Glaucoma

Longitudinal Observation of Border Tissue Configuration
During Axial Elongation in Childhood
Yong Woo Kim,1 Jin Ju Choi,2 Michael J. A. Girard,3,4 Jean Martial Mari,5 Dong Gyu Choi,2and Ki Ho Park1
1

Department of Ophthalmology, Seoul National University Hospital, Seoul National University College of Medicine, Seoul,
Korea
2
Department of Ophthalmology, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul, Korea
3
Department of Biomedical Engineering, National University of Singapore, Singapore
4Singapore Eye Research Institute, Singapore
5
University of French Polynesia, Tahiti, French Polynesia

Correspondence: Ki Ho Park,
Department of Ophthalmology,
Seoul National University College ofMedicine, 101 Daehak-ro,
Jongno-gu, Seoul 03080, Korea;
kihopark@snu.ac.kr.
Dong Gyu Choi, Department ofOphthalmology, Kangnam Sacred
Heart Hospital, Hallym University
College of Medicine, Seoul, Korea, 1,
Singil-ro, Yeongdeungpo-gu, Seoul,07441, Korea;
eyechoi@hallym.or.kr.
YWK and JJC contributed equally tothis work as first authors.
Received: October 4, 2020
Accepted: February 27, 2021
Published: April 7, 2021
Citation: Kim YW, Choi JJ, Girard
MJA, Mari JM, Choi DG, Park KH.
Longitudinal observation of bordertissue configuration during axial
elongation in childhood. Invest
Ophthalmol Vis Sci. 2021;62(4):10.
https://doi.org/10.1167/iovs.62.4.10

M

PURPOSE. To investigate the change of border tissue configuration during axial elongationin childhood.
METHODS. Fifty-four subjects (108 eyes; age range, 29.3–132.5 months) who had undergone a series of swept-source optical coherence tomography scans at intervals of 6months or longer were classified into stable axial length (AXL) eyes (n = 55; AXL changeof ≤0.36 mm) and elongating AXL eyes (n = 53; AXL change of >0.36 mm). The angle
between the Bruch’s membrane opening (BMO) reference plane and the border tissue
of Elschnig was defined as the border tissue angle (BTA). The border tissue angle, BMO
distance (BMOD) and minimum rim width (MRW) were measured in the temporal andnasal regions.
RESULTS. During 15.6 ± 7.2 months of follow-up, the AXL significantly increased from
22.8 ± 1.3 mm to 23.3 ± 1.4 mm (P < 0.001). Changes of border tissue angle and AXL
showed a significant correlation only in the temporal region of elongating AXL eyes (r =–0.409; P = 0.002), but not in stable AXL eyes. Both BMOD and nasal MRW significantly
increased from 1482.5 ± 153.0 to 1506.1 ± 154.6 μm and from 310.6 ± 83.2 to 324.6 ±95.6 μm, respectively (all Ps < 0.001). The changes of BMOD and nasal MRW showeda significant positive correlation with changes of AXL in elongating AXL eyes but not instable AXL eyes.
CONCLUSIONS. During the axial elongation in childhood, temporal border tissue configuration change, BMO enlargement, and nasal peripapillary tissue elevation showed asignificant correlation with changes in the AXL.
Keywords: myopia, axial elongation, lamina cribrosa, border tissue, optical coherence
tomography

yopia induces typical optic nerve head (ONH) changes
including optic disc tilt1,2 and torsion,3,4 peripapillary atrophy,5,6 and focal lamina cribrosa (LC) defects.7,8These structural changes render accurate ONH-based diagnosis of POAG difficult in myopic eyes.9 In addition, many
epidemiologic studies and meta-analyses have confirmedthat myopia increases the risk for POAG.10–12 Myopic optic
disc tilt and torsion is associated with development ofa retinal nerve fiber layer defect13,14 and its subsequent
progression in POAG eyes.15,16 These observations haveencouraged clinicians to explore the relationship of myopia
with POAG by elucidating the origin of optic disc tilt
or torsion and the process of myopic axial elongation inchildhood.
Kim et al.2 assessed serial optic disc images obtained
from Korean children (age range, 1–17 years) with myopic
shift and demonstrated progressive ONH tilting with thedevelopment and/or enlargement of peripapillary atrophy.

