Key words
myopia - axial length - children - biometry - refractive error - progressive myopia
Schlüsselwörter
Myopie - Achslänge - Kinder - Biometrie - Refraktionsfehler - fortschreitende Myopie
Introduction
Biometer devices for the assessment of eye biometry have a longstanding use in the
power calculation for intraocular lenses in mostly middle-aged or elderly patients
who undergo cataract
surgery or refractive lens exchange [1], [2]. More recently, biometers have become important tools for ophthalmologists that
focus on
myopia control management in children and adolescents [3], [4]. For myopia control management, it is important to detect any deviation
from a normal growth pattern in a childʼs eye as early and as safely as possible and
to start the appropriate treatment.
Myopia can be triggered by behavioral and environmental factors [5]. It usually first develops in childhood between the ages 5 and 10 [6]
(often called school myopia). It is contemplated that every childʼs eye that has a
normal growth pattern of a developing eye will eventually approach the state of emmetropia
in adulthood. Any
axial length (AL) growth that is in excess of this normal growth will cause axial
myopia and, if not reduced to normal, lead to the condition of progressive myopia
[7]. Treatment options that have been proven to be effective by randomized clinical
trials (RCTs) are low-dose atropine [8] and vision aids such as multifocal
contact lenses [9], orthokeratology lenses [10], and spectacle lenses having included lens segments [11], [12]. Myopia control management uses the therapeutic tools with the aim to decrease an
eyeʼs excessive axial growth to approach at a growth rate that
corresponds to the growth of children of the same age who become or stay emmetropic
[3], [4], [13]. Thus, one
essential element for proper myopia control management is the assessment of AL growth
rate [14] through the measurement of AL (mm) at two points of time. The AL
growth rate is then calculated at the time of the second measurement and referenced
to a 1-year period to give numbers in mm growth per year (mm/yr). For a good assessment
of AL growth, the
availability of a reliable and stable AL measurement is key.
With the introduction of new biometers that are specifically designed for AL measurement
in myopia management [Myopia Master, Oculus Optikgeräte, Wetzlar, Germany; by partial
coherence
interferometry (PCI), Myah, Topcon, Tokyo, Japan; by optical low-coherence interferometry
(OLCI), and novel software tools for myopia management on the Lenstar LS900 (Haag-Streit),
Koeniz,
Switzerland; by optical low-coherence reflectometry (OLCR)], the measurement of AL
in children is about to become the key method for optometrists and ophthalmologists
who are dedicated to
myopia management [15], [16]. Any optical biometry holds the advantage that it is independent of pupil size and
accommodation, as an AL
measurement can be performed before and after cycloplegia, leading to the same results
[17]. Yet, the IOLMaster 700 [Carl Zeiss Meditec, Oberkochen, Germany; by
swept-source optic coherence tomography (SS-OCT)] remains the standard instrument
for biometry for ophthalmologists concerned with cataract surgery or refractive surgery.
One problem may arise
from the fact that there are different biometers around, which even employ different
technologies of biometry. Children who are about to enter myopia management are sometimes
called for a
second opinion by a different optometrist or ophthalmologist who may then employ a
different optical biometer. It is thus important to know whether there is a good agreement
between the
different types of biometers. Pedersen and colleagues [18] have looked at this before but tested adult subjects instead of children. Furthermore,
as the assessment
of the axial growth rate becomes more important, it is important to know the minimum
time interval between two measurements to calculate the current axial growth rate,
giving a minimum axial
growth to be reliably detected. Repeated measurements on the same subject will inevitably
vary around the true value because of the measurement error. On the assumption that
the standard
deviation (SD) between repeated measurements is the same for all subjects [19], we can measure the size of the measurement error for repeated measurements, i.e.,
the intra-subject SD or repeatability.
The present work thus aimed to analyze the repeatability and reproducibility of biometric
data obtained with the IOLMaster 700, Myopia Master, Myah, and Lenstar LS900.
Methods
This retrospective analysis included a total of 44 eyes of 22 myopic children who
were scheduled for a routine ophthalmological examination at our clinic between June
2022 and August 2022. To
assess the repeatability of the biometers, a subset of 16 subjects who agreed to a
second measurement on at least one of the biometers was used. Patients with ocular
pathologies other than
refractive and/or axial myopia were excluded in this evaluation.
