Ⅰ. Introduction
The axial length (AL) is the most critical element of the eyeball that determines the refractive error of the eye. Many studies regard myopia as a result of the increase in axial length, but that is still indisputable.1) Recently, an announced report showed that the myopia prevalence of the youth aged 12 to 18 years in Korea was as much as 78.8%.2) Since refractive error of eye including myopia is the main cause of visual disturbance around the world,3) we are making much efforts to find the risk factor of myopia. In this regard, identifying the determinants of emmetropia and the risk factors of myopia are important in order to prevent the transition from emmetropia to myopia.4-7)
In addition, the AL is a critical factor that determines visual acuity after intraocular lens implantation for a cataract. Since congenital cataracts represent a high percentage (5~20%) of visual disturbances throughout the world, studies have been conducted continuously on several surgical techniques related to intraocular lens insertion for the improvement of vision in children.8) Even a small error in the AL measurement is known to have a significant effect on the degree of residual refractive error. For example, a difference of 0.1 mm in AL will cause the difference as much as 0.25-0.75 D in calculation of intraocular lens power.9) Shamrani et al8) stated that the measurement value of axial length should be used very carefully when calculating intraocular lens power in order to reduce errors.
On the other hand, the ability to measure the AL of children accurately with the development of non-contact precision equipment for AL has been reported. Most importantly, Chakraborty et al10) who measured the daily variation of the AL by using this equipment in 2011, reported that the AL of adults aged 18 to 30 years is the longest during the day and is the shortest during the night.11-13) Daily variation of this value will have tremendous impact on correctly identifying the measurement value and the refractive error of the eye. Benett and Rabbetts reported that the AL change of 0.032±0.018 mm is the same as the refractive power change of 0.083 D.10,14) In addition, since the AL of children is helpful for the diagnosis of glaucoma, the measurement value of normal AL is required.
Since there is no sufficient study reporting the difference of AL in accordance with the measurement time among children, the present study aimed to investigate the AL distribution of children aged 12 to 13 years and to compare the measurements between the morning and afternoon.
Ⅱ. Subjects and Methods
1. Participants
This cross-sectional study included 34 eyes of 34 urban school children aged 12~13 years (mean age: 12.4±0.3 years; 17 boys and 17 girls) in Seoul, South Korea (Table 1). Students with ocular pathology, strabismus, previous intraocular surgery or laser treatment, and retinal complications were excluded.
2. Measurements
The AL was measured by using partial coherence laser interferometry (IOLMaster 500, Carl Zeiss Meditec AG, Jena, Germany). The difference in the AL was determined by using two sets of measurements, the first taken at 9 AM and the second taken at 1 PM.
The refractive error (spherical equivalent; calculated as a sphere plus half of the negative cylinder) was estimated with an open-field autorefractor (SRW-5000, Shin-Nippon, Tokyo, Japan). All measurements were measured each five times, and the mean value was chosen.
3. Statistical analysis
All statistical analyses were performed by using SPSS version 18 for Windows (IBM, Inc., Armonk, NY). The initial analysis showed that the AL was not statistically different between the right and left eyes (AM, p=0.520; PM, p=0.338, paired t-test); therefore, only the right eye was evaluated. The Student’s t-test was used to compare the AL according to the measurement time or sex. Pearson correlation analysis was used to evaluate the correlations between AM and PM measurements. A linear regression model was used to classify the AL in the PM as the dependent variable and the AL in the AM as the independent variable. The results are expressed as the mean±standard deviation (SD) of five repeated measurements. A p value <0.05 was considered to indicate a significant difference.
Ⅲ. Results
1. Characteristics of the study participants
The characteristics of the 34 eyes of the 34 children included in the analysis are shown in Table 1. The mean SE was –1.12±0.88 D in the right eyes and -1.07±0.85 D in the left eyes. No statistically significant difference was found between the right and left eyes.
2. AL distribution
The distribution of the ALs measured in the morning and in the afternoon is shown in Fig. 1. The skewness of the morning and that of the afternoon were -0.063 and -0.011 respectively, while the kurtosis were -0.895 and -0.705 respectively, showing the most frequently measured value of the AL was 23~25 mm in both the morning and afternoon.