Another prospective cohort study from Korea evaluated the
deep ONH structure by spectral-domain optical coherence
tomography (OCT) in pediatric eyes (age range, 6.7–12.5
years).17 It demonstrated changes of temporal border tissue
from internally oblique to externally oblique but relatively
stable Bruch’s membrane opening (BMO) distance during
axial elongation in myopic eyes. In a subsequent study, thesame authors posited nasal horizontal shifting of LC during
myopic elongation based on the observation that the angle
between the major vascular arcades decreases and the position of the central vascular trunk is dragged to the nasal
side.18 They suggested that the oval shape of optic disccommon in myopic eyes originates from nasal LC shift but
not tilting itself.
However, the nasal LC shift theory is derived from theassumption that the central vascular trunk is embedded in
the LC, not from direct observation of LC insertion or its
configuration. The location of the central vascular trunk

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BT Change during Axial Elongation
might not be an ideal surrogate marker or even evidence
for nasal LC shift. Indeed, because the ONH is a threedimensional (3D) structure, tracking changes in blood vessel
or vascular trunk position in a two-dimensional manner
cannot grasp the actual phenomenon of ONH change during
myopic elongation. Instead, it is necessary to visualize the
LC directly to reach any appropriate conclusions on LC
shift.
The present study aimed to investigate longitudinalchanges of border tissue configuration during axial elongation in childhood, not only in myopic eyes, but alsoin emmetropic and hyperopic eyes. Swept source OCT(SS-OCT) scanning with image enhancement by the adaptive compensation technique enabled improved visualization of the neural canal opening structure in childhood
eyes.19

METHODS
The present study was a retrospective longitudinal observational study performed at Kangnam Sacred Heart Hospital
after approval by the Institutional Review Board of Hallym
University Medical Center (Institutional Review Board No.:
2020-02-024). The participants in the study were subjects
enrolled consecutively from April 2016 who had undergone
routine SS-OCT optic disc scans for ONH evaluation and met
the eligibility criteria. The study followed the tenets of the
Declaration of Helsinki (1964), and informed consent was
waived owing to the retrospective design and absence of
any additional medical intervention.

Study Subjects
For inclusion in the present study, subjects had to be
between 3 and 13 years of age and have a series of SSOCT scans and measurements of axial length (AXL) andcycloplegic refraction obtained at intervals of 6 months or
longer. The present study excluded subjects with (1) a bestcorrected visual acuity worse than 20/40, (2) evidence ofmacular or optic nerve abnormality, (3) a history of prematurity or systemic diseases, (4) a history of intraocular or
periocular surgery, or (5) poor cooperation for OCT examination.
Each of the subjects underwent complete ophthalmic
examinations including measurement of best-correctedvisual acuity (converted to logMAR), refraction, and assessment of ocular motility and alignment. The pupils weredilated with cyclopentolate 1% and tropicamide 1% at intervals of 15 minutes, and manual cycloplegic refraction as
well as autorefraction by autokeratorefractometry (KR-800A,
Topcon, Tokyo, Japan) was assessed 60 minutes after the
instillation of the first drop. Based on the refraction, eyes
were classified as emmetropia (refraction < +0.5 diopters[D] or ≥ –0.5 D), myopia (refraction < –0.5 D), or hyperopia (refraction ≥ +0.5 D). AXL was measured using partialcoherence interferometry (Lenstar LS-900, Haag-Streit AG,
Switzerland). The study subjects were further classified into
stable AXL eyes (AXL change of ≤0.36 mm) and elongating
AXL eyes (AXL change of >0.36 mm), and the criteria were
derived from the median AXL change of the entire study
population.

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SS-OCT Imaging
All eyes were dilated before undergoing SS-OCT (DRI OCT
Triton, Topcon) imaging. The 3D disc scans, in 6 mm×6
mm length and 512×256 sizes, were obtained. This protocol captures color fundus photos as well as optic disc scans,allowing for simultaneous evaluation between OCT B-scansand color optic disc photos with the built-in review program(IMAGEnet 6 Version 1.25, Topcon). The retinal nerve fiber
layer thickness, rim area, and disc area were automaticallymeasured with the built-in review program (IMAGEnet 6
Version 1.25, Topcon). All of the analyses were performed
according to the right-eye orientation. Only high-quality
scans with an image quality score of 60 or greater wereused for the analysis. Scans with defocus, poor centration,and motion artifacts (n = 10 eyes) were excluded from theanalysis.

