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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 112  |  Issue : 2  |  Page : 52-60

Correlation between central corneal thickness and axial errors of refraction


1 Ain Shams University
2 Memorial institute of ophthalmic research

Date of Submission22-Mar-2019
Date of Acceptance15-May-2019
Date of Web Publication19-Jul-2019

Correspondence Address:
Ahmad A Hassan
Egypt

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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ejos.ejos_18_19

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  Abstract 

Aim This study aimed to determine the correlation between central corneal thickness (CCT) and axial refractive errors (axial myopia and axial hyperopia).
Patients and methods A total of 84 eyes were included in this study. They were classified into three groups (group 1, which included 28 axially myopic eyes; group 2, which included 28 axially hyperopic eyes; and group 3, which included 28 emmetropic eyes). CCT was obtained by pentacam, axial length was obtained by A-Scan ultrasonography, and refraction was obtained with auto-refractometer after cycloplegia and being confirmed with trial lenses.
Results The study showed that the mean CCT in the myopia group (group 1) was 531 nm, mean CCT in the hyperopia group (group 2) was 523.5 nm, and mean CCT for the emmetropia group (group 3) was 555 nm. CCT was obtained by pentacam, axial length (AL) was obtained by ultrasound A-scan, and refraction was obtained by auto-refractometer after cycloplegia and being confirmed with trial lenses.
Discussion CCT was found to be thinner in axially ametropic eyes compared with emmetropic eyes. CCT did not differ statistically between different grades of the same refractive error. CCT decreased with age but not with sex or AL.
Conclusion The study revealed that axial errors of refraction had a correlation with CCT and that CCT of axially ametropic eyes showed thinner corneas than emmetropic eyes.

Keywords: axial length, central corneal thickness, refractive errors


How to cite this article:
Mourad MS, Rayhan RA, Moustafa M, Hassan AA. Correlation between central corneal thickness and axial errors of refraction. J Egypt Ophthalmol Soc 2019;112:52-60

How to cite this URL:
Mourad MS, Rayhan RA, Moustafa M, Hassan AA. Correlation between central corneal thickness and axial errors of refraction. J Egypt Ophthalmol Soc [serial online] 2019 [cited 2019 Aug 19];112:52-60. Available from: http://www.jeos.eg.net/text.asp?2019/112/2/52/263007


  Introduction Top


The determination of corneal thickness has gained re1evance in recent years, partly owing to the growing interest in refractive surgery [1]. Studies that have attempted to investigate the effect of refractive errors on central corneal thickness (CCT) and axial length have reported conf1icting results [2]. Chen et al. [3] and Nangia et al. [4] studied re1ationship between CCT and refractive error and reported no significant correlation. Meanwhile, Kadhim et al. [5] and Saxena et al. [6] studied the re1ation between CCT and refractive errors and reported a significant correlation.

In this study, we aimed at further investigating the correlation between CCT and axial refractive errors among a group of Egyptian patients.


  Patients and methods Top


Our study comprised 84 eyes. Those 84 eyes were subdivided into three main groups: group 1 included 28 myopic eyes, group 2 included 28 hyperopic eyes, and group 3 included 28 emmetropic eyes. Two of these groups (groups 1 and 2) were further divided into three subgroups according to the severity of axial refractive errors as follows: group 1, which included 28 myopic eyes, was subdivided into group 1a, which included nine low myopic candidates (<−3 D); group 1b, which included 10 moderate myopic candidates (−3 to −6 D); and group 1c, which included nine high myopic candidates (>−6 D). Group 2, which included 28 hyperopic eyes, was subdivided into group 2a, which included 10 low hyperopic candidates (≤+2 D); group 2b, which included nine moderate hyperopic candidates (+2 to +5 D); and group 2c, which included 9 high hyperopic candidates (>5 D). All candidates underwent careful full ophthalmological examination (adnexa, anterior segment and posterior segment). CCT was measured by pentacam. Axial length was measured by A-Scan ultrasonography. Refraction was measured by auto-refractometer under cycloplegia and after being confirmed by trial lenses in a latter visit whenever needed. All patients signed an informed written consent before investigations including type and technique of procedure.

