|Year : 2020 | Volume
| Issue : 4 | Page : 1150-1154
Characteristics of posterior corneal astigmatism in different stages of keratoconus
Abd E. E. Sarhan1, Marwa A Zaky1, Gehad A. F. Sakr2
1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
2 Department of Ophthalmology, Berkat Elsabaa Hospital, Menoufia, Egypt
|Date of Submission||18-Feb-2020|
|Date of Decision||18-Mar-2020|
|Date of Acceptance||28-Mar-2020|
|Date of Web Publication||24-Dec-2020|
Gehad A. F. Sakr
Berkat Elsabaa, Menofia
Source of Support: None, Conflict of Interest: None
The aim was to evaluate the magnitudes and axis orientation of anterior corneal astigmatism (ACA) and posterior corneal astigmatism (PCA) and the correlation between ACA and PCA in the different stages of keratoconus (KC).
PCA is an essential factor to be considered for accurate measurement of intraocular lens (IOL) power used in the treatment of cataract with KC.
Patients and methods
A retrospective case series study was conducted between August 2017 and July 2018 on a sample of 94 eyes with KC. Participants were divided into four subgroups according to Amsler–Krumeich classification. Pentacam was used to measure the magnitude and axis orientation of ACA and PCA. A correlation between ACA and PCA in the different stages of KC was also done.
The means of anterior and PCA were 3.5 ± 3.02 diopters (D) and 0.72 ± 0.53 D, respectively. With-the-rule astigmatism was the dominant type of astigmatism orientation at both anterior (80.9%) and posterior (87.2%) corneal surfaces, some were oblique, and there were no cases with against-the-rule astigmatism orientation. There was a strong correlation (P < 0.001) between ACA and PCA in the different stages of KC, and the correlation was lower in eyes with grade 2 (P = 0.002) and grade 3 (P = 0.01) KC.
The prevalence rates of with-the-rule orientation of the ACA and PCA were higher than those for oblique orientation, and there were no cases with against-the-rule astigmatism orientation. The corneal astigmatism values at the two surfaces were significantly associated with the severity of KC, and the ACA and PCA values were strongly correlated.
Keywords: corneal astigmatism, corneal elevation, keratoconus
|How to cite this article:|
Sarhan AE, Zaky MA, Sakr GA. Characteristics of posterior corneal astigmatism in different stages of keratoconus. Menoufia Med J 2020;33:1150-4
|How to cite this URL:|
Sarhan AE, Zaky MA, Sakr GA. Characteristics of posterior corneal astigmatism in different stages of keratoconus. Menoufia Med J [serial online] 2020 [cited 2021 Apr 19];33:1150-4. Available from: http://www.mmj.eg.net/text.asp?2020/33/4/1150/304508
| Introduction|| |
Keratoconus (KC) is an ectatic corneal disorder characterized by progressive corneal thinning that generates a corneal protrusion, irregular astigmatism, and decreased vision. This protrusion, which assumes a conical shape, is generated by degeneration of stromal tissue, leading to decrease in the mechanical strength of the corneal structure.
The onset of KC is common during the second decade of life with great variability in progression, which may last until the third or fourth decade of life. KC typically affects both eyes. Although the etiology is not fully understood, KC has traditionally been considered a noninflammatory disease. However, recent evidence suggests that proinflammatory factors are involved in KC pathogenesis, presenting some controversy about this issue.
The diagnosis of KC is typically made based on the combination of clinical signs, corneal topographic, and tomographic signs. The KC topographic patterns differ qualitatively and quantitatively from that of normal corneas.
Some biomicroscopic findings in KC, such as stromal thinning and Vogt's striae, suggest that the posterior surface of the cornea is altered independent of the anterior corneal shape. Although changes in the anterior curvature in KC have been frequently described in the literature, only limited information is available regarding the posterior corneal surface.
Corneal tomography has the ability to measure not only the anterior corneal surface but also the posterior surface, allowing a three-dimensional image of the cornea. This technology provides significantly more information than anterior surface topography, as tomography used data from anterior and posterior surfaces of the cornea, as well as pachymetric mapping. The Pentacam is an anterior segment tomography device, based on a rotating Scheimpflug camera.
Tomidokoro et al. found that both the anterior and posterior curvatures were affected in eyes with different stages of KC.
The aim of the study is to evaluate the magnitude and axis orientation of anterior corneal astigmatism (ACA) and posterior corneal astigmatism (PCA) and the correlation between ACA and PCA in the different stages of KC.
| Patients and Methods|| |
This was a retrospective case series study conducted on patients with KC who visited Tiba Eye Center Shebin Elkom Menoufia during the period from August 2017 to July 2018 and were assessed for corneal cross-linking, intracorneal ring implantation, or penetrating keratoplasty. The study included a sample of 94 eyes with KC. All the participants underwent corneal power measurement with a rotating Scheimpflug device (WaveLight, GmbH, Erlangen, Germany) after obtaining Menoufia Ethical Committee Approval. Inclusion criteria were first, patients with clinical KC (progressive decrease in vision commonly during second decade, irregular astigmatism and conical protrusion of the cornea with central thinning with the apex of the cone usually directed inferonasally), diagnosed with Pentacam; and second, patients with no visual dysfunctions other than KC.