Evaluation of Border Tissue Configuration
In the present study, three horizontal B-scans were selected
and analyzed from the center (middle one-third), superior(upper one-third), and inferior (lower one-third) horizontal meridians, and then averaged for the analysis. All of
the subsequent measurements were performed with ImageJsoftware (developed by Wayne Rasband, National Institutesof Health, Bethesda, MD; available at http://imagej.nih.gov/ij/). Serial OCT scans were linked automatically by the“Compare” function of the built-in review program. Thisfunction synchronizes the B-scans by detecting the locationof blood vessel in the shadowgram and allows for simultaneous evaluation and tracking of serial scan images at the same
location. OCT machine scaling was adjusted to a 1:1 micrometer scale before measurement. The obtained scan images
were enhanced by the moving average technique available
from the built-in review program. The averaged scan images
were further enhanced by applying adaptive compensationaccording to the relevant protocols (contrast exponent = 2;
threshold exponent = 6).20,21
The two BMOs were connected to define the BMO reference plane. The distance between the two BMO points was
defined as the BMO distance (BMOD). The angle between
the BMO reference plane and the border tissue of Elschnig
was defined as the border tissue angle (BTA). The BTA
was measured in the temporal and nasal regions, respectively. The border tissue configuration was classified into
three categories of obliqueness: (1) internally oblique, where
the border tissue extends internally to the anterior scleralcanal opening (ASCO) (BTA of <80°); (2) externally oblique,where the border tissue extends externally to the ASCO (BTAof ≥100°); and (3) nonoblique, where the border tissue isperpendicular relative to the ASCO (80 ≤ BTA < 100).22
The minimum rim width (MRW), defined as the minimum
distance from the BMO to the internal limiting membrane(ILM), was manually measured in both the temporal andnasal regions, respectively. To evaluate the intraobserver andinterobserver reproducibility of the BMOD, BTA, and MRW
measurements, two observers (Y.W.K. and J. J. C.) blind to thesubjects’ clinical information performed measurements in
20 randomly selected B-scans. The absolute agreement of asingle observer’s measurements as well as the two observers
were calculated with the intraclass correlation coefficientobtained from a two-way mixed-effect model. The overall
methodology for assessment of border tissue and LC configuration is illustrated in Figure 1.

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FIGURE 1. Measurements of BMOD, BTA, and MRW. The representative optic disc scan image with adaptive compensation is provided. The
two BMOs were connected to define the BMO reference plane. The distance between the two BMO points was defined as the BMOD. The
angle between the BMO reference plane and the border tissue of Elschnig (red dotted line) was defined as the BTA. The BTA was measured
in the temporal and nasal regions, respectively. The MRW was defined as the minimum distance from the BMO to the internal limiting
membrane (ILM) (light blue lines).

Data Analysis
Continuous variables for comparison of initial and final visitswere analyzed by the paired t test. Categorical variables
were compared using the χ 2 or Fisher’s exact test. The
false discovery rate was controlled for using the Benjamini–
Hochberg method. Except where stated otherwise, the data
are presented as mean ± standard deviations, and the levelof statistical significance was set at a P value of less than 0.05.
All of the statistical analyses were performed with R software (R version 3.6.2., available at: http://www.r-project.org;
accessed April 2020).