Instruments and measurements
The AL (in mm), steepK, flatK, and meanK values (each in D) were obtained using IOLMaster,
Lenstar, Myopia Master, and Myah. To minimize inter-operator variation, all measurements
were
performed by the same optometrist (A. M.) in a dim lit (15 lx) room. The measurements
were performed at all the biometers, IOLMaster, Myopia Master, Myah, and Lenstar,
before the full
ophthalmic exam. Both eyes of each subject were included in the evaluation and the
biometric measurements of each eye were considered as independent. All eyes (n = 44)
were measured with the
IOLMaster at least once. This biometer was chosen as the reference for the comparison
and to assess reproducibility (see [Table 1]). For both, the first and
second measurement, the order of biometers was randomized.
Table 1 Mean values and SD of the repeated measurements performed with all biometers.
|
Parameter
|
n
|
1st Measurement (Mean ± SD)
|
2nd Measurement (Mean ± SD)
|
P value
|
|
IOLMaster 700
|
AL (mm)
|
32
|
24.58 ± 1.15
|
24.58 ± 1.15
|
0.77
|
|
meanK (D)
|
31
|
43.79 ± 1.50
|
43.81 ± 1.45
|
0.36
|
|
J0 (D)
|
31
|
0.53 ± 0.48
|
0.54 ± 0.47
|
0.64
|
|
J45 (D)
|
31
|
0 ± 0.23
|
0.01 ± 0.24
|
0.62
|
|
Myopia Master
|
AL (mm)
|
25
|
24.61 ± 1.10
|
24.61 ± 1.11
|
0.60
|
|
meanK (D)
|
26
|
43.11 ± 1.20
|
43.14 ± 1.29
|
0.72
|
|
J0 (D)
|
26
|
0.33 ± 0.25
|
0.33 ± 0.30
|
0.75
|
|
J45 (D)
|
26
|
0.05 ± 0.13
|
0.05 ± 0.16
|
0.94
|
|
Myah
|
AL (mm)
|
30
|
24.44 ± 1.18
|
24.44 ± 1.18
|
0.95
|
|
meanK (D)
|
27
|
43.60 ± 1.52
|
43.58 ± 1.54
|
0.48
|
|
J0 (D)
|
27
|
0.55 ± 0.48
|
0.54 ± 0.46
|
0.74
|
|
J45 (D)
|
27
|
0 ± 0.20
|
− 0.02 ± 0.19
|
0.33
|
|
Lenstar
|
AL (mm)
|
29
|
24.56 ± 1.13
|
24.56 ± 1.13
|
0.30
|
|
meanK (D)
|
28
|
43.55 ± 1.48
|
43.55 ± 1.47
|
0.95
|
|
J0 (D)
|
28
|
0.54 ± 0.45
|
0.54 ± 0.45
|
0.99
|
|
J45 (D)
|
29
|
− 0.01 ± 0.21
|
0 ± 0.21
|
0.86
|
Analysis of the data
Data analysis was performed using Python (Python Software Foundation, Wilmington,
DE, USA). For reproducibility of the measurements (i.e., inter-device reliability),
the graphical method
described by Bland and Altman was adopted [20]. To measure the size of the measurement error of each biometer in our young cohort,
we calculated the intra-subject
SD based on the two consecutive AL measurements for each child. To assess repeatability,
the difference between two (consecutive) measurements for the same subject and the
true AL is
expected to be less than SQR(2)*1.96*SD or 2.77*SD for 95% of pairs of observations
(intra-subject repeatability) [19]. We calculated the minimum interval of time
that should lie between two AL measurements based on the respective intra-subject
repeatability found for each biometer. A paired Studentʼs t-test was used to compare
the first and the
second measurement of meanK, J0, J45, and AL and to compare the first measurements
of the Myopia Master, Myah, and Lenstar with the IOLMaster.
Corneal power K for the steep (steepK) and flat (flatK) corneal radius R was calculated using the
following equation:
where n’ is the refractive index of the cornea of 1.332 and n is the refractive index
of air with 1.
For the vectorial analysis, the corneal astigmatism was converted from the cylindrical
notation to power vector notation by applying a Fourier transformation using the following
equations
[21]:
where C is the negative cylindrical power calculated from steepK and flatK values and α is the cylindrical axis. J0 refers to cylinder power set at orthogonally 90° and
180°
meridians, representing Cartesian astigmatism. Positive values of J0 indicate a greater
refractive power and increased curvature along the vertical meridian than along the
horizontal. J45
refers to a cross-cylinder set at 45° and 135°, representing oblique astigmatism.
