Comparison of Central Corneal Thickness Measurements by Ultrasound Pachymetry, Konan Noncontact Optical Pachymetry, and Orbscan Pachymetry

Comparison of central corneal thickness: ultrasound pachymetry versus slit-lamp optical coherence tomography, specular microscopy, and Orbscan

Background

Central corneal thickness (CCT) can be measured by using contact and non-contact methods. Ultrasound pachymetry (US pachymetry) is a contact method for measuring CCT and is perhaps the most commonly used method. However, non-contact methods like scanning slit topography (Orbscan II), slit-lamp optical coherence tomography (SL-OCT), and specular microscopy are also used. Not many studies have correlated the measurement of CCT with all four modalities. The purpose of this study was to compare and correlate the CCT measurements obtained by US pachymetry with SL-OCT, specular microscopy, and Orbscan.

Method

This is a prospective, comparative study done in an institutional setting. Thirty-two eyes of 32 subjects with no known ocular disease and best-corrected visual acuity of 20/20 were enrolled. CCT measurements were obtained using SL-OCT, specular microscopy, scanning slit topography (Orbscan), and US pachymetry. Three measurements were made with each instrument by the same operator. Mean, standard deviation, and coefficient of variation were calculated for CCT measurements acquired by the four measurement devices. Bland–Altman plot was constructed to determine the agreements between the CCT measurements obtained by different equipment.

Results

The mean CCT was 548.16±48.68 μm by US pachymetry. In comparison, CCT averaged 546.36±44.17 μm by SL-OCT, 557.61±49.92 μm by specular microscopy, and 551.03±48.96 μm by Orbscan for all subjects. Measurements by the various modalities were strongly correlated. Correlations (r²) of CCT, as measured by US pachymetry compared with other modalities, were: SL-OCT (r²=0.98, P<0.0001), specular microscopy (r²=0.98, P<0.0001), and Orbscan (r²=0.96, P<0.0001). All modalities had a linear correlation with US pachymetry measurements.

Conclusion

In subjects with healthy corneas, SL-OCT, specular microscopy, and Orbscan (with correction factor) can be used interchangeably with US pachymetry in certain clinical settings. The four modalities showed significant linear correlations with one another.

Repeatability of pachymetric mapping using fourier domain optical coherence tomography in corneas with opacities

PURPOSE

To evaluate the repeatability of Fourier domain optical coherence tomography (OCT) pachymetric mapping in patients with corneal opacities and to assess the reliability of Fourier domain OCT with 830 nm wavelength as a pachymetric measurement tool in opaque corneas.

METHODS

A Fourier domain OCT system was used to map the corneal thickness of patients with corneal scars or dystrophy. A retrospective study of a consecutive series was conducted. The repeatability was measured using pooled standard deviation of repeated measurements. A slit-scanning tomography device provided pachymetric mapping for comparison.

RESULTS

Seventeen eyes of 12 patients with corneal scars (7 trauma and 3 post infection) or dystrophy (2 Reis-Bucklers and 5 granular dystrophy) were included. The posterior corneal boundary was detectable in all cases. The average corneal thickness measured by OCT was 536 ± 89 μm in central 2 mm area, 553 ± 76 μm in pericentral 2- to 5-mm area, and 508 ± 93 μm for the minimum corneal thickness. The slit-scanning tomography central corneal thickness, 433 ± 111 μm, was significantly lower than OCT readings (mean difference -91.1 ± 33.3 μm, P = 0.002). Repeatability of the OCT measurements was 2.1 μm centrally and 1.2 μm pericentrally.

CONCLUSION

Pachymetric mapping with Fourier domain OCT was highly repeatable. Fourier domain OCT is a reliable pachymetric tool in opaque corneas. In comparison, corneal thickness measured by the slit-scanning tomography is significantly thinner than those measured by the Fourier domain OCT in the presence of corneal opacities.

Central corneal thickness in patients with systemic sclerosis: a controlled study

PURPOSE

To compare the central corneal thickness (CCT) of patients with systemic sclerosis (SSc) and control subjects.

METHODS

The study group comprised 37 consecutive patients with SSc, and the control group comprised 23 healthy individuals similar in age and sex. CCT value was measured by ultrasound pachymetry.

RESULTS

In the SSc group, the mean CCT in the right eye was 534.9 ± 33.5 μm and 536.9 ± 32.4 μm in the left eye. In the control group, the mean CCT was 533.0 ± 32.9 μm in the right eye and 533.1 ± 33.6 μm in the left eye. The mean CCT was not significantly different in the SSc group compared with the control group for both the right (P = 0.83) and left (P = 0.67) eyes.

CONCLUSIONS

CCT measurements do not significantly differ in patients with SSc compared with healthy control subjects.

Comparison of corneal thickness measurements using Galilei, HR Pentacam, and ultrasound

PURPOSE

To assess the degrees of agreement in measuring corneal thickness in normal eyes between 2 noncontact systems based on the dual Scheimpflug system (Galilei; Ziemer) and rotating Scheimpflug imaging (HR Pentacam; Oculus) and also between each of these methods and the gold-standard method of ultrasound pachymetry.

METHODS

In a prospective study, measurement agreement was assessed in 47 eyes of 47 healthy subjects. All eyes were examined with each of the 3 devices. Measurements made with the Galilei and HR Pentacam were compared with those made with ultrasound. The central corneal thickness (CCT) and thinnest pachymetry of the Galilei and HR Pentacam were also compared.

RESULTS

The mean values of CCT obtained from Galilei, HR Pentacam, and ultrasound were 560.57 ± 29.10, 542.31 ± 30.50, and 548.61 ± 29.92 μm, respectively. The 95% limits of agreement of each of these devices with ultrasound were -11.03 and +34.94 μm for the Galilei and -33.67 and +20.74 μm for the Pentacam. The 95% limits of agreement of Galilei with Pentacam in measurement of central and thinnest corneal thickness were -3.39 and +33.11 and -10.40 and +27.29 μm, respectively. Although CCT measurements made with the HR Pentacam were thinner (P < 0.001), and those obtained with Galilei were thicker (P < 0.001) than those made with ultrasound, there were significant correlation between Galilei and ultrasound (r = 0.92; P < 0.0001) and between HR Pentacam and ultrasound (r = 0.89; P < 0.0001). The central and thinnest corneal thickness measurements made with the HR Pentacam were thinner (P < 0.0001) than those made with Galilei, but there was a significant correlation between Galilei and HR Pentacam in measuring CCT (r = 0.96; P < 0.0001) and thinnest corneal thickness (r = 0.95; P < 0.0001). Bland-Altman plots showed good agreement between these methods.

CONCLUSIONS

Although differences between the devices were statistically significant, there was good correlation and agreement between Galilei and Pentacam in measuring central and thinnest corneal thickness. The corneal thickness measurements made with the HR Pentacam and Galilei also showed good correlation and agreement with those made with ultrasound.