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Original Study

Comparison of Chinese and North American Tomographic Parameters and the Implications for Refractive Surgery Screening

Boyd, Brennan M. BS; Bai, Ji MD; Borgstrom, Mark PhD; Belin, Michael W. MD§

Author Information
Asia-Pacific Journal of Ophthalmology: March-April 2020 - Volume 9 - Issue 2 - p 117-125
doi: 10.1097/APO.0000000000000273
  • Open

Abstract

Tomographic analysis has become a mainstay in the evaluation of the preoperative refractive surgical candidate. Although multiple tomographic technologies exist [eg, Scheimpflug, scanning slit, optical coherence tomography (OCT)] one of the more commonly used devices is the Oculus Pentacam (Oculus GmbH, Wetzlar, Germany), and the most commonly used Pentacam screening display is the Belin/Ambrosio Enhanced Ectasia Display (commonly referred to as the “BAD” display).1–4 The BAD display uses 9 tomographic parameters in a proprietary regression analysis to give an overall evaluation of the cornea and its deviation from a “normal” cornea (final “D”) expressed as a standard deviation from normal. The data utilized in generating the regression analysis in the BAD display was created from combined North and South American refractive surgical patients.5,6

Previous published studies looked at the variation in tomographic parameters in different geographic and ethnic populations.7–9 Although the variations were relatively small, no previous work looked at the effect these variations could have on the overall analysis of the BAD display. Additionally, Chinese patients have been observed to have smaller corneal diameters than North American (white and African-American) patients.10,11 Multiple pachymetric progression parameters are presented on the BAD display (ARTmax, PImin, PIavg, PImax) and the pachymetric progression is shown graphically in both the Corneal Thickness Spatial Profile (CTSP) and Percent Thickness Increase (PTI) graphs. Each of these represents the rate of change in corneal thickness from the thinnest point to the periphery. Eyes with ectatic disease commonly show a more rapid change.12,13 It would be somewhat reasonable to assume that eyes with a smaller corneal diameter may have a higher rate of change, as the overall distance between the thinnest point and periphery is less.14 We therefore hypothesized that corneal diameter influences the pachymetric progression parameters more profoundly in Chinese patients (with smaller corneas) compared with North American patients (with larger corneas).

We undertook to determine whether there are significant clinical variations in the tomographic parameters utilized in the BAD display between Chinese and North American eyes and whether any of these changes are related to differences in corneal diameter.

METHODS

This was a retrospective observational study between Chinese and North American patients. The study conformed to the guidelines outlined in the Declaration of Helsinki. The study to analyze de-identified data was under local institutional review board exemption. All files were devoid of any patient identifiers and were generated from patients seeking refractive surgical correction with a minimum of 2-year uneventful follow-up. All files were re-analyzed using current Pentacam software (version 1–21r43) and evaluated at the University of Arizona College of Medicine in Tucson, AZ.

The North American database was an earlier established database utilized in previous published studies.11 All eyes were imaged with the Pentacam HR Eye Scanner. Both eyes were analyzed. All patients had a normal ocular examination that included biomicroscopy, fundus evaluation, Schirmer tear testing, and muscle balance. All eyes had a corrected distance visual acuity (CDVA) of 20/20 or better and no family history of ectatic disease. Only simple and compound myopic patients [range −0.50 to −10.00 diopters (D)] were included in this study. Patients were included whether they proceeded to refractive surgery or not so as not to bias the databases based on a preconceived selection prejudice. All patients independently analyzed by 2 fellowship trained refractive surgeons. When there was >1 examination, the most recent examination (preoperative for surgical patients) was chosen. Clinically uneventful follow-up was defined as no loss of ≥1 lines of vision, no increase in astigmatism by 0.50 D, and no need for an enhancement. Because this was a retrospective analysis, not every postoperative patient had follow-up tomography; however, only patients with postoperative clinical data (ie, refraction, CDVA, uncorrected distance visual acuity) were included. Similarly, nonsurgical patients were evaluated by a chart review, when available, or by telephone contact. To be included in the study, all paired eyes had to have at least 9.0 mm of coverage with a minimum of 8.0 mm nonextrapolated data points and an internal Pentacam quality analysis indicating no issues. The internal quality analysis includes checks for alignment, pupil, movement, blinking, coverage, and data consistency.

