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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 109  |  Issue : 1  |  Page : 41-46

Diagnosis of keratoconus with optical coherence tomography


Department of Ophthalmology, Sohag Faculty of Medicine, Sohag University, Sohag, Egypt

Date of Submission20-Nov-2015
Date of Acceptance04-Feb-2016
Date of Web Publication21-Oct-2016

Correspondence Address:
Mortada A Abozaid
Department of Ophthalmology, Faculty of Medicine, Sohag University, Sohag, 82155
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2090-0686.192741

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  Abstract 

Purpose
The aim of the study was to assess the accuracy of optical coherence tomography (OCT) (3D OCT-2000) in diagnosing keratoconus by measuring the central corneal thickness and the central curvature radius.
Patients and methods
In this observational study, 50 patients with keratoconus underwent corneal topography, ultrasound pachymetry, Scheimpflug imaging, and anterior-segment optical coherence tomography (AS-OCT).
Results
The mean corneal power measured by AS-OCT was 51.65 ± 0.78 D, that measured with corneal topography was 50.19 ± 0.64 D, and that with the Sirius Scheimpflug camera was 50.78 ± 0.82 D. The mean central corneal thickness measured by OCT was 486 ± 73 μm, that measured by ultrasound was 475 ± 49 μm, and that using the Sirius Scheimpflug camera was 481 ± 66 μm.
Conclusion
3D OCT-2000 may be a useful alternative for measuring the anterior corneal power and the central corneal thickness in keratoconic eyes.

Keywords: anterior segment OCT, keratoconus diagnosis


How to cite this article:
Abozaid MA, Mohammed AM. Diagnosis of keratoconus with optical coherence tomography. J Egypt Ophthalmol Soc 2016;109:41-6

How to cite this URL:
Abozaid MA, Mohammed AM. Diagnosis of keratoconus with optical coherence tomography. J Egypt Ophthalmol Soc [serial online] 2016 [cited 2022 Dec 1];109:41-6. Available from: http://www.jeos.eg.net/text.asp?2016/109/1/41/192741


  Introduction Top


Keratoconus is a bilateral, asymmetric, chronic disease of the eye caused by weakening of the cornea with a prevalence of one per 2000 in the population.[1] It is characterized by progressive thinning and steepening of the cornea, resulting in a cone-shaped cornea, which leads to increased astigmatism and high-order aberrations [2] and a loss of visual quality [3].

In the early stages of the disease, the use of spectacles or contact lenses might provide sufficiently functional visual quality to the patient.[4] However, the progressive corneal thinning and steepening usually lead to the need for corneal transplant in advanced stages [5],[6].

The diagnosis of advanced keratoconus is not complicated because of the typical biomicroscopic and topographic findings, but the detection of subclinical or forme fruste cases may impose difficulty [7].

It is particularly important to detect the disease among refractive surgery candidates, because keratorefractive procedures may worsen their condition [8].

Optical coherence tomography (OCT) has become a widespread tool in various fields of medicine [9],[10], particularly in ophthalmology, in which its high resolution and noninvasiveness have allowed multiple applications in the retina and anterior segment of the eye [11],[12],[13],[14],[15],[16],[17].

Although corneal topography is the standard method for early diagnosis and therapeutic control assessment, it has significant limitations, mainly because it requires a regular corneal surface and does not provide three-dimensional information regarding depth of the cornea and of the entire anterior segment. Other imaging methods, such as ultrasound biomicroscopy or Scheimpflug camera systems, were developed to assess quantitative parameters, including anterior chamber depth and corneal thickness [18],[19],[20].

OCT of the anterior segment was clinically validated in various conditions of the cornea and anterior eye segment and was recently optimized for imaging of the entire anterior segment, including parts of the lens and the chamber angle [19],[20],[21],[22].

In this study the accuracy of OCT (3D OCT-2000) in diagnosing keratoconus was evaluated by measuring the central corneal thickness and the central curvature radius.


  Patients and methods Top


Fifty eyes of fifty patients with keratoconus (27 men and 23 women) were recruited for this observational study from Sohag University Hospital. This study followed the principles laid down in the Declaration of Helsinki, had an institutional ethics committee approval, and written informed consent from all patients.

