Posters

Comparison of corneal epithelial thickness measurement between Fourier-domain optical coherence tomography and very high-frequency digital ultrasound

Poster Details

First Author: T.Yap UK

Co Author(s):    T. Archer   M. Gobbe   D. Reinstein        

Abstract Details

Purpose:

To compare measurements of corneal epithelial thickness using optical coherence tomography (OCT) and very high-frequency digital ultrasound (VHFDU).

Setting:

London Vision Clinic, London, UK.

Methods:

Retrospective analysis of 211 pre-operative and 191 post-operative eyes which had undergone measurement of corneal epithelial thickness with both the RTVue Fourier-domain OCT (tear film included) and Artemis VHFDU systems (tear film excluded). Data were obtained according to the zonal averages included in the standard RTVue mapping of epithelial thickness within the central 6-mm diameter zone. Zonal averages were calculated for the central 2-mm diameter zone, 8 zones equally distributed within an annulus between the 2 and 5-mm diameter rings, and 8 zones equally distributed within an annulus between the 5 and 6-mm diameter rings. The agreement between measurements was analyzed by calculating the mean difference (OCT - VHFDU), 95% limits of agreement (LoA, 1.96 standard deviation of the difference) and Bland-Altman style analysis.

Results:

In pre-operative patients, mean central epithelial thickness was 53.3±3.15 µm (range: 46-62 µm) with OCT and 54.0±3.06 µm (range: 47-62 µm) with VHFDU. OCT measured the central corneal epithelium thinner than VHFDU in 68% of eyes, with a mean difference of -0.71 µm and 95% LoA of ±3.96 µm (p<0.001). The smallest difference was in the superotemporal zone of the inner annulus (mean +0.02 µm, 95% LoA ±8.70 µm) while the largest difference was in the inferonasal zone of the outer annulus (mean -3.55 µm, 95% LoA ±9.46 µm). In post-operative patients, mean central epithelial thickness was 57.8±6.19 µm (range: 42-77 µm) with OCT and 60.3±6.62 µm (range: 41-49 µm) with VHFDU. OCT measured the central corneal epithelium thinner than VHFDU in 85% of eyes, with a mean difference of -2.51 µm and 95% LoA of ±5.39 µm (p<0.001). The smallest difference was in the inferior zone of the outer annulus (mean -1.14 µm, 95% LoA ±9.02 µm) while the largest difference was in the inferonasal zone of the inner annulus (mean -2.94 µm, 95% LoA ±5.41 µm). A larger difference between methods was more common when measuring thicker epithelium.

Conclusions:

Measurement of corneal epithelial thickness using OCT was found to be thinner than VHFDU. In contrast to VHFDU, the OCT measurement includes the tear film, so the true difference is likely to be 4-6 µm more than reported in the results. The difference was greatest in the inferior cornea and was also higher for post-operative eyes and eyes with thicker epithelium. FINANCIAL INTEREST: One of more of the authors... has significant investment interest in a company producing, developing or supplying product or procedure presented