The Chinese data were collected between January 2016 and January 2017 at the Chongqing Optometry and Ophthalmology Hospital in Yuzhong District, Chongqing, China, from patients seeking refractive surgery with a minimal 2-year uneventful follow-up. All files were initially reviewed (J.B.) and considered normal. Patients had imaging performed on both eyes. Only scans with both QS (Quality Specification) reported as “OK” and minimal corneal coverage (both anterior and posterior corneal surfaces) ≥7.5 mm were accepted. Patients with previous refractive surgery, previous corneal surgery, or evidence of corneal ectasia were excluded. Additionally, any patient with ≥2 parameters >2 standard deviations from the norm were re-reviewed and personally evaluated (M.W.B.) and excluded if the scan likely represented a non-normal cornea.

The investigated indices included the following: Corneal diameter, Kmax, flat and steep simulated keratometry (K1, K2 respectively), ARTmax, progression indices minimum, maximum and average (PImin, PImax, PIavg), minimal corneal thickness, anterior and posterior elevation at the thinnest point [best-fit-sphere (BFS) 8.0 mm] Df and Db (anterior and posterior elevation difference between enhanced and standard reference surfaces), and the final “D” (final overall reading from the BAD display).

Corneal diameter as measured by the Pentacam HR is a horizontal white to white measurement (HWTW) and only obtainable in instruments with the additional coaxial camera (all “newer” HR versions have this capability). This is the same camera used for pupil size determination. The measurement is taken from the most horizontal Scheimpflug image and determined using edge detection image analysis. The HWTW measurement is only displayed when there is sufficient coverage and all internal machine quality checks are acceptable. Although there is normal variation between horizontal and vertical measurements, only the HWTW is measured as the vertical measurement is typically unreliable due to lid position and typically more limited vertical coverage.

STATISTICAL ANALYSIS

Data were collected and verified using Microsoft Excel 2019, and analyses were performed using GraphPad Prism 7.0 for Mac OS X. Averages of OD and OS were taken for each variable in every patient, where n is number of patients to ensure independence in the data analysis. The following tests were performed: calculation of the mean, standard deviation, 95% confidence interval, and unpaired student t test with P values for each investigated index in both Chinese and North American populations.

Regression analyses were performed for all investigated indices with respect to corneal diameter, yielding coefficients of determination with P values to evaluate the magnitude and statistical significance of association. The resulting regression slopes for each investigated index were automatically compared between the Chinese and North American patients, with corresponding P values to determine whether the slopes were significantly different, thus evaluating whether corneal diameter exerts a different influence on the BAD display parameters between Chinese and North American patients. Statistical significance was defined as P < 0.05.

The regression data were converted into standard scores (z scores) to allow for graphical comparison among various parameters with different units and scales. This was accomplished by subtracting the mean for each variable from an individual raw score and dividing the difference by the standard deviation. The resulting variables all have a mean of zero and a standard deviation of 1. This linear transformation was performed across Chinese and North American groups so they would be on the same scale and group differences would be preserved, while simultaneously allowing for comparison among various parameters.

RESULTS

100 North American patients had examinations performed in both eyes, met entrance criteria, and were analyzed. 51 Chinese patients had examinations performed in both eyes and met initial criteria. On further review, 2 patients were excluded: 1 for previous refractive surgery (ARTmax 184, final D 6.5, anterior elevation of the thinnest point −11) and 1 for representing a non-normal cornea (ARTmax 187, final D 3.1, minimal corneal thickness 448). Thus, 49 Chinese patients were analyzed. The North American patients averaged 33.0 ± 8.0 years with a range of 18 to 58 years. The Chinese patients averaged 30.1 ± 7.0 years with a range of 21 to 52 years.

Direct comparison of BAD display parameters between Chinese and North American patients revealed statistically significant differences in anterior elevation at the thinnest point (P < 0.01) and Df (P < 0.01). Additionally, Chinese and North American patients demonstrated statistically significant differences in corneal diameter (P < 0.01) (Table 1, Fig. 1 ).