Ethics
This study was conducted with the approval of the Ethics Committee of the University
Hospital Jena (No.: 2019/1520) in accordance with national law and under the tenets
of the Declaration
of Helsinki in its latest revision. Informed consent was obtained from all participating
children and both their parents.
Results
The 22 myopic children had a mean age of 11.28 ± 2.4 yr [95% confidence interval (CI)
10.28 to 12.29 yr]; their mean spherical equivalent (SE) was − 3.53 ± 2.36 D (95%
CI − 4.52 to − 2.55
D).
Repeatability of the biometers
[Table 1] gives an overview of the first and second measurements for the corresponding parameters
collected (AL, meanK, J0, J45), where n gives the number
of eyes, as not all eyes were measured twice with each biometer. Paired Student t-test
did not show any significant difference between the measurements.
The intra-subject repeatability of AL measurements and the minimum time interval that
should lie between two consecutive AL measurements with the according 95% (CI) for
different
requirements of reliability, i.e., detection limits, are given in [Table 2].
Table 2 Overview of the intra-subject SD, intra-subject repeatability (2.77*SD), and the
calculated time interval required between two axial length measurements for
different axial growth rates to be reliably detectable.
|
Device
|
SD [mm]
(95% CI)
|
Repeatability (2.77*SD) [mm]
(95% CI)
|
Measurement interval required to reliably detect axial growth [months]
|
|
0.05 mm/yr
(95% CI)
|
0.1 mm/yr
(95% CI)
|
0.2 mm/yr
(95% CI)
|
0.3 mm/yr
(95% CI)
|
|
IOLMaster 700
|
0.02
(0.01 – 0.02)
|
0.05
(0.03 – 0.06)
|
11.3
(7.4 – 15.2)
|
5.6
(3.7 – 7.6)
|
2.8
(1.8 – 3.8)
|
1.9
(1.2 – 2.5)
|
|
Myopia Master
|
0.02
(0.01 – 0.03)
|
0.06
(0.03 – 0.08)
|
13.2
(8.0 – 18.4)
|
6.6
(4.0 – 9.2)
|
3.3
(2.0 – 4.6)
|
2.2
(1.3 – 3.1)
|
|
Myah
|
0.02
(0.01 – 0.03)
|
0.06
(0.04 – 0.08)
|
13.4
(8.6 – 18.2)
|
6.7
(3.6 – 9.1)
|
3.3
(2.1 – 4.5)
|
2.2
(1.4 – 3.0)
|
|
Lenstar
|
0.02
(0.01 – 0.02)
|
0.04
(0.03 – 0.06)
|
10.0
(6.4 – 13.7)
|
5.0
(3.2 – 6.8)
|
2.5
(1.6 – 3.4)
|
1.7
(1.1 – 2.3)
|
Reproducibility of the biometers
[Table 3] gives an overview of the measured parameters of all eyes that were measured at least
once with all of the biometers. Regarding the AL measurement,
Myopia Master and Myah were in close agreement with the IOLMaster, except for Lenstar,
which significantly deviated from the IOLMaster by 0.02 mm (p < 0.001). In the assessment
of corneal
power (meanK), only Myopia Master deviated from the IOLMaster by 0.21 D (95% CI: − 0.36
D to 0.78 D), on average (p < 0.001). The vector assessments (J0, J45) did not deviate
from each
other to a clinically relevant degree, i.e., differences were less than 0.10 D.
Table 3 Comparison of all eyes measured with all devices and the corresponding result of
the paired Studentʼs t-test with the values of the IOLMaster 700.
|
AL [mm]
|
n
|
Mean
|
SD
|
P value
|
|
n. s.: not significant, *p < 0.05, **p < 0.01, ***p < 0.001
|
|
IOLMaster 700
|
35
|
24.54
|
1.12
|
–
|
|
Myopia Master
|
24.52
|
1.11
|
n. s.
|
|
Myah
|
24.54
|
1.12
|
n. s.
|
|
Lenstar LS900
|
24.56
|
1.12
|
***
|
|
meanK [D]
|
|
IOLMaster 700
|
33
|
43.47
|
1.55
|
–
|
|
Myopia Master
|
43.26
|
1.44
|
***
|
|
Myah
|
43.44
|
1.49
|
n. s.
