Corneal Thickness

Abstract

We read with interest the articles by Li et al1Li E.Y. Mohamed S. Leung C.K. et al.Agreement among 3 methods to measure corneal thickness: ultrasound pachymetry, Orbscan II, and Visante anterior segment optical coherence tomography.Ophthalmology. 2007; 114: 1842-1847Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar and Li et al2Li H. Leung C.K.S. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar on the measurements of central corneal thickness (CCT) between ultrasound pachymetry (USP) and optical coherence tomography (OCT). Li et al1Li E.Y. Mohamed S. Leung C.K. et al.Agreement among 3 methods to measure corneal thickness: ultrasound pachymetry, Orbscan II, and Visante anterior segment optical coherence tomography.Ophthalmology. 2007; 114: 1842-1847Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar reported that Visante anterior segment OCT underestimated CCT by 14.74 μm compared with that measured with USP. Another Li et al2Li H. Leung C.K.S. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar compared the corneal thickness measurement by the 2 commercially available anterior segment OCT systems—Visante anterior segment OCT and slit-lamp OCT (SLOCT)—with the gold standard USP. They reported the Visante OCT automatic CCT measurements was 14.6 μm less than CCT with USP, whereas no difference was observed between SLOCT CCT measurements and USP (mean CCTs by USP, automatic Visante OCT, and automatic SLOCT were 550.3±31.1, 535.7±30.2, and 548.3±30.1 μm, respectively). We would like to share our experience with the authors. We prospectively studied 22 eyes from 22 patients during their preoperative visits for cataract operation on the measurement of CCT with USP and SLOCT (Heidelberg Engineering, Dossenheim, Germany). Although our results showed good correlations between USP and SLOCT (r = 0.919; P<0.001, Bland-Altman plot; mean difference, 10.5 μm; 95% limits of agreement between −16.4 and 37.4 μm), the SLOCT measurements have underestimated CCT to a similar degree as the Visante anterior segment OCT in our study. Our mean measurements of average CCT by USP and SLOCT were 542.86±31.70 and 532.36±34.84 μm, respectively. This clearly contradicts the results of Li et al.2Li H. Leung C.K.S. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar Despite the fact that we were using the same model of Heidelberg SLOCT as Li et al,2Li H. Leung C.K.S. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar we cannot explain the discrepancies between the 2 studies. In our case series, the mean age of our subjects is much older (76.14±9.10 years). It is not clear if older corneas alter the optical principles of SLOCT with respect to the detection of the anterior corneal surface. Other possible factors include the technique of the observers and alignment of the instruments. We hope that our results can further confirm the usefulness of SLOCT in the measurement of the corneal thickness and lead to further discussion on the discrepancies involved. Author replyOphthalmologyVol. 115Issue 12PreviewWe thank Pong et al for their interest in our work. A number of studies,1,2 including ours,3 have demonstrated that central corneal thickness (CCT) obtained from Visante anterior segment optical coherence tomography (OCT) is thinner compared with that measured by ultrasound pachymetry. However, our recent study comparing slit-lamp OCT (SLOCT) and Visante OCT did not reveal significant difference in CCT between SLOCT and ultrasound pachymetry,4 which is different from the finding by Pong et al. The discrepancy in CCT measurement between the 2 anterior segment OCT instruments could be attributed to the differences in the choice of refractive indexes in the calculation of corneal thickness and different algorithms for image analysis. Full-Text PDF