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 Table of Contents  
ORIGINAL ARTICLE
Year : 2020  |  Volume : 32  |  Issue : 1  |  Page : 45-50

Relationship between myopia and central corneal thickness – A hospital based study from South India


Department of Ophthalmology, PSG Institute of Medical Sciences and Research, Coimbatore, Tamil Nadu, India

Date of Submission22-Dec-2019
Date of Acceptance02-Jan-2020
Date of Web Publication17-Apr-2020

Correspondence Address:
Dr. K Divya
Department of Ophthalmology, PSG Institute of Medical Sciences and Research, Coimbatore - 641 004, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/kjo.kjo_95_19

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  Abstract 

Background and Purpose: The prevalence of myopia has been increasing steadily, and studies on the relationship between myopia and central corneal thickness (CCT) have shown conflicting results. The purpose of this study was to investigate the relationship between myopia and CCT in a hospital based population from South India. Materials and Methods: This prospective, cross-sectional study was carried out from a tertiary care institution in South India between January 2017 and June 2018. One hundred and ninety-four myopic individuals between 11 and 40 years of age with spherical equivalent (SE) ranging from -0.5 to -6 Diopters after cycloplegic refraction were enrolled in the study. CCT was measured in both eyes of each patient using Tomey EM-3000 noncontact specular microscope (Tomey Corporation, Nagoya, Japan). The relation between CCT and myopia was analyzed using Pearson's correlation coefficient. Statistical significance was set at P < 0.05. Results: A total of 388 eyes were analyzed. 50% (n=97) of study participants were females. The mean age of the study participants was 25.47 ± 7.25 years (range: 12–40 years). The mean SE was −-2.79±1.34 D (range: -0.5 to -6.0 D). The mean CCT was 540.53 ± 56.16 μm (range: 441–664 μm). No significant association was observed between CCT and age or gender of the study participants (P = 0.706 and 0.550, respectively). No correlation was observed between mean CCT and SE (Pearson, r = 0.095,P = 0.062). Conclusion: There was no correlation between CCT and degree of myopia. These data contribute to the ongoing discussion on the structural alterations in ocular coats and susceptibility to glaucoma in myopia.

Keywords: Central corneal thickness measurement, myopia, noncontact specular microscope, spherical equivalent


How to cite this article:
Divya K, Ganesh M R, Sundar D. Relationship between myopia and central corneal thickness – A hospital based study from South India. Kerala J Ophthalmol 2020;32:45-50

How to cite this URL:
Divya K, Ganesh M R, Sundar D. Relationship between myopia and central corneal thickness – A hospital based study from South India. Kerala J Ophthalmol [serial online] 2020 [cited 2020 Aug 5];32:45-50. Available from: http://www.kjophthal.com/text.asp?2020/32/1/45/282675




  Introduction Top


Myopia, referred to as “short sight,” is increasing in prevalence at an alarming rate and has emerged as a major cause for visual impairment globally. Estimates suggest that around 1.5 billion people or 22% of the present world population is myopic.[1] Apart from the ethnic and genetic factors, environmental and lifestyle changes have contributed significantly to the increasing prevalence of myopia. A recent meta-analysis on the prevalence of myopia in South-east Asia has shown an estimated pool prevalence of 4.9% (95% confidence interval [CI]: 1.6–8.1) in children and 32.9% (95% CI: 25.1–40.7) in adults.[2] The recent shift to younger age at the onset of myopia is even more worrisome considering its impact on the economic burden and quality of life.