RESULTS
Subject Demographics
The present study finally included 108 eyes of 54 subjects.
The median age of the participants was 80.1 months (agerange, 29.3–132.5 months), and all had been followed-up for15.6 ± 7.2 months (range, 6.3–32.1 months). The medians ofbaseline refractions and AXL were –0.125 D (range, –10.00
to +7.75 D) and 22.76 mm (range, 19.62–25.17 mm), respectively. The distribution of baseline age, follow-up period,
baseline refraction and AXL are provided in Supplementary
Figure S1. At baseline, 37 eyes (34.3%) were emmetropic, 35(32.4%) eyes were myopic, and 36 (33.3%) eyes were hyperopic.
During the study period, the AXL changed significantly
from 22.78 ± 1.28 to 23.28 ± 1.42 mm (P < 0.001). The
median of AXL change was 0.36 mm (range, 0–1.79 mm).
Based on this value, the subjects were classified into stable
AXL eyes (n = 55; AXL change of ≤0.36 mm) and elongating
AXL eyes (n = 53; AXL change of >0.36 mm). The elongating AXL eyes had significantly older age (88.7 ± 19.8 vs.
67.7 ± 23.6 months; P < 0.001) and greater baseline AXL(23.15 ± 1.09 vs. 22.43 ± 1.36 mm; P = 0.003) than stable
AXL eyes. However, there were no significant differences infollow-up periods, best-corrected visual acuity (in logMAR),refractions, peripapillary retinal nerve fiber layer thickness,
rim area, or disc area between the two groups (Table 1). The
distribution of AXL and refractions at baseline and final areprovided in Supplementary Figure S2.

TABLE 1. Subject Baseline Demographics

FU period, mo
Baseline age, moFemale sex
BCVA (logMAR)
Refraction, D
AXL, mm
RNFL thickness, μm
Rim area, mm2
Disc area, mm2

Stable AXL(n = 55)

Elongating AXL(n = 53)

P Value

15.4 ± 6.3
67.7 ± 23.6
30 (54.6)
0.01 ± 0.04
0.10 ± 3.60
22.43 ± 1.36
109.3 ± 11.2
1.50 ± 0.39
2.23 ± 0.35

15.9 ± 8.0
88.7 ± 19.8
26 (49.1)
0.01 ± 0.02
0.35 ± 2.45
23.15 ± 1.09
110.9 ± 9.8
1.60 ± 0.39
2.36 ± 0.41

0.72*
<0.001*
0.71†
0.37*
0.67*
0.003*
0.42*
0.20*
0.08*

Values are mean ± standard deviation or number (%).
* Comparison was performed using the Student t test.
† Comparison was performed using the χ 2 test.
Statistically significant P values after the Benjamini–Hochberg
procedure are shown in bold.
BCVA, best-corrected visual acuity; FU, follow-up; RNFL, retinal
nerve fiber layer.

icantly decreased from 120.0 ± 39.0° to 107.7 ± 43.2° (P <0.001) in the temporal region, and significantly increased
from 137.9 ± 31.0° to 139.9 ± 31.6° (P = 0.001) in thenasal region. However, in the subgroup analysis, only eyeswith elongating AXL (but not eyes with stable AXL) showeda significant BTA change both in the temporal and nasal
region (Table 2). The BTA change showed a significant negative correlation with AXL change in the temporal region (r= –0.555; P < 0.001) and a positive correlation in the nasal
region (r = 0.410; P < 0.001). In the subgroup analysis, elongating AXL eyes showed a significant correlation between
TABLE 2. Change of BTA during Axial Elongation
Baseline

Final

P Value*

95% CI

Border Tissue Configuration Change

Entire population (n = 108)
Temporal 120.0 ± 39.0 107.7 ± 43.2 <0.001 –17.0 to 7.8
Nasal
137.9 ± 31.0 139.9 ± 31.6
0.001 0.79 to 3.15
Stable AXL (n = 55)
Temporal 120.1 ± 38.4 118.4 ± 39.3
0.15
–4.2 to 0.7
Nasal
131.6 ± 34.2 130.8 ± 34.2
0.24
–2.4 to 0.6
Elongating AXL (n = 53)
Temporal 119.9 ± 40.1 96.5 ± 44.6 <0.001 –31.5 to –15.2
Nasal
144.5 ± 26.1 149.4 ± 25.7 <0.001
3.4 to 6.4

The BTA measurements by the two observers showed excellent intraobserver and interobserver reproducibility (Supplementary Table S1). During the study period, the BTA signif-

Values are mean ± standard deviation.
* Comparison was performed using a paired t test. Statisticallysignificant P Values after the Benjamini–Hochberg procedure areshown in bold.