History of previous ocular trauma or other corneal diseases, history of keratorefractive surgery, eyes with pellucid marginal degeneration, inferior corneal ectasia and thinning and inflammatory signs detected on slit-lamp biomicroscopy, and patients with pentacam images with inappropriate quality were the exclusion criteria of this study.
For each patient, the following data were collected: age, sex, eye side, visual acuity, manifest refraction by autorefractometer, intraocular pressure, anterior and posterior segment examination, stage of KC (according to Amsler–Krumeich classification), and data from Pentacam images, including corneal dioptric power in the flattest (K1) and steepest (K2) meridians, mean K, ACA, PCA, thinnest location (TL), TL-Y coordinate, pachy apex, anterior elevation, and posterior elevation.
The participants were divided into four KC stages according to the Amsler–Krumeich classification. The Amsler–Krumeich classification is based on astigmatism, myopia, keratometry, corneal transparency, and pachymetry. Stage 1 is characterized by eccentric steepening; myopia and induced astigmatism, both less than 5.00 D; and mean central K readings less than 48.00 D. Stage 2 is characterized by myopia and induced astigmatism, both greater than 5.00 D but less than 8.00 D; mean central K readings less than 53.00 D; absence of scarring; and minimal apical corneal thickness greater than 400 μm. Stage 3 is characterized by myopia and induced astigmatism, or both greater than 8.00 D but less than 10.00 D; mean central K readings greater than 53.00 D; absence of scarring; and minimal apical corneal thickness less than 400 μm but greater than 300 μm. Stage 4 is characterized by refraction not measurable, mean central K readings greater than 55.00 D, central corneal scarring, and minimal apical corneal thickness less than 300 μm.
The cornea was evaluated using a Scheimpflug imaging system that characterizes the anterior segment. The measurement was performed as follows: the patient was asked to place his/her chin on the chin rest of the Pentacam device, press the forehead against the forehead strap, and stare at a central target or fixation light; when the patient's eye and visual axis were aligned, the patient was asked to blink, and the image was captured. All measurements were based on data from a 3 mm diameter annular ring around the corneal apex.
ACA was classified as with-the-rule (WTR) when the steep meridian was within 60–120° and against-the-rule (ATR) when the steep meridian was within 0–30° or 150–180°. Otherwise, the remaining astigmatism was classified as oblique astigmatism. The diopteric power of the posterior corneal surface was negative, so the PCA was classified as WTR when the steep meridian was within 0–30 or 150–180° and as ATR when the steep meridian was within 60–120°. Otherwise, the remaining astigmatism was classified as oblique astigmatism, as described previously.
Data were collected, tabulated, and statistically analyzed using an IBM personal computer with SPSS software package version 22 (IBM Corp., Armonk, New York, USA).Qualitative data were presented in the form of numbers and percentages and quantitative data were presented in the form of mean, median, range (minimum and maximum), and SD. One-way analysis of variance test was used for comparison between more than two groups having quantitative variables and with independent parametric data. Kruskal–Wallis test was used for comparison between more than two groups having quantitative variables and with independent nonparametric data. Pearson correlation is used to study the correlation between normally distributed quantitative variable. P value at 0.05 was used to determine significance.
| Results|| |
A total of 94 eyes of 55 patients were included in our study (39 bilateral cases and 22 unilateral cases, 47 male and 47 female patients, and 49 eyes right and 45 eyes left) for which corneal topography with a rotating Scheimpflug device (WaveLight Allegro Oculyzer) was done [Table 1].
|Table 1: Distribution of the studied cases according to demographic data (n=94)|
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The results revealed a highly statistically significant increase in the magnitude of K1, K2, mean K, ACA, anterior elevation, and posterior elevation as the stages of KC progressed. The magnitude of PCA showed a statistically significant difference among the four groups.
According to axis orientation of ACA and PCA, the results revealed that the dominant type of astigmatism orientation was WTR at both anterior (80.9%) and posterior (87.2%) corneal surfaces, some were oblique, and there were no cases with ATR astigmatism orientation [Figure 1].
|Figure 1: Graphical representation shows the prevalence of with-the-rule, oblique, and against-the-rule astigmatism at the anterior and posterior corneal surfaces.|
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There was a highly statistically significant correlation between ACA and PCA [Figure 2] and anterior and posterior elevation in the different stages of KC. There was a statistically significant correlation between TL and PCA in the different stages of KC, and there was no significant correlation between TL-Y coordinate and PCA in the different stages of KC [Table 2].
|Figure 2: Graphical representation shows the correlations between the magnitudes of anterior corneal astigmatism and posterior corneal astigmatism in all patients.|
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|Table 2: Correlation coefficients between astigmatism and elevation values at both corneal surfaces in the different stages of keratoconus|
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| Discussion|| |
KC, the leading cause of corneal transplantation in developed countries, is an ectatic disorder that usually manifests at puberty when the cornea assumes a conical shape owing to a gradually progressive thinning of the corneal stroma. It is almost always a bilateral and asymmetrical condition that leads to irregular astigmatism and high myopia and, consequently, significant visual impairment.
