Variation Observed in Central Corneal Thickness Measurement Using Non-Contact Methods in Refractive Errors: A Comparative Study

 
Adeeba Hussain^1*, Radha I Dass²

¹Third Year Resident, Department of Ophthalmology, Shri MP Shah Medical College Jamnagar Gujarat, India
²Associate Professor, Department of Ophthalmology, Shri MP Shah Medical College Jamnagar Gujarat, India

*Correspondence author: Adeeba Hussain, Third Year Resident, Department of Ophthalmology, Shri MP Shah Medical College Jamnagar Gujarat, India; Email: [email protected]

Citation: Hussain A, et al. Variation Observed in Central Corneal Thickness Measurement Using Non-Contact Methods in Refractive Errors: A Comparative Study. J Ophthalmol Adv Res. 2025;6(3):1-8.

Copyright^© 2025 by Hussain A, et al. All rights reserved. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Table 1: Distribution according to baseline variables.

Table 2: Comparison of Central Corneal Thickness (CCT) between three measurement methods.

Table 3: Comparison of Central Corneal Thickness (CCT) between patients with myopia and hypermetropia across the three measurement method.

Table 4: Correlation of CCT measurements between various methods.

Figure 1: Mean central corneal thickness by various methods in study population.

Figure 2: Correlation of CCT measurements between the methods.

Figure 3: Speculated tear film thickness by AS OCT as measured from difference in CCT from other two methods.

Discussion

Accurate measurement of Central Corneal Thickness (CCT) is crucial in modern ophthalmic practice. CCT not only plays a key role in diagnosing and managing glaucoma but also serves as an essential parameter in preoperative evaluation for refractive surgery and corneal diseases [1,2]. Several modalities are available to measure CCT, including Anterior Segment Optical Coherence Tomography (AS OCT), Tonopachymetry and Specular Microscopy. Each method operates on different principles-optical interferometry, specular reflection and endothelial imaging respectively leading to variability in measurements. The choice of CCT measurement technique can significantly influence clinical decisions [10]. Dedicated Noncontact AS-OCT devices are non-invasive and non-contact procedures, which relies on the principle of interferometry to detect minute differences in tissue depth. They provide high resolution cross-sectional imaging of the cornea with both central and regional pachymetry. The OCT device captures a corneal sectional image, which is then analysed by software. In this case, the central corneal thickness value is calculated from tear film to corneal endothelium. Noncontact Tonopachymetry is a technique that combines the functions of tonometer and optical pachymeter. The pachymetry function measures CCT at the apex of the cornea by determining the distance between optical reflections from the anterior and posterior surfaces of the cornea. Non-contact specular microscopy measures CCT according to the principle of reflection of light from the anterior and posterior surfaces of the cornea, (tear film, corneal epithelium and aqueous humour) causes a light reflection due to a different refractive index. It obtains CCT values by considering the space between the tear film and the corneal endothelium [11,12].

In parallel, understanding the distribution of refractive errors, particularly myopia and hypermetropia, is increasingly important considering global trends.  Myopia has reached epidemic proportions in several parts of the world, especially among younger populations, while hypermetropia remains more prevalent in older individuals. According to the VISION 2020 Global Initiative and studies by Holden et al., the prevalence of myopia has surged in younger populations due to increase near work and reduced outdoor activity. The data presented in this study reveal no significant difference in central corneal thickness between myopic and hypermetropic groups. Although myopic eyes tend to have slightly thicker corneas on average, this finding contrasts with previous studies, such as those by Chang, et al., which suggested that myopic eyes often exhibit thinner corneas due to axial elongation. However, the relationship between refractive errors and corneal thickness has been shown to vary across populations and methodologies. Several studies have similarly found no significant correlation between refractive error and central corneal thickness.

The observed mean CCT values in this study are consistent with globally reported averages. Doughty, et al., reported a mean CCT of 535 ± 31 µm in a meta-analysis of 47 studies. Aghaian, et al., documented mean CCT of 555.6, 550.4, 550.6 and 548.1 micron in Chinese, Caucasian, Filipino and Hispanic population [195]. While our results appear slightly lower than these published averages, they remain within the expected physiological range of 500-560 µm. Differences in population ethnicity, age structure and environmental influences such as altitude and UV exposure may account for slight regional variation in CCT.