3. AL according to sex
The AL in accordance with sex is shown in Table 2. The mean ALs of boys and girls measured in the morning were 24.24±1.19 mm and 24.04 ±0.94 mm respectively, while the mean ALs of boys and girls measured in the afternoon were 24.29±1.13 mm and 24.18±0.85 mm respectively. No statistically significant difference was observed in the AL values by virtue of sex(p>0.05).
4. Difference in AL according to measurement time
The ALs in the morning and in the afternoon were highly correlated(r2=0.949, p<0.001). Linear regression analysis revealed a significant increase in the AL in the afternoon. The regression equation was y=0.876x+3.058(p<0.001)(Fig. 2). The ALs measured at 9:00 in the morning and at 1:00 in the afternoon were not significantly different (Table 3). However, the AL measured in the afternoon was 0.07±0.34 mm longer than that measured in the morning.
Ⅳ. Discussion
The development of the AL is complete before adolescence. During the period in which the AL increases, the sizes of several optical constants are adjusted continuously so that a clear image is generated on the retina. If this balance is not properly maintained, a refractive error will occur.15) Thus, the increase in the AL can be regarded as a change of the refractive components, and this plays the most important role in the development of myopia, which is a representative refractive error.16) Further, previous studies reported that the AL measurement could greatly facilitate diagnosing congenital glaucoma.16,17) Accordingly, continuous efforts have been made to improve the accuracy of the biometry measurement of the AL. However, biometry measurement data in Koreans are insufficient. In the present study, we analyzed the distribution of ALs in children aged 12 to 13 years and, in particular, we observed the difference in the measurements in the morning and in the afternoon.
In a previous study, the mean AL of 40 Korean kindergarten students aged 5 to 6 years was 22.51 mm,18) and that of elementary students aged 12 years was 23.60 mm for male students and 23.04 mm for female students.19) Zadnik et al20) asserted in their study targeting children over 10 years old that there was a difference in axial length by sex. However, most previous studies used ultrasound to obtain measurements. In present study, we used IOLMaster to measure AL, which is known to have high precision and reproducibility for non-contact bio measurements.21-23) By using this method, the ALs measured in the morning for boys and girls were 24.24 mm and 24.04 mm respectively and those measured in the afternoon for boys and girls were 24.29 mm and 24.18 mm, respectively, showing no statistically significant difference. On the other hand, recently Chakraborty et al10) recently reported a diurnal change in the AL measured by using this device. Targeting 30 subjects aged 18 to 30 years, this study showed that the diurnal change of axial length was 0.032 ±0.018 mm, showing the longest axial length at noon. Although the AL was longer at 1 PM than at 9 AM in the present study, this difference was not statistically significant. The ocular biometry measurement is critical for examining ophthalmological problems, and, in particular, the AL is the most important element for determining the intraocular lens power after cataract surgery. Since an AL error of 0.1 mm causes an average refractive power error of 0.25 D after surgery, the AL is the most important variable for determining the refraction status after the operation.9)
Directly comparing the results of this study to those of previous studies would be difficult because of the diverse measurement methods, ages of subjects, and refractive errors among the studies. Therefore, our reported values may be used as basic data for analyzing the ocular components of children because we have presented the distribution of the AL in children aged 12 to 13 years by virtue of measurement time. Eyes are known to exhibit diurnal physiological changes, resulting in variations in intraocular pressure and anterior chamber angle measurements.11,12,24,25) A limitation of this study is that we did not include the AL measurement during the evening. Therefore, that is not enough to understand the daily fluctuation in the AL. Future studies are required regarding diurnal changes of several ocular components. In summary, the ALs were not statistically significant difference that were observed between 9 AM and 1 PM.
Ⅴ. Conclusions
In conclusion, the AL according to measurement time in children aged 12 to 13 years was measured with the IOLMaster. The most frequent AL measurement in children aged 12 to 13 years was 23~25 mm. The AL did not differ in accordance with sex. The AL at 1 PM(24.25±1.00 mm) was longer than that at 9 AM(24.18±1.08 mm) by 0.07±0.34 mm; however, this difference was not statistically significant. Thus, a 0.07 mm difference in the axial length by virtue of the measurement time will have an impact to less than 0.25 D in IOL calculation.
