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INTRODUCTION

Intraocular pressure (IOP) is known to be an inde-pendent risk factor for glaucoma,1–4 and it is there-fore extremely important to measure IOP correctly in the diagnosis and follow-up of glaucoma. Central corneal thickness (CCT) affects the accuracy of IOP measurements made using applanation tonometry.5,6 Many studies have shown that there is a significant correlation between CCT and IOP.7–15 Increased CCT may lead to elevated IOP, or thinner CCT may lead to lower IOP. In addition, a thin cornea may be an independent risk factor for the development of pri-mary open-angle glaucoma in patients with ocular hypertension.16 These factors increase the importance of measuring CCT in clinical practice. Moreover,

although corneal refractive surgery is generally not recommended for individuals under 18 years of age, there has been discussion in recent years regarding whether this surgical procedure should be performed to treat anisometropia and to prevent aniseiko-nia. CCT is important for the determination of the residual corneal stromal thickness and the amount of laser ablation.17–20 Although CCT has been studied in detail in adults, there have been relatively few studies of CCT in children.20–26 Knowing the distribution of CCT and IOP in different communities is extremely important in the diagnosis and monitoring of diseases associated with CCT, particularly glaucoma.

To our knowledge there has been only one previous report of the distributions of CCT and IOP in Turkish school children.27 The present study was performed

Ophthalmic Epidemiology, 19(2), 83–88, 2012Copyright © 2012 Informa Healthcare USA, Inc.ISSN: 0928-6586 print/1744-5086 onlineDOI: 10.3109/09286586.2011.649227

Received 25 September 2010; revised 20 August 2011; accepted 22 September 2011

Correspondence: Yildirim Bayezit Sakalar, Dicle Universitesi Tip Fakultesi Goz Hastaliklari ABD 21280 Diyarbakir, Turkey. Tel: +90 412 2488001. Fax: +90 412 2488440. E-mail: ybsakalar@yahoo.com

25 September 2010

20 August 2011

22 September 2011

© 2012 Informa Healthcare USA, Inc.

2012

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10.3109/09286586.2011.649227

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ORIGINAL ARTICLE

Distribution of Central Corneal Thickness and Intraocular Pressure in a Large Population of

Turkish School ChildrenYildirim Bayezit Sakalar, Ugur Keklikci, Kaan Unlu, Mehmet Fuat Alakus,

Mine Yildirim, and Umut Dag

Department of Ophthalmology, Dicle University Faculty of Medicine, Diyarbakir, Turkey

ABSTRACT

Purpose: To determine the distribution of central corneal thickness (CCT) and intraocular pressure (IOP) in a population of Turkish school children and to evaluate the relationships between these values and both age and gender.

Methods: A total of 30,320 eyes from 15,160 healthy school children aged 5–18 years were examined. CCT and IOP were measured using an ultrasonic pachymeter and a non-contact tonometer, respectively. Measurements were compared with age, gender, and parental consanguinity of subjects.

Results: The mean age of the school children was 10.43 ± 2.54 years. For right eyes, mean CCT was 557.91 ± 34.26 µm and mean IOP was 14.15 ± 2.87 mmHg. Mean CCT was significantly thicker in males com-pared to females (P < 0.0001). Mean IOP was significantly higher in females compared to males (P < 0.0001). CCT in children aged 14 years and older was significantly thinner than that in the younger age groups (P < 0.0001). There was a significant difference among the age groups for IOP (P < 0.05). No significant differ-ences were observed in CCT or IOP related to parental consanguinity (P = 0.538, P = 0.319, respectively).

Conclusion: Mean CCT in Turkish school children is of comparable thickness to that in Western school children. CCT reached adult values around 14 years of age in our children. Furthermore, males had thicker CCT than females. IOP was lower in males than females, and increased with age.

Keywords: Central corneal thickness, Intraocular pressure, Non-contact tonometer, School children, Ultrasonic pachymeter

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to determine the distributions of CCT and IOP in a significant number of school children, and to identify variations in these values that may arise with age and gender.

MATERIALS AND METHODS

This was a prospective cross-sectional study conducted as part of an eye health screening project among school children, in which the Department of Ophthalmology of Dicle University participated. The Declaration of Helsinki was adhered to in all procedures and the approval of the local ethics committee of Dicle University was obtained before proceeding with the study. Written informed consent was obtained from the parents of all children prior to the start of the study. The study was performed between February 2008 and February 2009, and the examinations were conducted in a mobile medical unit.

