfaction was high and postoperative objective
visual quality performance was good.
Conclusion Box
Already known:
-
Pupil changes in dominant eyes are slightly different from those in nondominant eyes.
-
Accurate positioning of the excimer laser cutting center for dominant and nondominant
eyes is crucial.
-
Further research is needed to study angle kappa compensation and wavefront aberrations
of dominant and nondominant eyes.
Newly described:
-
Under mesopic conditions, the pupil center of the dominant eye is slightly shifted
to the temporal side.
-
In individualized corneal refractive surgery, adjusting the angle kappa percentage
in the nondominant eye to be higher than that of the dominant eye may be beneficial
for UDVA, predictability, effectiveness, safety, and quality of vision.
-
Further research is needed to study angle kappa compensation and wavefront aberrations
of dominant and nondominant eyes, and the digital correspondence between personalized
ablation of various modes remains to be further explored.
