Central Corneal Thickness Measurement

Abstract

We read with interest the recent article by Nam et al.1Nam S.M. Im C.Y. Lee H.K. et al.Accuracy of RTVue optical coherence tomography, pentacam, and ultrasonic pachymetry for the measurement of central corneal thickness.Ophthalmology. 2010; 117: 2096-2103Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar This article provides useful information on the central corneal thickness (CCT) measurements with spectral domain optical coherence tomography (SD-OCT) and compares it with the other CCT measuring instruments. The authors found the CCT measurement by RTVue SD-OCT to be greater than that obtained by ultrasonic pachymetry (USP). However, similar studies by Li et al2Li Y. Tang M. Zhang X. et al.Pachymetric mapping with Fourier-domain optical coherence tomography.J Cataract Refract Surg. 2010; 36: 826-831Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar and Chen et al3Chen S. Huang J. Wen D. et al.Measurement of central corneal thickness by high-resolution Scheimpflug imaging, Fourier-domain optical coherence tomography and ultrasound pachymetry.Acta Ophthalmol. 2010 Jun 18; ([Epub ahead of print])Google Scholar have reported the CCT measurements by RTVue to be thinner (P<0.05 in both studies) than those obtained by USP. Although the variability in the CCT measurement may be the likely reason for such differences, the authors' perspective regarding this would be of interest. We think that the use of the term “accuracy” used in the title might be slightly misleading. Accuracy is “how close the measurement is to the true value.”4Sackett D.L. Tugwell P. Guyatt G.H. Clinical epidemiology: a basic science for clinical medicine; The clinical examination: 19– 49.in: 2nd ed. Little, Brown, Boston1991: 33Google Scholar The authors are not actually measuring the true CCT here. Although USP is considered the “gold standard” for CCT measurement, it actually doesn't measure the true CCT and is only a surrogate measure at best. The lack of a true gold standard for CCT measurement is because of the technical difficulties of direct physical measurement of the human corneal thickness in vivo.5Moezzi A.M. Sin S. Simpson T.L. Novel pachometry calibration.Optom Vis Sci. 2006; 83: 366-371Crossref PubMed Scopus (17) Google Scholar In our opinion, the term “accuracy” should be avoided in this situation. Accuracy of RTVue Optical Coherence Tomography, Pentacam, and Ultrasonic Pachymetry for the Measurement of Central Corneal ThicknessOphthalmologyVol. 117Issue 11PreviewTo compare the reliability and the agreement in measuring central corneal thickness (CCT) using the following technologies: RTVue Fourier-domain optical coherence tomography (Optovue, Inc., Fremont, CA), Pentacam (Oculus, Inc., Wetzlar, Germany), and ultrasonic pachymetry (USP; Pocket-II; Quantel Medical, Inc., Bozeman, MT). Full-Text PDF Author replyOphthalmologyVol. 118Issue 5PreviewWe appreciate the interest in our article dealing with the accuracy of RTVue, Pentacam, and ultrasonic pachymetry (USP) in the measurement of central corneal thickness (CCT). We agree that we do not have a true method to measure the real CCT, yet. However, if we define the accuracy as the proximity of the measurement to the reference value of a gold standard, research on “accuracy” can still be performed. A gold standard does not necessarily mean perfection, especially in the medical field. For example, optical pachymetry was introduced for CCT measurement as early as 1951, but USP replaced optical methods because of its ease of use, portability, accuracy, and reproducibility. Full-Text PDF