Statistical analysis

MedCalc version 18.2.1 (MedCalc, Ostend, Belgium) was used for data entry, processing, and statistical analysis. Tests of significance (Kruskal–Wallis, χ2, logistic regression analysis, Spearman’s correlation, and receiver operating characteristic (ROC) curve analysis) were also used. Data were presented, and suitable analysis was done according to the type of data (parametric and nonparametric) obtained for each variable. P values less than 0.05 (5%) was considered to be significant statistically.


  Results Top


Descriptive data

The mean age of all patients was 33.75±7.56 years. Regarding sex of the patients, 53.6% of patients were females, whereas 46.4% were males. In addition, the right eye was 50% of the sample size and the left eye was the other 50% ([Table 1]).
Table 1 Basic ocular data among 84 patients

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Regarding refraction data, the mean sphere was −0.47±4.02 D, the mean cylinder was −1.23±0.98 D, the mean axis was 93±58.9 D, and the axial length was 23.6±1.59 mm ([Table 2]).
Table 2 Refraction data and axial length among 84 patients

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Regarding CCT, the mean CCT of all patients was 536.27±34.85 nm ([Table 3]).
Table 3 Cntral corneal thickness data among 84 patients

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Comparative studies

Concerning age of the participants in all groups, the age was significantly higher from the statistical point of view in the hyperopia group compared with the other groups (P=0.0021; [Table 4]).
Table 4 Comparison among the three groups regarding basic ocular data using Kruskal–Wallis and χ2-tests

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The female predominance was significantly higher from the statistical point of view in the myopia group compared with other groups (P=0.0021; [Table 4]).

However, regarding the laterality among the three groups, a nonsignificant statistical difference was observed (P>0.05; [Table 4]).

Regarding the axial length, it was significantly higher from the statistical point of view in the myopia group when compared with the other groups (P<0.01; [Table 5] and [Figure 1]).
Table 5 Comparison among the three groups regarding refraction data and axial length using Kruskal–Wallis test

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Figure 1 Ultrasound A-scan showing axial length of a high myopic eye.

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The three groups showed nonsignificant statistical difference regarding cylinder and axis (P>0.05; [Table 5]).

Regarding CCT, it was statistically significantly higher in the emmetropia group when compared with the other groups (P=0.023; [Table 6] and [Figure 2]).
Table 6 Comparison among the three groups regarding central corneal thickness data using Kruskal–Wallis test

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Figure 2 Pentacam of an emmetropic case with central corneal thickness (CCT) 600 nm.

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Comparing the CCT of the myopia group with that of the hyperopia group, it showed a nonsignificant statistical difference (P>0.05). However, when the myopia group CCT was compared with that of the emmetropia group statistically, it was significantly higher in the emmetropia group (P=0.031; [Table 6] and [Figure 3]).
Figure 3 Pentacam of a high myopic eye with central corneal thickness (CCT) 502 nm.

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As we compared the CCT of the hyperopia group with that of the emmetropia group, it was statistically higher in the emmetropia group (P=0.015; [Table 6] and [Figure 4] and Graph 1).
Figure 4 Pentacam of a moderate hyperopic eye with central corneal thickness (CCT) 554 nm.

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Comparative studies regarding the myopia group:

A comparison between the three myopic subgroups revealed a statistically nonsignificant difference regarding CCT (P>0.05; [Table 7] and [Figure 5]).
Table 7 Comparison among the three subgroups regarding central corneal thickness data using Kruskal–Wallis test

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Figure 5 Pentacam of a moderate myopic eye with central corneal thickness (CCT) 565 nm.

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Comparative studies regarding the hyperopia group:

A nonsignificant statistical difference was noted regarding CCT in the three hyperopic subgroups (P>0.05; [Table 8] and [Figure 6]).
Table 8 Comparison among the three subgroups regarding central corneal thickness data using Kruskal–Wallis test

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Figure 6 Pentacam of a high hyperopic eye with central corneal thickness (CCT) 512 nm.

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  Discussion Top


This study aimed at investigating the relationship between CCT and axial refractive errors. It comprised 84 eyes of variable Egyptian subjects aged from 18 to 45 years. Those 84 eyes were divided into three main groups: group 1 included 28 axially myopic eyes, group 2 included 28 axially hyperopic eyes, and group 3 included 28 emmetropic eyes.