The inclusion criteria for this study were age 18 years or older, at least one clinical sign of keratoconus, including slit-lamp findings of Munson's sign, hydrops, Vogt's striae, Fleischer's ring, apical scar, apical thinning, or Rizutti's sign, and topographic evidence of keratoconus.

The exclusion criteria for this study included signs or history of other corneal disease (e.g. corneal opacities or dystrophies), having undergone previous corneal surgery (e.g. corneal cross-linking), and use of contact lenses.

A commercially available OCT (3D OCT-2000; Topcon, Oakland, NJ, USA) was used in the study. For retinal imaging, the system has an axial resolution in tissue of 5 μm. To image the cornea, a corneal adaptor module was attached to the retinal scanner. The axial resolution for corneal imaging was also 5 μm because it is determined by the coherence length of the light source.

During the scanning, the room lights were on. The patients were asked to look straight ahead and fixate on the green light that serves as the internal fixation target of the OCT system. The light is coaxial with the optical axis of the OCT system and in this study was projected through the corneal adaptor lens. It appeared to the eye being imaged as a green cross-light straight ahead inside the adaptor lens. The corneal mapping pattern was repeated three times for each eye during the same visit.

On each meridional scan, the anterior corneal power was calculated as Ka = (n1 − 1)/Ra, where n1 is the refractive index of the cornea (1.376) and Ra is the anterior radius of curvature within a central 3.0-mm area. The posterior corneal power was calculated as Kp = (n2 − n1)/Rp, where n2 is the refractive index of aqueous (1.333) and Rp is the posterior radius of curvature within a central 3.0-mm area. The net corneal power was calculated as K = Ka+KpD × Ka × Kp/n1, where D is the central corneal thickness (CCT). The overall anterior, posterior, and net powers of the cornea were obtained by averaging over all meridians.

The corneal powers measured by OCT were compared with those of standard keratometry. The simulated K value from Placido ring topography and the Sirius Scheimpflug system was used as the standard ([Figure 1],[Figure 2],[Figure 3]).
Figure 1: Corneal topography of one patient with early keratoconus (Note the Sim K readings of 46.98 and 48.66 D. This patient had an ultrasonic pachymetry of 468 μm.).

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Figure 2: Scheimpflug image of the same patient in [Figure 1] 1(Note the Sim K reading 47.18 and 49.26 D and central corneal thickness of 463 μm with thinnest location of 454 μm.).

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Figure 3: Anterior-segment optical coherence tomography of the same patient in [Figure 1] and [Figure 2] (Note the K readings of 47.06 and 50.88 D and the central corneal thickness of 469 μm.). Chart 1 Mean values of the corneal power as measured by anterior-segment optical coherence tomography (AS-OCT), corneal topography, and Scheimpflug camera. Chart 2 Mean values of the central corneal thickness as measured by anterior-segment optical coherence tomography (AS-OCT), ultrasonic pachymetry, and Scheimpflug camera.

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The CCT measured by OCT was compared with that measured with ultrasound and the Sirius Scheimpflug system.

Statistical analysis

Patient's data were recorded in data collection sheets. Statistical analysis was performed using commercially available software (SPSS for Windows; SPSS Inc., Chicago, Illinois, USA). Quantitative data were presented as mean, SD, and confidence interval.

The paired t-test was used to evaluate the agreement between OCT measurements and the standard measurements. P values less than 0.05 were considered statistically significant.


  Results Top


This study included 50 keratoconic eyes of 50 patients: 11 eyes had apical scars, six eyes had Vogt striae, and the remainder had a clear cornea.

The mean corneal power measured by OCT (mean: 51.65 ± 0.78 D) was equivalent to standard K values measured by corneal topography (mean: 50.19 ± 0.64 D) and the Sirius Scheimpflug camera (50.78 ± 0.82 D), as shown in [Table 1] and [Figure 4].
Table 1: Mean values of the corneal power as measured by anterior-segment optical coherence tomography, corneal topography, and Scheimpflug camera

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Figure 4: Mean values of the corneal power as measured by anterior-segment optical coherence tomography (AS-OCT), corneal topography, and Scheimpflug camera.

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Also the mean CCT measured by OCT (486 ± 73 μm) was similar to that measured by ultrasound (475 ± 49 μm) and the Sirius Scheimpflug camera (481 ± 66 μm), as shown in [Table 2] and [Figure 5].
Table 2: Mean values of the central corneal thickness as measured by anterior-segment optical coherence tomography, ultrasonic pachymetry, and Scheimpflug camera

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Figure 5: Mean values of the central corneal thickness as measured by anterior-segment optical coherence tomography (AS-OCT), ultrasonic pachymetry, and Scheimpflug camera.