For inclusion in this study, children had to be healthy, between the ages of 5 and 18 years, and to be coop-erative with the CCT and IOP examinations. Exclusion criteria included prior eye surgery, ocular trauma, the use of contact lenses, uveitis, any corneal or anterior segment abnormalities and an inability to cooperate with CCT and IOP measurements. Measurements were performed initially on the right eye, followed by the left eye, by one of three doctors from the Ophthalmology Department. Instruments were calibrated daily. All children underwent detailed ocular examinations prior to the measurements. IOP measurements were made using a non-contact air-puff tonometer (NT-2000; Nidek, Gamagori, Aichi, Japan). Three consecutive measure-ments were made for each eye and the average was used in analysis. Children with an IOP measurement of 22 mmHg or above were referred to hospital for fur-ther evaluation. Prior to the CCT measurements, topi-cal corneal anesthesia was achieved using one drop of 0.5% proparacaine hydrochloride ophthalmic solution (Alcaine; Alcon, Fort Worth, TX). CCT was measured using an ultrasonic pachymeter (US-4000; Nidek). The pachymeter probe was held perpendicular to the cornea and directed toward the center of the undilated pupil. Three pachymetry measurements were obtained for each eye, and an additional measurement was taken if any of these measurements deviated by 5% or more from the others. The smallest value obtained was used for analysis, as this represented the most perpendicular path through the cornea. Parental marriage status was obtained by written declaration from the parents. The results of CCT and IOP measurements were recorded together with age, gender, and consanguinity of the children’s parents. The children were divided into the following age groups prior to statistical analysis: 5–7 years, 8–9 years, 10–11 years, 12–13 years, 14–15 years, and 16–18 years, to determine the relationships between age and CCT and IOP values.

Statistical analysis

SPSS software, version 11.01 (SPSS, Inc., Chicago, IL), was used for data analysis, and P < 0.05 was consid-ered statistically significant. Data are presented as means ± standard deviation. The paired-sample t test was used to compare CCT and IOP values for right and left eyes. To compare CCT and IOP values between the age groups, one-way ANOVA variance analysis was used and post hoc comparisons were made using the Tukey HSD test. The Student’s t test was used to com-pare CCT and IOP values between genders. The rela-tionships between CCT or IOP values and age, gender, and parental consanguinity were evaluated by linear regression analysis for right eyes.

RESULTS

A total of 15,235 school children were considered for enrollment, however, 75 children were excluded due to the criteria outlined above. In total, the study included 30,320 eyes of 15,160 healthy school children aged between 5 and 18 years. Mean age was 10.43 ± 2.54 years. The study population consisted of 7,978 (52.6%) males and 7,182 (47.4%) females. Mean CCT values were 557.91 ± 34.26 µm and 558.74 ± 34.35 µm for right and left eyes, respectively. Mean IOP values were 14.15 ± 2.87 mmHg and 14.21 ± 2.95 mmHg for right and left eyes, respectively. There were statistically sig-nificant differences in mean CCT and IOP between right and left eyes (P < 0.0001 and P < 0.0001, respectively).

In total, 29% (n = 4,403) of the children had consan-guineous parents. There were no significant correla-tions between parental consanguinity and the CCT or IOP values (P = 0.538, R2 = 0.000; P = 0.319, R2 = 0.000, respectively).

CCT distribution in right and left eyes by age groups and gender are presented in Table 1. CCT was signifi-cantly thinner in the 14–15 and 16–18 years age groups compared to the younger age groups (P < 0.0001). There were no significant differences between the other groups. There were also no significant differences between the 14–15 and 16–18 years age groups (P > 0.05) (Figure 1). Linear regression analysis revealed a significant cor-relation between CCT and age (P < 0.001, R2 = 0.005). Measured CCT decreased by 0.889 µm for every year increase in age.

Mean IOP values by age groups and gender are pre-sented in Table 2. There was no significant difference between the 14–15 years age group and the 16–18 years age group (P = 0.911). However, there were significant differences between the younger age groups in IOP. A positive correlation was observed between age and IOP using linear regression analysis (P < 0.001, R2 = 0.005). Mean IOP rose slightly from the age of 5 years, and the change was significant up to the age of 14 years (Figure 2); IOP did not change after 14 years of age.

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Distribution of Corneal Thickness in Turkish Children 85

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Mean CCT values in males were 559.46 ± 34.28 µm and 560.27 ± 34.51 µm for right and left eyes, respectively. Mean CCT values in females were 556.19 ± 34.15 µm and 557.04 ± 34.10 µm for right and left eyes, respectively. CCT was significantly thicker in males than females (P < 0.0001). Mean IOP values for

males were 14.07 ± 2.86 mmHg and 14.12 ± 2.94 mmHg for right and left eyes, respectively. Mean IOP values in females were 14.24 ± 2.87 mmHg and 14.31 ± 2.96 mmHg for right and left eyes, respectively. There was a sig-nificant difference in IOP between males and females (P < 0.0001).

