Assessment of corneal epithelial thickness by Fourier domain OCT

This method was found to be reproducible, reliable for measurement of epithelial thickness at the vertex.

Amar Agarwal

The corneal epithelium plays an important role in the optics of the eye. The thickness of the epithelium variably contributes to the optical power of the cornea. The corneal epithelium influences tear film instability and is associated with local irregularities of the corneal topography created after excimer ablation surgeries. Anterior segment optical coherence tomography has been used in recent years to evaluate corneal epithelial thickness. We evaluated the repeatability, inter-user and intra-user, of Fourier domain OCT to measure epithelial thickness.

OCT unit

All patients underwent anterior segment Fourier domain OCT with the Cirrus HD-OCT (Carl Zeiss Meditec). The machine takes 27,000 A-scans per second. It has an axial resolution of 5 µm in tissue and transverse resolution of 15 µm in tissue. The A-scan depth is 2 mm in tissue. The optical source is a superluminescent diode with a scan beam wavelength of 840 nm. The exposure power at the cornea is less than 720 µW.

The test

The patient was positioned on the headrest. The infrared image of the cornea was seen directly on the examination screen. All scans were performed with the patient’s eye wide open with his own effort. No topical anesthesia or lubricating drops were used. Anterior segment five line raster mode was used. It has a scan angle of 0°, spacing of 0.25 mm between the lines and line length (scan length) of 3 mm. A repeat scan was taken if the first scan was not satisfactory (decentered due to patient’s eye movement, poor corneal apex reflection or head tilt). The output screen consisted of the captured iris image, the five scan images on the left side and the magnified view of the selected image on the right side (Figures 1 and 2). Inbuilt software, the “caliper tool,” was used to measure the total corneal thickness, epithelial thickness and corneal thickness excluding the epithelium (referred to as “non-epithelial thickness” in the rest of this article). The same image was used for all three calculations. The images were stored as screenshots, with one having the caliper reading for epithelial and non-epithelial corneal thickness (Figure 1) and one having the caliper reading for total corneal thickness (Figure 2). The non-epithelial thickness consisted of Bowman’s membrane, stroma, Descemet’s membrane and endothelium together.

Figure 1. Screenshot of output screen consisting of iris capture, five scans of the raster scan and magnified image of the selected scan with caliper tool showing measurements for the epithelium (56 µm in cornea) and non-epithelial region (432 µm in cornea). The measurement values were noted in real time, and screenshots were stored for analysis. Images: Agarwal A

Figure 2. Screenshot of output screen consisting of iris capture, five scans of the raster scan and magnified image of selected scan with caliper tool showing measurement for total corneal thickness (488 µm in cornea). The measurement values were noted in real time, and screenshots were stored for analysis.

Discussion

Various corneal diseases (Figures 3 and 4) can occur, and OCT can be helpful in detecting these conditions. OCT works on the principle of low coherence interferometry. The major advantages of Fourier domain systems include faster acquisition speed (leading to lesser motion artifacts) and better resolution compared with the time domain-based method. However, the reliability and reproducibility of Fourier domain systems for epithelial thickness measurement were not established.

The mean epithelial thickness noted in our cases (58.5 ± 2 µm) was similar to that noted by Tao and colleagues (52.5 ± 2.4 µm) on Fourier domain OCT, Sin and Simpson on time domain OCT (52 ± 3 µm) and Reinstein and colleagues on Artemis 1 (ArcScan) very high-frequency digital ultrasound (53.4 ± 4.6 µm).

Figure 3. Epithelial ingrowth.

Figure 4. Corneal dystrophy.

The corneal thickness at the vertex was 519.5 ± 31.1 µm in our study. In a previous study done by us on a different Fourier domain platform (Optovue), the mean central corneal thickness was comparable (521.12 ± 34.5 µm). A study by Ishibazawa showed the mean central Fourier domain pachymetry to be 530 ± 33 µm. Therefore, racial- and instrument algorithm-based differences may occur in pachymetric analysis in general, and the same can apply to isolated epithelium measurement.

Good reliability and reproducibility values were seen for epithelial thickness, as well as for the mean non-epithelial thickness and overall thickness. The high R2 values suggested a good data fit in a linear fashion, suggesting that both types of repeat measurements (by the same user and by different users) produced a good concordance with the initial values.

The values for reproducibility were slightly less compared with that achieved for reliability. However, on paired tests for mean differences, this did not show a significant difference in the mean of the two different users. It remains to be seen whether this difference is exaggerated in pathological corneas at extreme ends of the Gaussian curve.

The indices for coefficients of repeatability and reliability were the best for epithelial measurement, which can be a manifestation of the fact that the absolute range of values was less for epithelial measurement. Once the pachymetric values were considered, not just the standard deviations (using coefficients of variation), the results got reversed. Therefore, it should be noted by clinicians that comparing the reliability of a device for a measurement should be done with a corresponding measurement only, eg, comparing the epithelium measurement reliability of one device or user with that of another epithelium measurement and not with total measurement; 95% limits of agreement also indicate the amount of data spread. As with the other measures of reliability, the values of intra-user retesting were better than that with inter-user testing.

Conclusion

To conclude, the Fourier domain anterior segment is reproducible and reliable for measurement of epithelial thickness at the vertex. A slight, non-significant advantage for intra-user reliability was noted compared with inter-user reproducibility in our study. Future comparative studies between time- and Fourier domain-based epithelial thickness measurement, between normal and pathological corneas, and between adults and less cooperative patients such as the pediatric age group may reveal whether the higher resolution and faster acquisition speeds of the Fourier domain platforms help in maintaining this good reproducibility and reliability.

References:

• Reinstein DZ, Archer TJ, Gobbe M, Silverman RH, Coleman DJ. Epithelial thickness in the normal cornea: three-dimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg. 2008;24(6):571-581.
• Sin S, Simpson TL. The repeatability of corneal and corneal epithelial thickness measurements using optical coherence tomography. Optom Vis Sci. 2006;83(6):360-365.
• Tao A, Wang J, Chen Q, et al. Topographic thickness of Bowman’s layer determined by ultra-high resolution spectral domain-optical coherence tomography. Invest Ophthalmol Vis Sci. 2011 1;52(6):3901-3907.

• Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Prof. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; email: dragarwal@vsnl.com; website: www.dragarwal.com.
• Disclosure: Prof. Agarwal is a paid consultant to Bausch + Lomb, STAAR Surgical and Abbott Medical Optics. Dr. Prakash has no relevant financial disclosures.