With an increasing prevalence of myopia, there has also been a tremendous rise in the number of laser refractive procedures for the correction of myopia. Central corneal thickness (CCT) is a major factor determining the choice of refractive surgery for myopia, and a thin cornea is a recognized risk factor for corneal ectasia postrefractive surgery.[3] Corneal refractive surgery for myopia results in reduced corneal thickness and thereby leads to the underestimation of intraocular pressure.[4],[5] This is relevant in the diagnosis and treatment of any glaucoma later in life, since both myopia and reduced CCT are well-known risk factors for glaucoma.[6],[7],[8]

Studies evaluating the relationship between myopia and CCT have revealed conflicting results. While few studies have found a correlation between CCT and myopia,[9],[10],[11] some have reported no significant correlation.[12],[13],[14],[15] This can be attributed to the differences in ethnicity, characteristics of the population studied as well as varied methods of the measurement of corneal thickness. Two population-based studies using ultrasonic pachymetry (UP) from India have reported thinner corneas in Indians than that of other populations.[12],[16] Relatively fewer studies have evaluated CCT using noncontact specular microscopy, which is a noninvasive and accurate method with good reproducibility of CCT measurements.[17],[18],[19]

Given the increasing prevalence of myopia from India and conflicting evidence on the association between CCT and myopia, we attempted to study the relationship between myopia and CCT among patients attending the refraction clinic of a tertiary care institute using a noncontact specular microscope (NCSM).


  Materials and Methods Top


This prospective, hospital-based, cross-sectional study was carried out from a tertiary care institution in South India between January 2017 and June 2018. Individuals between 11 and 40 years of age who presented with visual impairment and found to have myopic refractive error (spherical equivalent [SE] more than −0.5 D) on cycloplegic refraction were included in the study. The exclusion criteria were myopia >−6 D, prior refractive or other ocular surgery, contact lens wear, trauma, corneal diseases, cataract, glaucoma, and ocular hypertension. Pregnant and lactating females and individuals with systemic diseases such as diabetes mellitus and collagen vascular disease were also excluded from the study. The study was performed according to the principles of the Declaration of Helsinki after prior approval from the Institutional Human Ethics Committee. Informed consent was obtained from all the participants.

All the study participants underwent a comprehensive ophthalmic examination, including slit-lamp biomicroscopy and fundus evaluation using +90 D lens and indirect ophthalmoscopy. Cycloplegia was achieved with three topical applications of 1% cyclopentolate at 15-min intervals. Cycloplegic refraction was performed 30–60 min after the first instillation of drug and spherical equivalent (SE, sphere + half-cylindrical component) was calculated.

CCT was measured by a single experienced ophthalmologist using Tomey EM-3000 NCSM (Tomey Corporation, Nagoya, Japan). The participants were instructed to fixate on the central target with chin and forehead positioned appropriately. Then, the participants were asked to blink completely once before measurements to spread an optically smooth tear film over the corneal surface. The measurements were taken using the auto-alignment and capture mode, thereby eliminating observer bias. Three consecutive valid measurements were taken, and the average of the three readings was used for the analysis. The CCT values were considered valid when the endothelial cells were focused clearly and bright corneal reflections were centered on the monitor. All measurements were performed in midday (between 12 and 4 PM) to eliminate diurnal variation.[20]

Statistical analysis

Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS, SPSS Inc., Chicago, LA, USA) software version 24. Descriptive statistics were presented as mean ± standard deviation. Discrete variables were presented as numbers (n) and percentages (%). Patients were classified into the following three groups according to SE: Group 1 included patients with SE -0.5 to <-2 D; Group 2 consisted of patients with SE -2 to < -4 D; and Group 3 individuals with SE -4 to < -6 D. The normality of CCT distribution was determined by the Shapiro–Wilk test. One-way analysis of variance followed by post-hoc Bonferroni was used for the analysis between the groups. The student's t-test was used to analyze the mean difference between two independent samples. Pearson's coefficient was used to study the correlation between variables. P < 0.05 was considered to be statistically significant.


  Results Top


This study included 388 eyes of 194 individuals with varying grades of myopia. 50% (n = 97) of the study participants were females. The mean age of the study participants was 25.47 ± 7.25 years (range: 12–40 years). The mean SE was − 2.79 ± 1.34 D (range: -0.5 to -6.0 D). The mean CCT was 540.53 ± 56.16 μm (range: 441–664 μm).