FIGURE 2 Age-specific intraocular pressure in right eyes.FIGURE 1 Age-specific central corneal thickness in right eyes.

TABLE 1 Mean CCT distributions by gender for the different age groups investigated

Age group (years)

Females Males

n (%)CCT (µm)*

n (%)CCT (µm)*

Right eye Left eye Right eye Left eye5–7 997 (13.88) 557.15 ± 33.19 558.04 ± 33.20 1080 (13.54) 562.26 ± 33.68 563.10 ± 33.828–9 2007 (27.95) 557.59 ± 34.21 558.38 ± 34.06 2006 (25.15) 562.03 ± 33.41 563.06 ± 33.9610–11 1803 (25.10) 555.96 ± 31.98 557.12 ± 32.22 1824 (22.86) 561.30 ± 33.84 561.87 ± 34.4012–13 1541 (21.46) 556.47 ± 35.11 557.27 ± 34.71 1855 (23.25) 557.79 ± 34.14 558.45 ± 33.8014–15 750 (10.44) 552.15 ± 37.87 552.73 ± 37.72 1026 (12.86) 553.31 ± 35.91 554.14 ± 36.3916–18 84 (1.17) 547.19 ± 32.82 545.56 ± 35.58 187 (2.34) 547.83 ± 36.39 550.13 ± 34.83Total 7182 (100) 556.19 ± 34.15 557.04 ± 34.10 7978 (100) 559.46 ± 34.28 560.27 ± 34.51CCT, central corneal thickness.*Data are means ± standard deviation.

TABLE 2 Mean IOP distributions by gender for the different age groups investigated

Age group (years)

Females Males

n (%)IOP (mmHg)*

n (%)IOP (mmHg)*

Right eye Left eye Right eye Left eye5–7 997 (13.88) 13.53 ± 2.71 13.64 ± 2.80 1080 (13.54) 13.47 ± 2.83 13.52 ± 2.908–9 2007 (27.95) 14.51 ± 2.96 14.50 ± 3.04 2006 (25.15) 14.25 ± 2.86 14.28 ± 2.9510–11 1803 (25.10) 14.15 ± 2.91 14.26 ± 3.04 1824 (22.86) 13.96 ± 2.84 14.02 ± 2.9512–13 1541 (21.46) 14.21 ± 2.83 14.24 ± 2.93 1855 (23.25) 14.04 ± 2.87 14.06 ± 2.9114–15 750 (10.44) 14.74 ± 2.68 14.84 ± 2.73 1026 (12.86) 14.49 ± 2.81 14.59 ± 2.8916–18 84 (1.17) 14.67 ± 2.36 14.74 ± 2.56 187 (2.34) 14.83 ± 2.73 14.81 ± 2.82Total 7182 (100) 14.24 ± 2.87 14.31 ± 2.96 7978 (100) 14.07 ± 2.86 14.12 ± 2.94IOP, intraocular pressure.*Data are means ± standard deviation.

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A positive relationship was observed between CCT and IOP (P = 0.001, R2 = 0.001). Linear regression anal-ysis indicated that for every 100 µm increase in CCT, measured IOP increased by 0.2 mmHg.

DISCUSSION

Differing mean CCT results have been observed in different ethnic populations. In the present study, mean CCT values in school children were 557 µm and 558 µm in right and left eyes, respectively. Mean CCT has been reported to be between 548 and 564 µm in Western white school children.20–23,26 Previous stud-ies were not identical to our study in terms of age distribution, but the demographics were similar with the mean age investigated being 10 years.21,22 In all of these studies, including the present study, ultra-sonic pachymetry was used for measurement of CCT. Haider reported that mean CCT was 549µm for both eyes in all ethnicities. They also reported that CCT was thinner in African-American children than in other ethnic groups.26 In Asian school children, mean CCT was reported to be between 513 and 550 µm.28–30 These results suggest that mean CCT in Turkish school chil-dren is thicker than that of African-American children, similar to or slightly thinner than that of white children, and slightly thicker than that in Asian populations. High myopia may be associated with decreased CCT, however refractive status was not evaluated in the pres-ent study. The rate of myopia has been reported to be between 2.56% and 9.7% in Turkish children.31–33 Ergin reported that mean refractive error was −1.81 ± 0.47 diopters in myopic Turkish children.32 Thicker CCT in Turkish children compared to Asian children may be related to the reduced number of cases of myopia in this population.