The study showed that median CCT in the myopia group was 531 µm (510–544.5), median CCT in the hyperopia group was 523.5 µm (502.5–545.5), and median CCT in the emmetropia group was 555 µm (518–573.5). Therefore, a correlation was found between CCT and axial refractive errors as CCT was found to be statistically higher in the emmetropia group compared with the myopia and the hyperopia groups (P=0.023)

Kadhim et al. [5] studied the correlation between CCT and refractive errors where CCT was evaluated by ultrasound pachymeter. They reported significantly thinner corneas in myopia (539.5 nm) than emmetropia (550.47 nm), which correlated with our results.

Chang et al. [7] studied the cornea in myopic adults. CCT was evaluated by specular microscopy. They found that mean corneal thickness was 533 (SD 29) µm and reported thinner corneal thickness in more myopic eyes (r=0.16, P=0.021) and in cases with longer axial lengths, which correlated with our results.

Saxena et al. [6] studied the important considerations of CCT in ophthalmology clinic. CCT was evaluated with ultrasound pachymeter. They reported thinner CCT in myopic individuals (522.87±18.034 nm) than hyperopic individuals (536.39±17.753 nm), which contradicted our results, as we found thinner corneas in the hyperopia group compared with the myopia group (median CCT measured 523 and 531 µm for the hyperopia and the myopia groups respectively), but this difference was not statistically significant (P>0.05).

Chen et al. [3] studied the correlation between CCT and refractive error. CCT was evaluated by ultrasound pachymeter. No significant statistical different between mean CCT obtained (558±29 nm in males and 554±32 nm in females) and refractive errors was reported (P=0.445), which contradicted our results, as we found thinner CCT in the hyperopia and myopia groups compared with the emmetropia group (523.5, 531, and 555 µm, respectively), with significant statistical difference (P=0.023).

Nangia et al. [4] studied CCT and its association with ocular and general parameters. CCT was obtained by ultrasonography. No significant association was reported. Their result did not correlate with ours, which found a significant increase in CCT in the emmetropia group compared with both the myopia and the hyperopia groups, with a significant statistical difference (P=0.023).

Chen et al. [8] studied the correlation between CCT and myopia in Taiwan. CCT was obtained with Orbscan. No significant association between CCT and degree of myopia was found. In addition, CCT of myopia and emmetropia showed no significant difference. Those two results partially contradicted our study results as we found CCT to be thinner in the myopia group compared with the emmetropia group, with a significant statistical difference (P=0.031). However, they correlated with our study in that no correlation was found between CCT and degree of myopia (P=0.851).

The difference in results among studies could be related to different sample sizes, different ages in each study, which in turn would influence CCT measurements, different devices used to measure CCT, or human error in obtaining accurate results.

As we compared median CCT of the myopia group with that of the hyperopia group, it was much thinner in the myopia group, but statistically, this difference was not significant (P>0.05). This result came in agreement with Kadhim et al. [5] who found that mean CCT for myopia was 539.05 nm and that for hyperopia was 550.54 nm.

As we compared the CCT of the hyperopia group with that of the emmetropia group statistically, a significant difference was found (P=0.015), which correlated with Nomura et al. [9] who found median CCT for hyperopia 512.5 nm and for emmetropia 516 nm.

This study also showed that the median CCT measurements for the myopia subgroups were 526 nm (510–540) for mild myopia, 536 nm (511–565) for moderate myopia, and 531 nm (508–543) for high myopia. This discrepancy was also found in a study conducted by Nomura et al. [9] who found that mean CCT measurements for mild, moderate, and high myopia were 515.4, 525, and 520.6 nm, respectively. However, this difference in our study regarding CCT was found to be statistically nonsignificant (P>0.05). This result correlated with other studies such as Chen et al. [8] who reported no correlation between CCT and degree of myopia, and Solu et al. [2], who studied the correlation between CCT and the degree of myopia. CCT was evaluated with ultrasound pachymeter. They concluded that CCT had no correlation with the degree of myopia (P=0.96). On the contrary, some studied contradicted this result, such as Das et al. [10] who studied the correlation between CCT and axial length in myopes of different grades. CCT was evaluated with ultrasound pachymeter. A significant difference between CCT among moderate to high grades of myopia (530.56±38.28 and 494.4±45.79 nm for moderate and high myopia respectively) was reported.