|
|
Lenstar LS900
|
43.48
|
1.50
|
n. s.
|
|
J0 [D]
|
|
IOLMaster 700
|
33
|
0.47
|
0.43
|
–
|
|
Myopia Master
|
0.38
|
0.33
|
**
|
|
Myah
|
0.52
|
0.44
|
*
|
|
Lenstar LS900
|
0.53
|
0.43
|
*
|
|
J45 [D]
|
|
IOLMaster 700
|
33
|
− 0.02
|
0.21
|
–
|
|
Myopia Master
|
0.01
|
0.14
|
n. s.
|
|
Myah
|
− 0.01
|
0.19
|
n. s.
|
|
Lenstar LS900
|
− 0.01
|
0.21
|
n. s.
|
[Fig. 1] shows the Bland-Altman plots for reproducibility of the Myopia Master, Myah, and
Lenstar, with the IOLMaster as the reference, for AL, meanK, J0, and
J45, with the respective mean difference and limits of agreement (LoA) shown in [Table 4].
Fig. 1 Bland-Altman plots of AL (a – c), meanK (d – f), J0 (g – i), and J45 (j – l) for Myopia Master, Myah, and
LenstarLS900 with the IOLMaster 700. The solid line represents the mean difference
and dashed lines, the lower and upper and limits of agreement from − 1.96 SD to + 1.96 SD,
with values
shown in [Table 4].
Table 4 Mean difference and limits of agreement 9FG90ROCFfrom − 1.96 SD to + 1.96 SD for
Bland-Altman plots in [Fig. 1].
|
IOLMaster 700 and Myopia Master
Mean (1.96*SD)
|
IOLMaster 700 and Myah
Mean (1.96*SD)
|
IOLMaster 700 and Lenstar LS900
Mean (1.96*SD)
|
|
AL [mm]
|
0.01 (− 0.06/0.08)
|
0 (− 0.05/0.05)
|
− 0.02 (− 0.06/0.02)
|
|
meanK [D]
|
0.21 (− 0.36/0.78)
|
0.04 (− 0.23/0.31)
|
0 (− 0.24/0.24)
|
|
J0 [D]
|
0.10 (− 0.25/0.45)
|
− 0.05 (− 0.27/0.17)
|
− 0.04 (− 0.30/0.22)
|
|
J4 [D]
|
− 0.03 (− 0.25/0.19)
|
− 0.01 (− 0.19/0.17)
|
− 0.02 (− 0.22/0.18)
|
Discussion
To monitor myopia progression in young children and adolescents, it is well established
to assess the refractive status of the eye. Recently, the biometric measurement of
the AL of the eye
and the corneal curvature became the more important means [14]. Biometric and refractive measures taken together also allow for a differentiation
between mere
refractive and axial myopia [22]. In this study, we analyzed four optical biometry devices with the same myopic children
to see whether there were clinically
significant differences in the outcomes of the measurements that might lead to confusion
or deviating interpretations regarding the current status and progression of the childʼs
myopia. A
reliable measurement of AL and assessment of the AL growth rate from two consecutive
AL measurements are required for a proper evaluation of a current or future therapeutic
intervention that
aims to reduce excessive axial growth and thus reduces or prevents further myopic
progression.
Previous comparisons of biometers were mostly published on the parameters of IOL power
prediction for use in cataract surgery. Jeon et al. evaluated the agreement between
ocular biometry
outcomes in 112 eyes of patients undergoing cataract surgery measured by the IOLMaster
and Lenstar and found high agreement with narrow 95% LoA [23]. A comparison
of the Myah, Pentacam AXL, and IOLMaster in myopic children was performed by Sabur
and Takes [24]. Rauscher et al. evaluated the feasibility and repeatability of
Lenstar biometry measurements in a pediatric population and found that repeatability
improved with age [25]. Ye et al. evaluated the accuracy of the Myopia Master
in terms of AL, keratometry, and refractive measurement in children with ametropia
and concluded that this three-in-one device provides the desired values with high
efficiency and accuracy
[26].
In our analysis, all biometers showed good repeatability in AL measurement, with values
ranging between 0.04 to 0.06 mm. Any AL measured with the Lenstar was, on average,
longer by 0.02 mm
compared to the AL measured by the IOLMaster (p < 0.001). This is considered a small
offset between the devices and would become relevant only if both devices are used
to assess AL and
axial growth in one subject. As long as the same device is used for the longitudinal
analysis of the same subjects in follow-up visits, the observed offset is of no importance.