The CCT measurements were found to be nonnormally distributed. [Figure 1] shows the distribution of CCT readings among the study participants. [Figure 2] shows the distribution of SE among study participants.
Figure 1: Distribution of central corneal thickness readings among the study participants

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Figure 2: Distribution of spherical equivalent among the study participants

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[Table 1] shows the association of gender of study participants with CCT. No significant differences were noted in the mean CCT between males and females (P = 0.550, Student's t-test).
Table 1: Association between the gender of study participants and central corneal thickness

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[Table 2] shows the association between mean CCT and age of study participants. No significant differences were noted in mean CCT values between various age groups (P = 0.706).
Table 2: Association between mean central corneal thickness and age of study participants

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[Table 3] shows the mean CCT values among the SE groups. [Figure 3] shows the correlation between mean CCT and SE. No correlation was observed between mean CCT and SE (Pearson, r = 0.095, P = 0.062).
Table 3: The mean central corneal thickness values among spherical equivalent groups

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Figure 3: Scatter plot showing the correlation between mean central corneal thickness and spherical equivalent; no correlation was observed between mean central corneal thickness and spherical equivalent (Pearson, r = 0.095, P = 0.062)

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  Discussion Top


In this hospital-based study from South India, we evaluated the CCT measurements of young myopic individuals with mild-to-moderate myopia using NCSM and found no correlation between mean CCT and degree of myopia.

Population-based studies from India using UP for CCT measurements have reported significantly thinner corneas among Indians compared to Africans (530 μm),[21] Nepalese (539 μm),[22] Caucasians (540 μm),[23] and Chinese (542 μm).[24] Nangia et al. evaluated the distribution of CCT among adults aged 30+ from rural Central India using UP. Mean CCT among 9370 eyes studied was 514 ± 33 μm (range: 290–696 μm). CCT was associated significantly with younger age, male gender, and lower corneal refractive power (P < 0.001). However, no statistically significant association was found between CCT and refractive error (P = 0.54).[12] Vijaya et al. evaluated 6754 adults aged 40+ from South India using UP and found the mean CCT to be 511.4 ± 33.5 μm.[16]

A hospital-based, prospective study involving 156 participants from South India, with a mean age of 29.27 years and mean refractive error of −3.10 D revealed a mean CCT value of 533.87 μm using UP. CCT had a positive correlation with axial length and negative correlation with SE, suggesting that axial myopes tend to have thicker corneas.[10] Kunert et al. measured the CCT of 615 myopic patients of Indian origin (aged 18+) before laser in situ keratomileusis with UP as well as Orbscan scanning-slit topography/pachymetry.[25] Mean CCT reported was 519.92 ± 33.36 μm (range: 420–640 μm) using ultrasound method and 518.23 ± 31.03 μm (range: 428–607 μm) using Orbscan. No significant differences were noted in the CCT values measured using these two methods. Lower CCT readings were observed in individuals with myopia −0.5 to −4.9 D compared to those with myopia ranging from −5 to −9.9 D.

Mean CCT measurements obtained with EM-3000 (NCSM) in our study were 540.53 ± 56.16 μm (range: 441–664 μm). Módis et al. reported that the CCT values obtained using EM-3000 NCSM were higher than that found using ultrasound pachymetry (P < 0.0001).[26]

It would not be appropriate to compare our CCT measurements with other studies from India because we measured CCT using noncontact specular microscopy EM-3000 while others have utilized ultrasound pachymetry. The operating principles behind these two instruments are entirely different which explains the variation in measured values. While UP measurements depend on the reflection of ultrasonic sound waves from the anterior and posterior corneal surfaces, NCSM measurements rely on the reflection of light. CCT may be under or overestimated due to high inter-device variations, and studies have shown poor inter-device agreement among CCT measurements taken with different instruments.[27],[28]

Although ultrasound pachymetry is the gold standard for CCT measurements, it is a contact method that needs topical anesthesia and user skill with regard to the placement of the probe. Furthermore, studies have reported alteration in corneal thickness values after the application of local anesthetic eye drops.[29],[30] Unlike ultrasound pachymetry, NCSM is noninvasive and provides reproducible measurements of CCT.