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BT Change during Axial Elongation
TABLE 3. Change of BMOD During Axial Elongation

Entire population (n = 108)
Stable AXL (n = 55)
Elongating AXL (n = 53)

Baseline

Final

P Value

95% CI

1482.5 ± 153.0
1462.4 ± 156.7
1503.5 ± 147.6

1506.1 ± 154.6
1472.3 ± 160.2
1541.1 ± 141.8<0.001
0.002<0.001–29.2 to –17.8–16.2 to –3.7
–45.8 to –29.4

Values are mean ± standard deviation.
* Comparison was performed using a paired t test. Statistically significant P values after the Benjamini–Hochberg procedure are shownin bold.

changes of BTA and AXL only in the temporal region (r =–0.409; P = 0.002), but not in the nasal region (r = 0.082; P= 0.56). The correlation between changes of temporal BTA
and AXL was still significant when adjusted with baselineage of the subjects (P = 0.005). In eyes with stable AXL,
there was no significant correlation between changes of BTA
and AXL, both in the temporal and nasal regions, (all Ps >0.05) (Supplementary Figure S3).
At baseline, the most predominant type of border tissue
configuration in the temporal region was internally oblique
in hyperopic (86.1%) and emmetropic (70.3%), but was lesspredominant in myopic eyes (60.0%). Instead, in myopic
eyes, the externally oblique border tissue was more predominant (25.7%) than in hyperopic (2.8%) or emmetropic(18.9%) eyes. The externally oblique border tissue in thetemporal region had become more prominent at the final
visit (P < 0.001; Supplementary Table S2). In the nasal
region, the most predominant border tissue configuration
at baseline was internally oblique in all groups. Myopic eyes
had no externally oblique border tissue, whereas 19.4% of
hyperopic eyes exhibited an externally oblique border tissue
in the nasal region. There was only one case of an externally oblique border tissue in emmetropic eyes at baseline in the nasal region (Supplementary Table S2). The eyespecific changes in BTA and border tissue configurations are
animated in Supplementary Video S1.

BMOD and MRW Change
The BMOD and MRW measurements by the two observers
showed excellent intraobserver and interobserver reproducibility (Supplementary Table S1). During the study
period, the BMOD significantly increased from 1482.5 ±153.0 μm to 1506.1 ± 154.6 μm (P < 0.001). In the subgroup
analysis, both elongating and stable AXL eyes showed significant increase of BMOD (mean of the differences, –37.6 [P< 0.001] and –9.9 μm [P = 0.002], respectively) (Table 3).
The increase in BMOD had a significant positive correlation
with the increase in AXL (r = 0.524; P < 0.001). However,this correlation was only significant in elongating AXL eyes(r = 0.358; P = 0.008), but not in stable AXL eyes (r =0.176; P = 0.20) (Supplementary Fig. S4). The correlation
between changes of BMOD and AXL were still significant
when adjusted with baseline age of the subjects (P < 0.001).
During the study period, the nasal MRW significantly
increased from 310.6 ± 83.2 μm to 324.6 ± 95.6 μm (P< 0.001). However, there was no significant change in thetemporal MRW during the same period (Table 4). In thesubgroup analysis, the elongating AXL eyes showed a significant increase in nasal MRW (mean of the differences =21.8 μm; P < 0.001), but no change in temporal MRW (P= 0.11). The stable AXL eyes did not show any significant
changes in neither nasal nor temporal MRW during the same
period (Table 4). There was a significant correlation between

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TABLE 4. Change of MRW during Axial Elongation
Baseline

Final

Entire population (n = 108)
Temporal 216.7 ± 44.4 214.8 ± 44.7
Nasal
310.6 ± 83.2 324.6 ± 95.6
Stable AXL (n = 55)
Temporal 213.2 ± 49.9 213.5 ± 51.0
Nasal
315.4 ± 91.0 321.9 ± 101.0
Elongating AXL (n = 53)
Temporal 220.4 ± 38.1 216.2 ± 37.6
Nasal
305.6 ± 74.8 327.3 ± 90.7

P-value*

95% CI

0.26<0.001–1.4 to 5.2
7.4 to 20.6

0.90
0.11

–4.6 to 4.0–0.9 to 9.1

0.16<0.001–1.5 to 8.5
11.1 to 32.4

Values are mean ± standard deviation.
* Comparison was performed using the paired t test. Statisticallysignificant P values after the Benjamini–Hochberg procedure areshown in bold.