Our findings on
the differences between the IOLMaster and Lenstar go along with the study of Jeon
et al. [23], who found in a subgroup analysis that Lenstar measures a longer AL
than IOLMaster only in longer (rather myopic) eyes and described that Lenstar may
be more influenced by the media factor since it uses the principle of reflectometry
through the medium of the
object. The IOLMaster uses a 1050 nm wavelength laser, where the Lenstar uses an 820 nm
super luminescent diode laser. The difference in the transmittance of the wavelength
due to the
turbidity of the medium and the error caused by the increase in the length of the
measurement object are combined [23]. The largest variance between AL measurements
was found when comparing IOLMaster and Myopia Master (95% LoA − 0.06 to 0.08 mm).
While AL is the primary biometric determinant of an eyeʼs refractive error, the dimension,
curvature, and refractive index of each individual ocular structure contribute to
the refractive
state [27]. Here, the software of the Myopia Master also holds a tool to analyze which part
of the eye differs from an age-dependent Gullstrand eye and to tell
whether a childʼs myopia is either caused by a high refractive power of the cornea
or lens, or rather mostly or exclusively by an increased axial elongation of the eye
bulb [28]. Regarding lens thickness, Jos et al. showed that the onset of myopia can be delayed
by a decrease in the central thickness of the lens [29]. However, as this segment of the eye was only analyzed with the Lenstar and IOLMaster,
it was not further evaluated with regard to repeatability and reproducibility in this
study.
Regarding anterior corneal power prediction, no statistically significant difference
was found between the first and the second measurement with the same biometer. When
comparing the
measurements with the IOLMaster, the Myopia Master measures meanK significantly lower
by a mean of 0.21 D. This was also described by Pedersen et al., who found that the
mean corneal curvature
was significantly flatter when measured with the Myopia Master than with the IOLMaster
in a cohort of subjects between the ages of 19 to 41 years [18]. The
IOLMaster uses a telecentric method to measure the curvature of the cornea by projecting
a light source with 18 points in a distance of 1.5, 2.4, and 3.2 mm from the center
of the cornea [30], where the Myopia Master uses four equally spaced points and a ring projected onto
the cornea to measure the central corneal curvature [31]. It is likely that these differences in measurement methods have caused a slight
difference in the keratometry results.
The normal growth pattern of a 16-year-old child shows an axial elongation of less
than 0.05 mm/year [15]. According to our results, this axial growth rate can
only be reliably detected, i.e., at a probability of 95%, if the two measurements
will be about 10 months (e.g., for Lenstar) to 13.4 months (e.g., for Myah) in time
apart from each other. In
other words, if the two measurements are less than this time interval apart, the assessment
of the axial growth rate will not be sufficiently reliable. From [Table
2], one can also draw, for each biometer employed, how far two consecutive AL measurements
must be apart to reliably detect a certain change in AL growth. For example, a 6-year-old
myopic child has a true AL growth of 0.3 mm/yr, which is assumed to be about 0.1 mm/yr
above normal age-matched eye growth of emmetropes (cf. 0.2 mm/yr is 50th percentile
annual growth for
6-year-olds in the data of Truckenbrod et al. [15]). At what point can this increased growth be detected with the biometers? Answer:
to reliably detect an AL growth
of 0.1 mm/yr, the child should not be scheduled earlier for a second AL measurement
than 5 to 6.7 months after the first one. Our study provides insight in the actual
reliability of AL
measurements with the biometers investigated. For the practitioner, it is helpful
to know what reliability from the measurements are to be expected. This is of particular
importance for the
practitioner who will use two consecutively measured AL values to determine the subjectʼs
current axial growth rate in myopia management. In a practical approach, if a reduction
in axial
growth due to the childʼs myopia treatment intervention of at least 0.05 mm/yr is
to be reliably detected, the two consecutive AL measurements of the child should be
not less than 12 months
(i.e., 11 months to 13.4 months) apart.
Conclusion Box
Already known:
Newly described:
-
The intra-subject repeatability of AL measurements in children is comparable to the
repeatability in adults.
-
In myopia control management, childrenʼs individual axial eye growth should be monitored
in a time interval not shorter than 6 months.
