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 Table of Contents  
ORIGINAL ARTICLE
Year : 2013  |  Volume : 3  |  Issue : 3  |  Page : 98-102

Agreement and repeatability of central corneal thickness measurements made by ultrasound pachymetry and anterior segment optical coherence tomography


Department of Ophthalmology, National University Hospital, Taipei, Taiwan

Date of Web Publication22-Aug-2013

Correspondence Address:
Jehn-Yu Huang
Department of Ophthalmology, National Taiwan University Hospital, Number 7, Zhongshan South Road, Taipei 100
Taiwan
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Source of Support: None, Conflict of Interest: None


DOI: 10.1016/j.tjo.2013.04.007

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  Abstract 


Purpose: To evaluate repeatability within and between examiners and to assess agreement in corneal thickness measurements obtained by anterior segment optical coherence tomography (ASOCT) and ultrasonic pachymetry (USP).
Methods: This was a prospective, observational study. Fifty-one patients who visited a glaucoma service with suspected glaucoma, glaucoma, or cataract were enrolled. Patients with a history of corneal surgery or abnormalities and those who were uncooperative for examinations were excluded. Each patient underwent ASOCT followed by USP, with each test performed by two different examiners. Measurement repeatability was evaluated using intraclass correlation coefficient (ICC) values. Agreement in corneal thickness measurements was evaluated by the Bland and Altman plot method. Linear regression analysis was used to assess the relationship between ASOCT and USP measurements.
Results: USP andASOCTcorneal thickness measurements revealed high intraexaminerand interexaminer repeatability (ICCs > 0.978). ASOCT corneal thickness measurements of a central 2-mm zone showed higher intraexaminer (ICC = 0.999) and interexaminer (ICC = 0.999) repeatability than USP measurements or ASOCT measurements of the vertex. The 95% limit of agreement (LoA) between the vertex-centered ASOCT and the USP scan was between 3.68 and 24.76 μ m. The 95% LoA between the central 2-mm zone ASOCT and the USP scan was between 3.75 and 23.39 μm. Average ASOCT corneal thickness was less than USP measurements, but ASOCT thickness could be converted to USP thickness through linear regression equations.
Conclusion: Both devices have good intraexaminer and interexaminer repeatability, though ASOCT has slightly better interexaminer repeatability. ASOCT accurately and reliably measures corneal thickness in a noninvasive manner.

Keywords: agreement, anterior segment optical coherence tomography, central corneal thickness, repeatability, ultrasound pachymetry


How to cite this article:
Lin CW, Wang TH, Huang YH, Huang JY. Agreement and repeatability of central corneal thickness measurements made by ultrasound pachymetry and anterior segment optical coherence tomography. Taiwan J Ophthalmol 2013;3:98-102

How to cite this URL:
Lin CW, Wang TH, Huang YH, Huang JY. Agreement and repeatability of central corneal thickness measurements made by ultrasound pachymetry and anterior segment optical coherence tomography. Taiwan J Ophthalmol [serial online] 2013 [cited 2022 Jun 26];3:98-102. Available from: https://www.e-tjo.org/text.asp?2013/3/3/98/203883


  1. Introduction Top


Central corneal thickness (CCT) is an important measurement in the clinical management of glaucoma. Many studies have reported that high or low corneal thickness can lead to overestimation or underestimation of intraocular pressure (IOP).[13] Although adjusting IOP for CCT is not necessary, CCT can help ophthalmologists properly perform risk assessments for the development of glaucoma.[4] The results of the Ocular Hypertension Treatment Study (OHTS) suggest that patients with thinner corneas may be at a greater risk of developing glaucoma.[4]

Central corneal thickness is an important evaluation before laser refractive surgery and is a predictive factor of postoperative kera-toectasia.[5] Overestimating corneal thickness may lead to excessive ablation and potentially result in iatrogenic keratoectasia. Knowledge of the corneal thickness is also critical in avoiding endothelial injury during collagen cross-linking treatment for progressive keratoconus.[6]

All CCT data in the above-mentioned studies were obtained with ultrasound pachymetry (USP).[4],[5],[6] This is currently the most frequently used clinical technique and the gold standard to evaluate CCT. Ultrasound pachymetry has some advantages, including its ease of use, portability, and low cost. However, the accuracy of this method is heavily examiner dependent. Any misalignment or variation in probe placement can introduce measurement error. Furthermore, USP probes must be placed directly onto the cornea, which introduces risk of corneal abrasion and infection transmission.[7],[8] Because of these drawbacks, devices and methods for noninvasive measurement of CCT, such as the Orbscan (Bausch & Lomb Inc., Rochester, NY, USA), the Pentacam (Oculus Optikgerate GmbH, Wetzler, Germany), and anterior segment optical coherence tomography (ASOCT), were invented. Anterior segment OCT evaluates anterior segment structure and measures corneal thickness in a noncontact manner. According to previous studies, CCT readings obtained by ASOCT were significantly lower than those obtained by USP.[9],[10],[11],[12],[13],[14]