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  Discussion Top


OCT is not the only instrument that can directly measure the curvatures of both the anterior and posterior corneal surfaces. Slit-scanning instruments such as the Orbscan II, Pentacam, and the Galilei dual Scheimpflug camera (Ziemer Group) also have that capability. However, the OCT system is faster, with higher resolution.

According to the Munnerlyn formula [23] a 10-μm axial sag error within the central 3.0-mm diameter could produce an anterior corneal power calculation error of ∼3.0 D. This was the primary reason for the poor repeatability of corneal power measurements by the lower-speed time-domain OCT system [24].

Anterior-segment optical coherence tomography (AS-OCT) is fast enough to obtain direct corneal power measurements with acceptable repeatability. Because the technology does not rely on an assumed fixed geometric relationship between the anterior surface and the posterior surface, it may be a more robust method for measuring corneal power in surgically modified eyes (e.g. post-LASIK) and pathologically distorted eyes (e.g. keratoconus).

Because the OCT can directly measure both anterior and posterior corneal curvatures, it may be a more valid instrument for use in keratoconic eyes, which have lower than normal anterior–posterior curvature ratios. In addition OCT combined with videokeratography may be more useful for differentiating mild forms of keratoconus than videokeratography alone [25].

Standard keratometry measures the slope of the cornea over an annular area (or a portion of) centered on the vertex, whereas OCT measures the curvature of the cornea over a circular area centered on the pupil. These two methods should yield the same results if the cornea is perfectly spherical; however, any deviation from sphericity could produce measurement differences.

This study compared the accuracy of 3D OCT-2000 and standard keratometry in measuring the anterior corneal shape in keratoconic eyes. The corneal power measurements by the two methods agreed well on the average (P = 0.03). There was still a considerable range of differences between the two methods for individual eyes, which might be due to some basic differences between the two methods.

At a reasonable working distance, the 5-μm resolution of the OCT device is much higher than that possible with slit scanning. Thus, the OCT system gives more accurate corneal thickness measurements than the Orbscan II in the presence of corneal haze or opacity [26].

Corneal thickness obtained from the OCT technique is based on the refractive index (RI) of the interface. For normal corneas, the RI of corneal stromal layers is assumed to be constant. However, for keratoconic eyes, posterior displacement of Bowman's membrane or stromal scarring leads to abnormal reflectivity in OCT images [27].

Wang et al. [28] found good correlation between the CCT values obtained from RTVue-OCT and Visante-OCT in keratoconic eyes. However, small systematic differences were observed between the two OCT systems and therefore data from the two instruments cannot be used interchangeably [28].

In the current study, a high correlation was noted between OCT and both ultrasonic pachymetry and the Scheimpflug system in CCT measurements (P = 0.01). The measurement is taken from the front and back boundary curves of the cornea on a delineated cross-sectional image.

Unlike the OCT, the ultrasound contact probe displaces the tear film and compresses the corneal surface, and the exact location of the posterior corneal reflection is unknown; it could possibly be between the Descemet membrane and the anterior chamber [29]. In addition, misplacement of the probe might result in considerable measurement errors, although five measurements were taken and averaged in ultrasonic pachymetry.

Regarding the comparison between the CCT measurements using the OCT and the Scheimpflug system, the two methods were closely correlated. Doors et al.[30] showed that Pentacam significantly overestimated CCT measurements compared with the Visante AS-OCT by a mean of 19.2 mm. Our result differs from theirs. However, different results were reported by Prospero Ponce et al.[31], who demonstrated that there was no significant difference in Pentacam and Visante AS-OCT with respect to CCT measurements.

Limitations of this study include the small number of cases, lack of classification of keratoconus cases according to severity, and lack of data regarding reliability of AS-OCT (3D OCT-2000) in imaging of keratoconus.

In conclusion, there was no statistically significant difference between the AS-OCT and the standard tests as regards the mean corneal power and the CCT in patients with keratoconus. Hence, 3D OCT-2000 may be a useful alternative for measuring the anterior corneal power and the CCT in these patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

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

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