TABLE 1
TABLE 1:
Comparison of Tomographic Parameters
FIGURE 1
FIGURE 1:
Graphs comparing tomographic parameters between Chinese and North American patients. Data represent mean ± 95% confidence interval with P values by unpaired student t test.
FIGURE 1 (Continued)
FIGURE 1 (Continued):
Graphs comparing tomographic parameters between Chinese and North American patients. Data represent mean ± 95% confidence interval with P values by unpaired student t test.

In the Chinese patients, regression analyses revealed statistically significant correlations between corneal diameter and all investigated indices, with the exception of minimal corneal thickness. In the North American patients, regression analyses revealed statistically significant correlations between corneal diameter and all investigated indices, with the exceptions of minimal corneal thickness and anterior elevation at the thinnest point (Table 2, Fig. 2   ).

TABLE 2
TABLE 2:
Regression Analyses With Respect to Corneal Diameter
FIGURE 2
FIGURE 2:
Regression analyses with respect to corneal diameter. y Axes represent z scores for the investigated indices in view. Each graph shows coefficients of determination with p values to evaluate the magnitude and statistical significance of association. The regression slopes for each investigated index were automatically compared between the Chinese and North American patients, with corresponding P values (far right in bold), to determine whether the slopes were significantly different between populations.
FIGURE 2 (Continued)
FIGURE 2 (Continued):
Regression analyses with respect to corneal diameter. y Axes represent z scores for the investigated indices in view. Each graph shows coefficients of determination with p values to evaluate the magnitude and statistical significance of association. The regression slopes for each investigated index were automatically compared between the Chinese and North American patients, with corresponding P values (far right in bold), to determine whether the slopes were significantly different between populations.
FIGURE 2 (Continued)
FIGURE 2 (Continued):
Regression analyses with respect to corneal diameter. y Axes represent z scores for the investigated indices in view. Each graph shows coefficients of determination with p values to evaluate the magnitude and statistical significance of association. The regression slopes for each investigated index were automatically compared between the Chinese and North American patients, with corresponding P values (far right in bold), to determine whether the slopes were significantly different between populations.
FIGURE 2 (Continued)
FIGURE 2 (Continued):
Regression analyses with respect to corneal diameter. y Axes represent z scores for the investigated indices in view. Each graph shows coefficients of determination with p values to evaluate the magnitude and statistical significance of association. The regression slopes for each investigated index were automatically compared between the Chinese and North American patients, with corresponding P values (far right in bold), to determine whether the slopes were significantly different between populations.

The regression slopes for each investigated index were automatically compared between the Chinese and North American patients, with corresponding P values (far right column in Table 2 and far right box in Fig. 2   ), to determine if the slopes were significantly different between populations, thus evaluating if corneal diameter exerts a different influence on the BAD display parameters between Chinese and North American patients.

Comparison of the regression slopes between Chinese and North American patients revealed a statistically significant difference in slope for the influence of corneal diameter on ARTmax (P = 0.04) and was nearly significant for final D (P = 0.06). Thus, although corneal diameter is correlated with most indices in both populations, corneal diameter exerts a more profound influence on ARTmax (and likely on final D) in the Chinese compared with the North Americans (Table 2 and Fig. 2   ).

When graphically comparing the various parameters represented as standard scores (z scores), it becomes visually apparent that corneal diameter has the most profound effect on the pachymetric progression parameters and final D (Table 2 and Fig. 2   ).

DISCUSSION

The purpose of our study was to determine whether the observed anatomical differences between Asian and white eyes resulted in different normal values for multiple tomographic parameters and whether these differences reached clinical significance when analyzed by the BAD display. Additionally, our second objective was to determine the influence of corneal diameter on the individual parameters and the final “D” from the BAD display. Although the current versions of the Pentacam are capable of measuring corneal diameter, the current BAD display does not incorporate corneal diameter in its analysis.