In this study, we used Fourier-domain OCT and USP to measure corneal thickness. The agreement both between and within measurement modalities was examined and the possibility of converting results from one imaging modality to the other through linear regression analysis was explored.


  2. Materials and methods Top


2.1. Patients

This prospective, cross-sectional, observational study adhered to the tenets of the Declaration of Helsinki and was reviewed and approved by the institutional review board of the National Taiwan University Hospital. Informed consent was obtained from all patients before any study procedure or examination was performed. The study population was composed of patients who were seen at the glaucoma service of National Taiwan University Hospital due to suspected glaucoma(n = 23), glaucoma(n = 16),orcataract(n = 12). Patients were recruited randomly between August 2009 and June 2010. Patients who had a history of corneal surface surgery or corneal abnormalities and those who were uncooperative were excluded from participation. Study measurements were made in a total of 51 eyes from 51 volunteers (20 men, 31 women) who enrolled in this study. Mean patient age was 56.5 ± 14.3 years (range: 21-89 years).

2.2. Sample size estimation

Based on the results of a previous study,[15] we calculated the necessary sample size to be 34 eyes with nQuery advisor (version 4.0; Statistical Solutions, Boston, MA, USA). A sample size of 34 had 80% power to detect a difference in mean corneal thickness of 5 μm. We assumed a standard deviation of 10 μm and that comparisons between means would be made with a two-tailed paired t-test with a 0.05 significance level.

2.3. Central corneal thickness measurements

The central corneal thickness (CCT) for each patient’s right eye was measured using the ASOCT (RTVue Model RT100; Optovue Inc., Fremont, CA, USA) first and then USP (PacScan 330P; Sonomed Inc., Lake Success, NY, USA). For intraexaminer and interexaminer repeatability testing, the first examiner performed two consecutive measurements. Then, the second examiner performed two consecutive measurements with the same instrument. Both examiners completed the ASOCT measurement before the USP measurement. Between the two ASOCT measurements, patients were given the opportunity to sit back and leave the chin rest. Between the two USP measurements, the probe was moved away from the corneal surface so that two independent readings could be made.

The RTVue OCT is a Fourier-domain OCT that uses an 840-nm super illuminated diode as its scanning source. The transverse resolution is 15 μm, and the axial resolution is 5 μm. When corneal-anterior module (CAM) adapters are installed on the RTVue OCT, images of the cornea and anterior chamber can be obtained. The two types ofCAM adapters are the CAM-L and the CAM-S. The CAM-L was selected for use in this study because it allows a corneal pachymetry map to be obtained. For this study, an OCTcorneal pachymetry map was obtained by selecting the “Pachymetry + CPwr (corneal power)” and aiming the measurement beam at the pupil center. The built-in software (version 3.5) automatically generated a pachymetry map [Figure 1]A and divided it into the central (0-2 mm), pericentral (2-5 mm), and transitional (5-6 mm) zones. The two outer zones were further subdivided into eight sectors each by radial lines. Average corneal thickness for the central zone and each subzone is provided on the ASOCT report, as well as the thickness of the corneal vertex [Figure 1]B. Both the central zone corneal thickness (CCTc) and the corneal vertex thickness (CCTv) were compared to USP central corneal thickness (CCTu) measurements.
Figure 1: (A) Pachymetry map obtained with anterior segment optical coherence tomography (ASOCT). Average corneal thickness of each region is shown. (B) The ASOCT report also provides information on corneal thickness at the corneal vertex.

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After ASOCT measurements were completed, the cornea was anesthetized with one drop of 0.5% proparacaine (Alcaine, Alcon Inc., Fort Worth, TX, USA), followed by 10-second eyelid closure. To avoid a possible increase in corneal thickness secondary to topical anesthesia, USP measurements were started after 90 seconds of normal blinking. The ultrasound probe was placed as close to the center of the cornea as possible, perpendicular to the surface, as patients gazed at a distant target. The measurements were used only if the probe was within 10° of perpendicular. Using the default settings of the PacScan 330P USP, five consecutive measurements were averaged to obtain a single value for CCT.

