From the Department of Ophthalmology, University of North Carolina School of Medicine, Chapel Hill, North Carolina.
Supported by Research to Prevent Blindness, Inc., New York, New York.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Graham W. Lyles, MD, Department of Ophthalmology, 5151 Bioinformatics Bldg., CB #7040, Chapel Hill, NC 27599-7040. E-mail: firstname.lastname@example.org
The management and treatment of Fuchs’ endothelial dystrophy has been dramatically altered by the advent and evolution of Descemet’s stripping endothelial keratoplasty (DSEK).1 Ophthalmologists will increasingly observe and treat more patients who have undergone this procedure. In the immediate postoperative period, successful graft positioning and adherence are of primary concern. With time, the ophthalmologist is concerned with graft survival and function, assessed by clarity and central corneal thickness (CCT). Ultrasound pachymetry is the gold standard for measurement of CCT. It is a technique that can be performed quickly and obtains multiple measurements to reduce measurement error.2
We present the case of a patient who underwent DSEK, in which ultrasound pachymetry measurement was lower than expected. Therefore, Visante optical coherence tomography (OCT) (Carl Zeiss Meditec, Inc., Dublin, CA) was used to image and measure the CCT of the host cornea and donor tissue.
A 63-year-old man with Fuchs’ dystrophy was referred to the cornea service for decreasing vision in both eyes during the day and glare with decreased vision at night making driving difficult. On initial examination, his best-corrected visual acuity in each eye was 20/40-. Ultrasound pachymetry of the right and left eyes revealed a CCT of 698 and 654 μm, respectively. Slit-lamp examination showed confluent guttae and stromal edema in each cornea and 2+ nuclear sclerotic cataracts. The remainder of the slit-lamp examination and the ophthalmoscopic examination were normal. Intraocular pressure was 12 mm Hg in each eye.
The patient underwent successful DSEK and phacoemulsification of cataract with insertion of a posterior chamber intraocular lens in the right eye. Using ultrasound pachymetry, the eye bank measured the thickness of the posterior donor lamella at 180 μm. One month after DSEK, uncorrected visual acuity was 20/70-. Ultrasound pachymetry measured CCT at 549 μm. The graft remained in the correct position with only mild edema in its periphery.
The ultrasound pachymetry measurement was thought to be an artifact. Therefore, cross-sectional corneal images were obtained using Visante OCT. Analysis of OCT images demonstrated complete interface adherence between host stroma and donor graft tissue (Fig. 1). The CCT at the corneal vertex measured with the Visante flap tool was 769 μm. Visante OCT automated pachymetry measurement of the CCT was 770 μm (Fig. 2).
Figure 1. High-Resolution Anterior Segment Optical Coherence Tomography Demonstrates Adherent Donor Tissue Following Descemet’s Stripping Endothelial Keratoplasty. Caliper Measurement Arms Are Positioned at the Corneal Vertex and Donor Endothelium.
Figure 2. Visante Optical Coherence Tomography (Carl Zeiss Meditec, Inc., Dublin, CA) Global Pachymetry Map of the Right Eye Following Descemet’s Stripping Endothelial Keratoplasty Is Shown. The Estimated Central Corneal Thickness of 770 μm Is Based on 16 Radial Scans Incorporating 2,048 Corneal Thickness Measurement Points.
In our patient, the 1-month postoperative ultrasound measured CCT of 549 μm was deemed an artifact because approximately 30 μm of Descemet’s membrane and endothelium were removed from the recipient and 180 μm of donor cornea were inserted.3 Even if the donor endothelium were functioning normally to produce a maximally thin cornea, the CCT measured with ultrasound should have been at least 700 μm. This was confirmed by the OCT measured CCT thickness of 769 μm using the flap tool (Fig. 1) and 770 μm by global pachymetry mapping (Fig. 2).
To monitor corneal health after DSEK, reproducible measurements of CCT are necessary. In our case, at 1 month after DSEK, an apparently unreliable CCT was measured with ultrasound. Ultrasound pachymetry may not be the ideal modality after DSEK. It is dependent on the different physical properties of the cornea and aqueous, in particular the density of the medium through which the ultrasound is passing.4 When sound crosses from one medium to another medium with a different impedance, an acoustic interface is formed; this creates an echo or reflection that is transmitted back to the probe tip.4 Following DSEK, the host–graft interface may produce an acoustic impedance different from normal host cornea, leading to erroneous pachymetric measurement. This may be secondary to fluid in the interface.
Anterior segment OCT provides qualitative and quantitative information about the state of the cornea.2 OCT imaging employs infrared light and measures the time-of-flight delay as light is reflected from anterior segment tissue structures. The reflected light is interferometrically correlated with a reference beam; the strict phase relationship between the two beams provides higher depth resolution and more sensitivity than other imaging modalities.5,6 After DSEK, this allows visualization of the host–graft interface and a CCT measurement. OCT software includes a flap tool that outlines the anterior and posterior surfaces of the cornea. A horizontal measuring arm can be positioned at the corneal vertex and at the stroma–donor graft interface or at the donor endothelium. This generates a measurement of the entire CCT, as well as the thickness of the individual layers.
In addition, the Visante software provides a global pachymetry map with a CCT measurement. Measurements are based on 16 radial scans centered on the corneal vertex; each radial scan is 10 mm in length and is the summation of 128 A-scans per radial line scan. This provides a total of 2,048 measurement points and employs an algorithm that interpolates both radially and circumferentially (personal communication with Carl Zeiss Meditec, June 30, 2009).
Anterior segment OCT is estimated to locate corneal boundaries to within ±2 μm.5 Previous studies found that CCT measurement by OCT and ultrasound are highly correlated.7–9 Most evidence suggests that OCT measurements are lower than those obtained by ultrasound; however, some authors found no difference or even higher measurements of CCT by OCT.9 These studies are part of an evolving literature dedicated to determining the systematic differences between the two modalities used to measure CCT. Our case report does not compare the systematic differences between OCT and ultrasound. However, it suggests that the ideal measurement tool for CCT after DSEK is likely to be situation specific.
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- Wong AC, Wong CC, Yuen HS, Hui SP. Correlational study of central corneal thickness measurements on Hong Kong Chinese using optical coherence tomography, Orbscan and ultrasound pachymetry. Eye. 2002;16:715–721. doi:10.1038/sj.eye.6700211 [CrossRef]
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- Byrne SF. Basic principles of ultrasound. In: Byrne SF. A-Scan Axial Eye Length Measurements: A Handbook for IOL Calculations. Mars Hill, NC: Grove Park Publishers; 1995:1–8.
- Huang D, Wang J, Lin CP, Puliafito CP, Fujimoto JG. Micron-resolution ranging of the cornea/anterior chamber by optical reflectometry. Lasers Surg Med. 1991;11:419–425. doi:10.1002/lsm.1900110506 [CrossRef]
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- Li EY, Mohamed S, Leung CK, et al. Agreement among 3 methods to measure corneal thickness: ultrasound pachymetry, Orbscan II, and Visante anterior segment optical coherence tomography. Ophthalmology. 2007;114:1842–1847. doi:10.1016/j.ophtha.2007.02.017 [CrossRef]
- Zhao PS, Wong TY, Wong WL, Saw SM, Aung T. Comparison of central corneal thickness measurements by Visante anterior segment optical coherence tomography with ultrasound pachymetry. Am J Ophthalmol. 2007;143:1047–1049. doi:10.1016/j.ajo.2007.01.050 [CrossRef]
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