AS-OCT consistently measured CCT values ~5-6 µm higher than both specular microscopy and tonopachymetry which is in line with Gokcinar, et al., Literature suggests that AS-OCT measurements are often 5-10 µm higher, likely due to inclusion of the tear film and better visualization of stromal boundaries [17-22]. These systematic differences might be related to measurement methodology, technology precision or operator variance. AS-OCT uses optical interferometry, measuring from the air-tear film interface to the posterior corneal boundary, potentially capturing more accurately the actual anatomical boundaries due to high resolution, as it includes tear film thickness too, thus slightly thicker measurements. OCT systems take nearly 26,000 A-scans per second with 5 µm axial resolution. This high-speed scanning makes ocular movements negligible during measurements, giving a minimal corneal variation. Regular calibration checks and adherence to strict imaging protocols can mitigate systematic biases.

The study confirms excellent agreement between specular microscopy and tonopachymetry and a consistent positive bias in AS-OCT readings. This minor but systematic difference underscores the need for method-specific normative data and highlights the importance of using a single, consistent modality in serial measurements, particularly in refractive surgery, where corneal thickness determines eligibility and safety and Glaucoma management, where CCT is used to adjust Intraocular Pressure (IOP) readings. The correlation analysis underscores very high agreement among all instruments despite systematic differences, confirming each method’s reliability for clinical use. The highest correlation was found between Specular microscopy and Tonopachymeter (R=0.984). The consistently high correlations between the three CCT measurement techniques (Specular microscopy, Tono pachymeter and AS OCT) across refractive error groups highlight the robust agreement and reliability among these modalities. Despite earlier observed systematic differences especially the higher values obtained by AS OCT these strong correlations suggest that all three instruments can reliably measure corneal thickness and are suitable for clinical applications. The findings suggest clinicians can reliably choose among these methods based on patient comfort, availability, clinical context or procedural considerations, albeit acknowledging systematic differences in absolute measurements.

Clinically, while inter-instrument agreement in relative measurements is excellent, slight method-specific differences must be accounted for when making clinical decisions-particularly in scenarios where precise corneal thickness values critically influence surgical or therapeutic choices, such as glaucoma management, refractive surgery or keratoconus screening. Such differences emphasize the importance of device-specific baselines and consistent measurement techniques in clinical follow-up and surgical evaluation.

By comparing AS-OCT with the other two modalities, the speculated tear film thickness was estimated to be around 5.3-5.6 µm, supporting the idea that AS-OCT captures an additional anterior Tear optical layer absent in other methods. Our study had certain limitations. First, small sample size and cross-sectional nature of the study limit the ability to assess longitudinal changes in CCT that would be more informative for understanding how it changes with age or refractive error evolution. Myopia is more commonly observed in younger individuals, while hypermetropia tends to be prevalent in older adults (>45 years). Central Corneal Thickness (CCT) decreases with age, it is theoretically expected that myopic patients would have thinner corneas and hypermetropic patients thicker ones. However, our study demonstrated a contrasting pattern, likely influenced by age-related confounding, which was not assessed in our study. The study population may lack ethnic diversity and since CCT varies significantly across ethnic groups (e.g., thinner corneas in African descent, thicker in East Asians), the results may not be generalizable to all populations.

Conclusion

AS-OCT consistently records slightly higher CCT values (~5-6 µm) compared to other modalities, likely due to its high optical resolution and inclusion of the tear film, whose estimated thickness ranges from 5.3 to 5.6 µm when inferred from AS-OCT measurements. AS-OCT, Specular Microscopy and Tonopachymetry show excellent agreement, with the highest inter-instrument correlation, supporting their reliability in clinical settings. No significant difference in CCT was observed between myopic and hypermetropic groups in our cohort, suggesting that refractive error type alone may not dictate corneal thickness. Some limitation are, studyIt hashad a modest, cross-sectional sample with limited ethnic diversity and unequal refractive-error distribution, restricting longitudinal interpretation and generalizability across populations. The detectable difference in CCT between AS-OCT and other modalities suggests tear film thickness can be calculated and   can be helpful in Dry Eye Disease. All three methods demonstrated very strong correlations across refractive error subgroups, suggesting clinical interchangeability, though method-specific biases must be considered when making surgical or diagnostic decisions. All three methods are non-contact and quick methods, which generally leads to good patient comfort, but Tonopachymetry and Specular Microscopy tend to be slightly faster, while AS-OCT offers more detailed imaging at the expense of a bit longer scan time.

Conflict of Interest

The author declares no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding Details

No funding was received for this review.

Author’s Contributions

All authors have contributed equally to this work and have reviewed and approved the final manuscript for publication.

Consent For Publication

Not applicable.

Ethical Statement

Not Applicable.

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