2.4. Statistical analysis

For intraexaminer repeatability, we compared two measurements of the same method by the same examiner. For interexaminer repeatability, we compared the first measurement made by each examiner. The repeatability was evaluated by the intraclass correlation coefficient (ICC) value, which was interpreted as follows: 0–0.2 indicated poor agreement, 0.3–0.4 indicated fair agreement, 0.5–0.6 indicated moderate agreement, 0.7–0.8 indicated strong agreement, and >0.8 indicated almost perfect agreement.

Agreement between ASOCT and USP measurements was evaluated by the Bland-Altman method. Ninety-five percent limits of agreement (LoA) were defined as the mean ± 1.96 standard deviations (SDs). Difference between ASOCT and USP measurements was evaluated by the Wilcoxon signed ranked test. The relationship between ASOCT and USP corneal thickness measurements was determined using linear regression analyses, where USP measurements were compared individually to ASOCT vertex scans and central 2-mm zone results.


  3. Results Top


3.1. Intraexaminerrepeatability ofcorneal thickness measurements

The ICCs for the measurement of CCTv, CCTc, and CCTu by the first examiner were 0.998, 0.999, and 0.995, respectively. The ICCs for the measurement of CCTv, CCTc, and CCTu by the second examiner were 0.996, 0.999, and 0.978, respectively[Table 1]. Both ASOCT and USP showed good intraexaminer repeatability. In addition, ICC for corneal thickness measurement by ASOCT was similar between the first and second examiners. However, USP ICCs for corneal thickness measurements were lower for both the first (0.995) and the second examiner (0.978) than for ASOCT measurements.
Table 1: Intraexaminer repeatability of optical coherence tomography and ultrasound pachymetry measurements of corneal thickness.

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3.2. Interexaminer repeatability ofcorneal thickness measurements

We also analyzed interexaminer repeatability of corneal thickness measurements made by the ASOCT and USP by comparing the first measurement made by each examiner. Inter-examiner repeatability of both devices was acceptable for clinical practice and the ICCs for CCTv, CCTc, and CCTu were 0.998, 0.999, and 0.980, respectively. The CCTc measurement showed the highest inter-examiner repeatability and the CCTu measurement showed the lowest interexaminer repeatability [Table 2].
Table 2: Interexaminer repeatability of optical coherence tomography and ultrasound pachymetry measurements of corneal thickness.

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3.3. Agreement between optical coherence tomography and ultrasound measurements

Mean corneal thickness and the 95% LoA for both ASOCT and USP measurements are summarized in [Table 3]. Average ASOCT corneal thickness was statistically less than USP measurements for both vertex and central OCT measurements [Table 3]. Differences in individual patient ASOCT and USP corneal thickness measurements are shown in the Bland-Altman plots of [Figure 2] and [Figure 3]. Vertex CCT was thinner than CCTu in all but one case and CCTc was always thinner than CCTu. The mean difference between CCTu and CCTv measurements was 14.22 ± 5.38 μm (95% LoA = 3.68–24.76 μm, ICC = 0.984, p < 0.001 ) and the mean difference between CCTu and CCTc measurements was 13.57 ± 5.01 μm (95% LoA = 3.75–23.39 μm, ICC = 0.986, p < 0.001).
Figure 2: Blande–Altman plots comparing ultrasound pachymetry and anterior segment optical coherence tomography (vertex-scan) corneal thickness measurements.

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Figure 3: Blande–Altman plots comparing ultrasound pachymetry and anterior segment optical coherence tomography (central 2-mm zone) corneal thickness measurements. C.-W. Lin et al. / Taiwan Journal of Ophthalmology 3 (2013) 98–102 101

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Table 3: Interdevice agreement of optical coherence tomography and ultrasound pachymetry measurements of corneal thickness.

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3.4. Relationship between optical coherence tomography and ultrasound measurements

There was a strong linear relationship between OCT and USP measurements of corneal thickness [Figure 4] and [Figure 5]. Using linear regression analyses, we found the equations of the best-fit lines to convert OCT results to USP results. These equations are as follows (all CCT measurements in μm):
Figure 4: Relationship between ultrasound pachymetry and anterior segment optical coherence tomography (vertex-scan) corneal thickness measurements. Equation: CCTu = 7.360 + 1.011 × CCTv (adjusted r2 = 0.974).

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Figure 5: Relationship between ultrasound pachymetry and anterior segment optical coherence tomography (central 2-mm zone) corneal thickness measurements. Equation: CCTu = 10.124 + 1.006 × CCTc (adjusted r2 = 0.973).