changes of AXL and nasal MRW during the study period (r =0.424; P < 0.001). However, this correlation was only significant in elongating AXL eyes (r = 0.466; P < 0.001) and not in
the stable AXL eyes (P = 0.12) (Supplementary Fig. S5). The
correlation between changes of nasal MRW and AXL werestill significant when adjusted with the baseline age of thesubjects (P < 0.001).
A representative case showing changes in the BTA,
BMOD, and MRW during myopic axial elongation isprovided in Figure 2. A schematic illustration of longitudinalchange of deep ONH structure in myopic eyes is provided
in Figure 3.

DISCUSSION
The present study investigated longitudinal changes in theborder tissue during axial elongation in childhood, including
myopic, emmetropic, and hyperopic eyes. During the study
period, the AXL significantly increased with the medianvalue of 0.36 mm. The BTA significantly decreased in thetemporal region (i.e., the border tissue changed to externallyoblique) and showed a significant negative correlation with
AXL change. The BMOD and nasal MRW showed a significant positive correlation with AXL changes especially in eyeswith AXL change of more than 0.36 mm.
During myopic axial elongation, the macula moves posteriorly, causing tensile stress to the ONH and neural canal
opening. Because the neural canal opening is connectedto the superonasal region of the eyeball, the tensile stressto the ONH will be more marked in the temporal region
than in the nasal region. This phenomenon is quite analogous to the tensile stress applied to the ONH during adduction of the eyeball. A recent magnetic resonance imaging study demonstrated optic nerve sheath straightening
and tethering of the eyeball during adduction.23 A finite
element model study predicted shearing deformation of
peripapillary tissues during horizontal eye movements; inBT Change during Axial Elongation

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FIGURE 2. Representative case of longitudinal changes of border tissue configuration. Optic disc scan of the left eye of 7-year-old girl atbaseline (A–D) and final (E–H, 18 months later) visits. Enhanced images with adaptive compensation are provided (C and G). The baseline
refraction was –7.25 diopters and changed to –9.125 diopters at the final visit. The AXL increased from 25.05 to 26.14 mm. During thefollow-up period, the BMOD increased from 1622 to 1712 μm (B and F). The temporal BTA decreased from 22.3° to 15.4°, while the nasal
BTA changed from 169.4° to 170.8°. The nasal MRW increased from 251 to 268 μm (C and G). A 3D-rendered OCT image revealed elevationof nasal peripapillary tissues during the follow-up period (D and H).

FIGURE 3. Illustration showing longitudinal changes of border tissue configuration in myopic eyes. Note the decreased temporal BTA andrelatively stable nasal BTA, and elevation of nasal peripapillary tissue during axial elongation.

adduction, temporal peripapillary tissues are taut posteriorly, whereas nasal peripapillary tissues are elevated anteriorly, and vice versa in abduction.24 Their subsequent in vivo
spectral-domain OCT study exhibited progressive, but tran-

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sient, temporal ONH tilting and BMO displacement (anterior displacement of nasal BMO and posterior displacementof temporal BMO) during adduction of the eyeball.25,26 The
pattern of ONH change was consistent with the present

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FIGURE 4. Schematic diagram showing nasal LC tilting during axial elongation in myopic eyes. The previous nasal LC shift theory is depictedin the upper row. The new hypothesis for optic disc tilting in myopic eyes is proposed in the lower row. During axial elongation, the macula
moves posteriorly and the BMO stretches to the temporal side (initial BMO size depicted in black dotted line) with elongation of temporalborder tissue and configuration change to externally oblique. Owing to the tensile stress in the temporal direction, the nasal neural canalbecomes more constricted, and the nasal peripapillary tissue elevates, causing the steeper major vessel and nasal LC to have a pronounced
curvature, resulting in the ‘lying L’ shape and temporal tilt of optic disc.