There was no significant association between CCT and gender or age in our study. The role of gender on CCT has been reported to be variable, with few studies showing thicker CCT in males,[18],[31] while others have reported no significant differences between males and females.[32] Few studies in patients older than 40 years of age have reported a negative association between CCT and age.[16],[21],[33]

The effect of refractive error on CCT has been analyzed by several investigators with contrasting results. Our study results demonstrate that CCT does not correlate with the myopia of different grades. Our observations are in agreement with the majority of studies elsewhere.[12],[13],[14],[15] Chen et al. correlated the CCT measurements and degree of myopia among 528 Taiwanese adults enrolled for myopic laser refractive surgery.[13] No statistically significant association was found between CCT and refractive error (P = 0.49). The authors also studied the relation between CCT and myopia by intraindividual comparison in myopic anisometropia. The mean myopic anisometropia was 3.09 ± 1.06 D in their series, and the CCT in more myopic eye was not significantly different from the other less myopic eye. The authors have concluded that the cornea does not thin in the same way as the sclera in myopic eyes. Similarly, Fam et al. evaluated 714 consecutive Chinese patients with mean SE of − 5.3 D (range: −17.5 to −0.625).[15] No correlation was found between CCT and the degree of myopia (r = −0.13, P = 0.719). Ortiz et al. studied the relationship between CCT as well as mid-peripheral corneal thickness (PCT) in 175 myopic participants using Orbscan II.[34] Participants were divided into three groups, namely myopia <−6 D (n = 76), -6 to -12 D (n = 72), and more than −12 D (n = 27). No statistically significant differences were found in both CCT and PCT between the groups (P> 0.05).

A recent study comparing the thickness of preocular tear film, corneal layers (epithelium, Bowman layer, stroma and Descemet's membrane-endothelium complex in the central cornea), and anterior sclera using anterior segment spectral-domain optical coherence tomography found no statistically significant differences between emmetropia and those with moderate-to-high myopia (P > 0.05).[35]

However, few authors have found a significant correlation between CCT and myopic refractive error. Kadhim and Farhood evaluated CCT measurements of 418 eyes of 209 healthy individuals using UP.[9] The mean CCT was 543.95 ± 32.58 μm. CCT measurements significantly negatively correlated with age and myopes were noted to have significantly thinner corneas than emmetropic individuals (P = 0.019). Chang et al. evaluated a total of 216 participants, with a mean age of 22.2 years and mean refractive error of −4.17 D using UP.[36] A significant correlation was observed between CCT and SE (r = 0.16, P = 0.021). Corneas tended to be thinner in more myopic eyes. The authors have concluded that a decrease in corneal thickness is one among the changes in the anterior segment of the eye as myopia progresses. In contrast, Pedersen et al. measured CCT in 57 emmetropes and 48 high myopes using a high-precision Haag-Streit optical low coherence reflectometry pachymeter.[37] No statistically significant differences were noted between myopic participants and emmetropes (527.7 ± 35 μm versus 538.6 ± 32.1 μm), suggesting that the process by which myopia progresses does not influence the central cornea to a measurable degree.

The wide discrepancy in the observed results can be attributed to the varied age groups of patients, smaller sample sizes, nonelimination of contact lens wearers as well as the influence of diurnal variation, and use of pachymeters with variable reproducibility. The relatively new NCSM EM-3000 (Tomey, Japan) used in this study has been suggested to have high intraoperator repeatability in normal corneas.[27]

Our study has a few limitations. Observations are from a single-center, and axial length was not measured to differentiate axial myopia from other causes for myopia. However, we intentionally excluded patients older than 40 years to eliminate index myopia. To the best of our knowledge, this is the first study from India using NCSM to evaluate CCT measurements among myopic individuals.


  Conclusion Top


In summary, we evaluated CCT measurements using noncontact specular microscopy in a cohort of young individuals with mild-to-moderate myopia. The mean CCT values did not correlate with myopic SE. Further studies with a larger sample size and different grades of myopia are needed to better understand the relation between myopia and CCT.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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