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CCTu = 7.360 + 1.011 × CCTv (adjusted r2 = 0.974) (1)

CCTu = 10.124 + 1.006 × CCTv (adjusted r2 = 0.973) (2)


  4. Discussion Top


Central corneal thickness is an important parameter in determining whether or not patients are candidates for corneal refractive surgery and in assessing a patient’s risk when glaucoma, or the suspicion of glaucoma, is present. Adjustment ofIOP measurements for CCT may lead to decision changes regarding medical therapy, laser treatment, and surgical intervention in 8.5%, 2.1%, and 3.2% of patients, respectively.[16] Therefore, an accurate and precise CCT measurement is helpful for physicians in the clinical management of their patients.

Currently, USP is the standard device for CCT measurement, but with the introduction of ASOCT, ophthalmologists have an alternative, noncontact method to measure CCT. Measuring CCT without touching the cornea reduces the risk of iatrogenic corneal injury and infection transmission. Additionally, we have shown that ASOCT is less examiner-dependent than USP.

In agreement with previous studies,[10],[11],[12] our ASOCT CCT measurements were lower than USP measurements, with differences ranging between 6.4 and 49.4 μm. Possible explanations for these differences include topical anesthesia-induced corneal edema and uncertain patient-to-patient variations in the speed of sound through the cornea, for USP measurements, and tissue index of refraction, for ASOCT measurements.[10]

Good measurement repeatability depends on short data acquisition time, consistent positioning between scans, and minimal corneal thickness variation along neighboring points.

Based on our results, both intra- and inter-examiner repeatability were higher for OCT than for USP measurements. This indicates that OCT may be more suitable for repeated CCT measurements over time. The procedure of measuring CCT with ASOCT is less examiner-dependent than the process of obtaining measurements using USP. While taking OCT measurements, a realtime corneal cross-sectional image is visible during the examination, and examiners can easily adjust the scanning position to ensure that the measurement is being made perpendicular to the corneal surface. If the location of the corneal vertex does not change over time, this method could serve as a location registration to make serial measurements at the same location possible. The ability to quickly acquire corneal thickness data is another advantage of ASOCT over USP. The RTVue OCT has an acquisition speed of 0.32 seconds, which is much faster than the minutes it takes to obtain USP measurements. Rapid data acquisition is essential in minimizing both patient and machine motion artifacts, and the higher axial resolution of the RTVue OCT may contribute to better repeatability.

The intraexaminer and interexaminer repeatability with the ASOCT central 2-mm zone method (CCTc) was slightly better than the vertex method (CCTv). The central 2-mm zone measurement is the result of averaging corneal thickness within the central 2-mm zone. Because it is not a single point measurement, errors due to misalignment are minimized, but CCTc does not provide the corneal thickness of a single central point.

The range of 95% LoA between CCTc and CCTu was narrower than the one between CCTv and CCTu. According to a previous study,[17] the 95% LoA between RTVue OCTand USP measurements is broad, and the corneal thickness measurements cannot be used interchangeably in either the clinical or research setting. However, our study demonstrated that reliable USP and ASOCT measurements are linearly related to each other and that it is possible to convert between the two types of CCT measurements. Linear regression analyses showed that adjusted R-squared values ofboth regression equations (CCTv vs. USP, CCTc vs. USP) were greater than 0.97. Therefore, it is possible to use ASOCT, instead of USP, to measure corneal thickness in healthy individuals. This may even be better because of the higher intraexaminer and interexaminer repeatability, and the noncontact nature, of ASOCT.

There were some limitations to our study. First, the sample size was relatively small and more corneal thickness measurements are needed in a larger group of patients to validate our conversion equations. Second, all patients enrolled in this study had normal corneas, so it is unknown how ocular and/or corneal alterations/ pathologies would have affected our results. Further studies including patients who have undergone refractive surgery (i.e., LASIK) or who have corneal abnormalities (i.e., keratoconus, corneal opacity) are needed because changes in corneal curvature and/or structure may lead to different results.

Corneal thickness as measured by ASOCT was consistently thinner than corneal thickness as measured by USP. Both measurement modalities had good intraexaminer and interexaminer repeatability, but ASOCT was slightly better. Using linear regression analysis to find the equation of the best-fit line, it was possible to convert between ASOCT and USP measurements. Therefore, ASOCT is a useful and precise technology that can reliably measure corneal thickness in a noncontact manner.

Acknowledgments

The authors indicated no financial supportorconflicts ofinterest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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