findings observed during myopic axial elongation. Although
myopic axial elongation and adduction of the eyeball aredifferent situations, they share the phenomenon of tensilestress to the temporal region of the ONH.27 In this sense, it
can be postulated that ONH change observed during adduction of the eyeball is analogous to ONH change during
myopic axial elongation.
The tensile stress in the temporal ONH during myopic
axial elongation can lead to permanent externalization andelongation of the temporal border tissue.22,28 Recently, Honget al.29 investigated the 3D neural canal direction andobliqueness by assessing the ASCO offset relative to BMO.
From a cross-sectional observation of 362 healthy eyes,
an increase in the AXL was associated with ASCO/BMO
offset magnitude, direction, and neural canal obliqueness.
In their subsequent study of 69 highly myopic eyes, theBMO and ASCO areas were significantly larger and more
elliptical in highly myopic eyes than controls.30 The neural
canal obliqueness was also greater in highly myopic eyes
than controls. The present study further demonstrated axial
elongation-related changes in the temporal BTA, BMOD,
and nasal MRW in myopic eyes. Although the present study
differs in that the measurements were made in three, nonradial B-scans per eye and the age range of participants isdifferent, it is quite relevant to the previous two studies inthat the BMO can move relative to the underlying ASCO.
Considering all of the past and present data together, it canbe postulated that, with elongation of the temporal bordertissue and its configuration changing to externally oblique,the BMO can be stretched to the temporal side, the nasal
neural canal becomes more constricted, and the nasal peripapillary tissue elevates, thereby causing temporal tilt of the
optic disc (Fig. 4).
This study has the following limitations. First, owing to its
retrospective design, the follow-up interval differed among
the subjects. However, there was no significant inter-groupdifference in the follow-up period. Additionally, the analysis
focuses on the effect of AXL changes on outcome parameters, so differences in follow-up periods between subjects
may be less affected. Second, the present study protocol
did not acquire repeated scans for the outcome parameters
so that it is difficult to tell whether the observed changes

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exceed the test–retest variability. In this sense, although thechanges in the nasal BTA were statistically significant (meanof the differences = –1.96°; P = 0.001), it is difficult to tellwhether the change was clinically significant in exceeding
the test–retest variability. Third, magnification correction canbe achieved by DRI Triton OCT by entering the subject’s
AXL before acquiring the image. However, the acquisition
protocol of the present study used a default AXL value of
24.39 mm and did not enter the eye-specific AXL values.
To account for the magnification errors, a post hoc adjustment was performed by using the formula of Bennett etal.31 From this post hoc adjustment, the BMOD still showedsignificant change during axial elongation in overall study
population (mean of the differences = 12.7; P < 0.001) andelongating AXL eyes (mean of the differences = 19.5; P <0.001) but not in stable AXL eyes (mean of the differences= 6.2; P = 0.08). The nasal MRW also showed significant
increase (P = 0.043) as well as significant correlation with
AXL change (P = 0.003). This trend was only significant inelongating AXL eyes and not in stable AXL eyes. However,
we decided to keep presenting the original data because
this post hoc compensation uses Gullstrand’s schematic eye
as a model and may introduce errors by oversimplifying theocular optical system.32 Fourth, the subjects in the present
study were all Koreans, and it is well-known that the biomechanical properties of the ONH can differ by race; thus,
the present findings cannot be generalized to other ethnicities.33,34 Last, the present study included only healthy childhood eyes, and so its findings cannot be extrapolated toglaucoma pathogenesis. To understand the role of myopic
ONH changes in glaucoma development, more extensive studies involving patients with myopic glaucoma areneeded.
In conclusion, the present study investigated, for the firsttime, changes of the border tissue configuration during axial
elongation in childhood, including myopic, emmetropic, andhyperopic eyes. During this period, the changes in AXL
showed a significant correlation with changes of temporal
BTA, BMOD, and nasal MRW. The process of myopic axial
elongation is a 3D phenomenon and should be monitored
as such. The present study broadens the understanding of
ONH changes during axial elongation in childhood.

BT Change during Axial Elongation

Acknowledgments
The authors thank YoungHyun Yun (Department of Anatomy,
Seoul National University College of Medicine) for the preparation of the excellent graphics.
Disclosure: Y.W. Kim, None; J.J. Choi, None; M.J.A. Girard,
None; J.M. Mari, None; D.G. Choi, None; K.H. Park, None

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