It has been reported in some research that toric IOL implantation could be used to correct astigmatism in patients with KC and cataract. Recently, the importance of PCA has been recognized when toric IOL is considered, because selecting toric IOL based on anterior corneal measurements and neglecting PCA could lead to an incorrect estimation of total corneal astigmatism.
Mean PCA values in the normal population that could range from 0.26 to 0.78 D have been reported in some studies, but the few studies that have evaluated PCA in eyes with KC have reported mean values that are significantly higher than those in the normal population [Table 3]. Thus, the presence of PCA cannot be ignored when considering toric IOL implantation, particularly in eyes with KC, and the assumption of total corneal astigmatism instead of ACA may be helpful for selecting a more accurate toric IOL power, especially for KC.
|Table 3: Published values for mean posterior corneal astigmatism in patients with keratoconus|
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In our study, we evaluated the magnitude and axis orientation of ACA and PCA and the correlation between ACA and PCA in the different stages of KC.
In the present study, we found that the mean magnitudes for ACA and PCA in our study population were in line with the results of Aslani et al., who evaluated 161 patients with KC using Oculus Pentacam and reported that the mean magnitudes for ACA and PCA were 4.08 ± 2.21 and 0.86 ± 0.45 D, respectively.
Naderan et al. evaluated 1273 patients with KC using Pentacam images and reported that the mean ACA and PCA magnitudes of 4.49 ± 2.16 and 0.90 ± 0.43 D, respectively. Kamiya et al. evaluated ACA and PCA in 137 patients with KC using the Pentacam HR and reported mean respective magnitudes of 3.93 ± 2.73 and 0.93 ± 0.64 D. Orucoglu et al. also evaluated ACA and PCA in patients with KCN using the Pentacam and reported mean magnitudes of 3.05 ± 1.97 and 0.71 ± 0.44 D, respectively.
Our findings are similar to those of Aslani et al. regarding that the prevalence rate of WTR astigmatism was approximately the same at the anterior and posterior corneal surfaces. In contrast, Kamiya et al. found that WTR astigmatism was more prevalent at the anterior corneal surface (65.7%) and oblique astigmatism was more prevalent at the posterior corneal surface (78.8%). Our findings are also different from those of Naderan et al., who reported that ATR astigmatism was more prevalent at the anterior corneal surface (57.4%) and that WTR astigmatism was more common at the posterior corneal surface (63.2%) in their patients with KC.
Our results showed a high prevalence of WTR ACA and WTR PCA in different stages of KC. In contrast, Aslani et al. reported that there was a high prevalence of WTR ACA in eyes with early stage KC, which changed to ATR astigmatism in eyes with stage 3 or 4 KC, and there was a high prevalence of ATR PCA in stages 1 and 2, which changed to WTR astigmatism in eyes with stage 3 or 4 KC. Kamiya et al. reported a high prevalence of WTR ACA and ATR PCA in the different stages of KC, which gradually decreased with progression through the stages of KC, without any change in the most prevalent axis orientation.
Our findings regarding correlation between the magnitude of ACA and PCA were in accordance with the reports by Naderan et al., Kamiya et al., and Aslani et al., who reported significant correlation between them. We also found that our results again were consistent with the findings of Naderan et al., who reported a gradually significant increase in the magnitude of ACA and PCA in eyes with stages 1, 2, and 3 KC. In contrast, Aslani et al. and Kamiya et al. did not find any significant increase in ACA and PCA with the progressive stages of KC.
We found that there were no cases with ATR astigmatism orientation unlike the previous studies. Similar to our findings, Aslani et al. did not find any relationship between change in axis orientation and severity of KC. However, Aslani et al. reported a decrease in the prevalence of anterior and posterior oblique astigmatism as the severity of KC increased, and Naderan et al. reported a decrease in oblique astigmatism at both corneal surfaces also.
Aslani et al. reported that the ACA was more affected than the PCA as the severity of KC increases, and this was similar to our results.
Tomidokoro et al. reported that both the anterior and the posterior corneal curvatures were affected in KC and that these changes could be observed even in early stage KC, but they did not report in detail on whether the PCA changed with increasing disease severity.
| Conclusion|| |
The prevalence rates of WTR orientation of the ACA and PCA were higher than those for oblique orientation, and there were no cases with ATR astigmatism orientation. The corneal astigmatism values at the two surfaces were significantly associated with the severity of KC, and the ACA and PCA values were strongly correlated.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]