Multiple pachymetric progression parameters are presented on the BAD display (ARTmax, PImin, PIavg, PImax) and the pachymetric progression is shown graphically in both the Corneal Thickness Spatial Profile and Percent Thickness Increase graphs. Each of these represents the rate of change in corneal thickness from the thinnest point to the periphery. Eyes with ectatic disease commonly show a more rapid change.12,13 It would be somewhat reasonable to assume that eyes with a smaller corneal diameter may have a higher rate of change, as the overall distance between the thinnest point and periphery is less.14 We therefore hypothesized that corneal diameter influences the pachymetric progression parameters more profoundly in Chinese patients (with smaller corneas) compared with North American patients (with larger corneas).

Consistent with previous work,10 Chinese patients in the study had smaller corneal diameters than North American patients (P < 0.01). In both populations, corneal diameter had the highest magnitude and statistical significance of correlation with the pachymetric progression parameters and final D. This makes sense, as the pachymetric progression parameters are highly weighed in the computation of the final D. Corneal diameter exerted a more profound influence (steeper slope Fig. 2   ) on ARTmax (P = 0.04) and on final D (P = 0.06) in the Chinese compared with the North Americans. Additionally, at the lower values for corneal diameter, there is a lower ARTmax and a higher final D in the Chinese study set (Fig. 2   ).

Although previous published studies looked at the variations in tomographic parameters in different geographic and ethnic populations, no previous work examined the effect these variations could have on the BAD display. One study showed that anterior chamber depth did not vary significantly by country, except for being smaller in a New Zealand population.7 Previous work looking at international variations for anterior and posterior elevation was clinically insignificant but did show that country-specific parameters for these variables may better reflect the test populations in China, Egypt, and India. Additionally, data reported for anterior and posterior elevation at the thinnest point in the Chinese are very similar to our findings.8 A study looking at several measures of central pachymetry of importance to refractive surgeons showed that international differences for minimal corneal thickness were clinically insignificant. The authors still found that it remains preferable to establish normative values in ethnic groups where possible. Mean minimal corneal thickness was 546 μm in the United States (vs 548 μm in North Americans in our study) and 525 μm in the Chinese (vs 545 μm in the Chinese in our study).9 A study comparing Asian and white anterior segment dimensions measured by OCT found that Asian eyes had smaller anterior segments compared with whites. Corneal diameter was found to be significantly narrower in Asians versus whites (0.5 mm narrower vs 0.2 mm narrower in our study). However, corneal diameter measurements for both populations were >1 mm wider in this study compared with our data.10 A study of Chinese and Caucasian ocular tomography suggested that anatomical differences between populations should be considered in soft lens design.15 A study looking at OCT corneal thickness maps in normal Chinese schoolchildren found that corneal thickness increases gradually from the center to the periphery and that central corneal thickness was associated with corneal curvature radius, but not with sex, age, axial length, or refraction.16 Our findings were consistent with the existing literature on the variations in tomographic parameters in different geographic and ethnic populations; however, there was a paucity of data for how these variations affect the BAD display.

The changes in pachymetric parameters between North American and Chinese were somewhat anticipated, and the highly significant differences in anterior elevation parameters (anterior elevation at the thinnest point and Df) were not. Although corneal diameter affects both populations (North American & Chinese) and most profoundly on the pachymetric progression, corneal diameter has a more influential effect in the Chinese study population. Although North Americans with smaller corneal diameters have similar finding to the Chinese, the proportion of Chinese individuals with smaller HWTW is significant.

This small pilot study suggests that incorporating corneal diameter as an additional variable may make the BAD display more universally applicable. The differences in anterior elevation parameters, however, would also suggest that ethnic/geographic specific normative values for the BAD display regression formulas may be beneficial for pre-operative evaluation of refractive surgical candidates. Currently, many Chinese refractive surgical candidates may be excluded if North American based screening parameters are applied to the Chinese population. Incorporating both corneal diameter and race/geography may make the BAD display more universally applicable.

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Keywords:

anterior elevation; Asian and white; Belin/ambrosio enhanced ectasia display (BAD display); corneal diameter; pachymetric progression and final D

Copyright © 2020 Asia-Pacific Academy of Ophthalmology. Published by Wolters Kluwer Health, Inc. on behalf of the Asia-Pacific Academy of Ophthalmology.