In addition, this study showed that CCT measurements for the hyperopia subgroups were 545 nm (503–557) for mild hyperopia, 521 nm (510–545) for moderate hyperopia, and 512 nm (489–527) for high hyperopia. This decrease in CCT as severity of hyperopia gets higher correlated with Solu et al. [2] who found that mean CCT was 551.44±30.15 nm for hyperopia (+2 D ≤+4 D) and 542.66±40.62 nm for hyperopia (>+4 D). However, this difference in our study regarding CCT was found to be statistically nonsignificant (P>0.05)


  Conclusion Top


In this study, we found that CCT was related to axial refractive errors and that there were thinner corneas in axially ametropic eyes compared with emmetropic eyes, proving that the changes in ocular tunics of axially ametropic eyes would involve the CCT.

However, much thinner corneas were found in the axially hyperopic eyes compared with the axially myopic and emmetropic eyes. Therefore, we believe that other factors other than the axial refractive errors such as genetic [11] or racial [12] factors may be influencing CCT.

Financial support and sponsorship

Nil.

Conflicts of interest

The author has no proprietary or financial interest in the materials presented.

 
  References Top

1.
American Academy of Ophthalmology. Basic and clinical science course. External disease and cornea, section 8. San Francisco, CA: American Academy of Ophthalmology; 2015. pp. 6–21.  Back to cited text no. 1
    
2.
Solu T, Baravaliya P, Patel I, Kamble S et al. Correlation of central corneal thickness and axial length in myopes, emmetropes, and hypermetropes. Int J Sci Study 2016; 3: p 207.  Back to cited text no. 2
    
3.
Chen MJ, Liu YT, Tsai CC, Chen YC, Chou CK, Lee SM. Relationship between central corneal thickness, refractive error, corneal curvature, anterior chamber depth and axial length. J Chin Med Assoc 2009; 72:133–137.  Back to cited text no. 3
    
4.
Nangia V, Jonas JB, Sinha A, Matin A, Kulkarni M. Central corneal thickness and its association with ocular and general parameters in Indians: the Central India Eye and Medical Study. Ophthalmology 2010; 117:705–710.  Back to cited text no. 4
    
5.
Kadhim YJ, Farhood QK. Central corneal thickness of Iraqi population in relation to age, gender, refractive errors, and corneal curvature: a hospital-based cross-sectional study. Clin Ophthalmol 2016; 10:2369–2376.  Back to cited text no. 5
    
6.
Saxena AK, Bhatnagar A, Thakur S. Central corneal thickness: important considerate in ophthalmic clinic. Austin J Clin Ophthalmol 2017; 4:1076.  Back to cited text no. 6
    
7.
Chang SW, Tsai IL, Hu FR, Lin LL, Shih YF. The cornea in myopic adults. Br J Ophthalmol 2001; 85:916–920.  Back to cited text no. 7
    
8.
Chen YC, Kasuga T, Lee HJ, Lee SH, Lin SY. Correlation between central corneal thickness and myopia in Taiwan. Kaohsiung J Med Sci 2014; 30:20–24.  Back to cited text no. 8
    
9.
Nomura H et al. The relationship between Intraocular pressure and refractive errors adjusting for age and central corneal thickness. Ophthal Physiol Opt 2004; 24:41–45.  Back to cited text no. 9
    
10.
Das A, Bangal SV, Gupta V. Central corneal thickness and axial length in myopia of various grades. VIMS Health Science Journal 2016; 3:163–166.  Back to cited text no. 10
    
11.
Toh T, Liew SH, MacKinnon JR, Hewitt AW, Poulsen JL, Spector TD et al. Central corneal thickness is highly heritable: the twin eye studies. Invest Ophthalmol Vis Sci 2005; 46:3718–3722.  Back to cited text no. 11
    
12.
Nemesure B, Wu SY, Hennis A, Leske MC. Corneal thickness and intraocular pressure in the Barbados eye studies. Arch Ophthalmol 2003; 121:240–244.  Back to cited text no. 12
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]



 

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