The relationship between CCT and age is con-troversial. Although some studies have shown that there is no significant relationship between CCT and age,7,22,23,25,26,28,34–36 others have suggested that such a rela-tionship exists. It has been reported that CCT increases with age in western societies. In contrast, studies in some Eastern societies suggested that CCT decreases with age.30,37–40 In the present study, CCT decreased from the age of 5 to 14 years, but the decrease was slow. After the age of 14 years, the decrease in CCT was more pronounced and CCT was thinner than in the younger age groups. Axial growth of the eye can be divided into three growth phases: a rapid postnatal phase, a slower infantile phase, and a slow juvenile phase.41 Similarly, retinal surface area increases markedly during gestation and more gradually over the first two postnatal years. Thereafter the rate of increase is small42 and supports our results. Ehlers suggested that CCT reaches adult levels at around 3 years of age.24 However, optical pachymetry was used for CCT measurements, which is different from the method used in the present study.

Hussein suggested that corneal thickness increases slowly with time and reaches adult levels between 5 and 9 years old.20 Our results suggested that CCT probably reaches adult levels around 14 years of age in school children. Hussein used ultrasonic pachymetry, but sub-jects ranged in age between 7 months and 14.7 years. We observed CCT thinning in the 14–18-year-olds: Hussein would not have made this observation due to the age limit in their study.

Some studies have presented data showing that there is no relationship between CCT and gender in children,21,22,26,27 however, Tong reported that mean CCT in males was 6.4µm thicker than that in females,29 and Yildirim found that mean CCT was thicker by 5 µm in male than in female Turkish school children.27 In the present study, CCT in males was thicker than that in females by 3.27 µm and 3.23 µm in right and left eyes, respectively.

It has been reported that there are no significant dif-ferences in mean CCT between right and left eyes.20,22 In the present study, mean CCT was thicker in left eyes compared to right eyes, however, there was no expla-nation for the variance in CCT between the two eyes. Measurements were taken first in the right eye followed by the left; this difference was likely caused by the order of measurements, and we feel that this difference is not clinically significant.

Yildirim measured IOP in Turkish school children using a non-contact tonometer, and reported a mean value of 16.7 ± 2 mmHg.27 In the same study, mean IOP values were found to be 16.21 ± 2.71 mmHg in females and 17.39 ± 2.88 mmHg in males. The age range of subjects and IOP measurement methods used in the study of Yildirim were similar to those of the present study, however, mean IOP values in the present study were lower than those measured by Yildirim. In the present study, IOP was found to increase slightly with age. Yildirim did not observe any significant correlation between age and IOP.27 In a study performed in Iran, mean IOP for subjects aged 10–19 years was found to be 14.1 ± 2.2 mmHg, and there was an increase in IOP from age 10 to 70 years.43

Alsbirk reported that CCT among Greenland Eskimos showed a resemblance between first degree relatives, indicating a fairly high degree of genetic influence, in addition to socioeconomic and environmental factors.44 Therefore, we considered the influences of parental consanguinity on CCT and IOP as these subjects are predicted to have similar genetic and environmental backgrounds. To our knowledge, the relationships between parental consanguinity and CCT and IOP have not been examined previously. Approximately one third of the parents were consanguineous in our study population. Our results indicated that there are no significant relationships between parental consan-guinity and CCT or IOP.

Our study had the limitation that we only examined children between 5 and 18 years of age. CCT and IOP

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distributions in children below 5 years of age were not determined; if they had been included, it would have provided an opportunity to evaluate CCT and IOP dis-tributions with age throughout the whole of childhood. Moreover, further studies are needed to determine whether there is continuation after the age of 18 years of the significant thinning in CCT, which initially appeared at the age of 14 years. Another limitation of our study was that a non-contact tonometer was used for measur-ing IOP. Although Goldmann applanation tonometry is the gold standard for measuring IOP, it is especially difficult for children to comply with the procedure as it takes a relatively long time and it is time consuming to screen large groups using this method. Non-contact tonometry was used in our study as it requires relatively less compliance and is less time consuming. However, non-contact tonometry is influenced by CCT to a greater extent than Goldman applanation tonometry and tends to give higher measurements.45

In conclusion, CCT declines slowly from the age of 5 years, and this thinning reaches significant levels by the age of 14 years. Unlike CCT, IOP increases with age during childhood. Furthermore, CCT in females may be thinner than in males of the same age and IOP may be higher. For these reasons, factors such as age, gender, and ethnicity, which may have an effect on CCT and IOP, should be taken into consideration prior to measur-ing IOP, diagnosing glaucoma, or performing refractive surgery.

ACKNOWLEDGMENTS

This study was supported by T.R. Prime Ministry Southeastern Anatolia Project Regional Development Administration, Governorship of Diyarbakir and McDonald’s Children’s Foundation.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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