Marian Fathy, Mohammed H El Agooz, Alahmady H Samman and Engy M Mostafa*
Affiliation of authors: Ophthalmology Department, Sohag University, Sohag, Egypt
Received: 26 December, 2015; Accepted: 04 January, 2016; Published: 09 January, 2016
Engy Mohamed Mostafa, MD, PhD, Ophthalmology Department, Sohag University, Nasr City, Sohag, 82524, Egypt, +2/01005643587; E-mail:
Fathy M, El Agooz MH, Samman AH, Mostafa EM (2016) Anterior Segment Characteristics of Keratoconus Eyes Using Scheimpflug-Placido Topography. J Clin Res Ophthalmol 3: 003-007. 10.17352/2455-1414.000027
© 2015 Fathy M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Purpose: To assess anterior segment parameters of keratoconus (KC) eyes at different stages of the disease using Scheimpflug-placido topography (Sirius, CSO, Italy) in a sample of the southern Egyptian population.
Methods: retrospective study of 400eyes of different grades of keratoconus along with 50 normal eyes as a control group were assessed for the following parameters: corneal thickness at apex (CCT), thinnest corneal thickness (TCT), anterior chamber depth (ACD), corneal volume (CV), corneal keratometry, corneal asphericity (Q), corneal elevation (both anterior and posterior) with aspherotoric surface as a reference, total root mean square (RMS), RMS Coma, RMS spherical aberration (SA), RMS Astigmatism, , Baiocchi Calossi Versaci front index (BCVf) and BCV back index (BCVb). They underwent Sirius Schemipflug placid topographer evaluation.
Results: eyes were subdivided into: normal group (50 eyes), suspect KC (82eyes), grade 1 KC (80eyes), grade 2 KC (142eyes), and grade 3 KC (96eyes). The thinnest corneal thickness corneal thickness at apex and posterior corneal elevation were significantly different in all comparison groups. ACD values showed inconsistent differences between groups. Corneal volume was proved to be significant in comparing normal vs suspect KC group as well as in normal vs mild KC and normal vs moderate KC. Total RMS, RMS coma and BCV f and BCV b were significant for most groups.
Conclusion: Corneal thickness and high order aberrations proved to be able to diagnose different grades of keratoconus while corneal volume did not prove to have a significant role.
The increasing volume of patients interested in refractive surgery and the new treatment options available for keratoconus have generated a higher interest in achieving a better characterization of this pathology . Keratoconus is a bilateral non-inflammatory progressive disorder characterized by corneal ectasia and thinning [2-4]. Detecting moderate and advanced keratoconus is not difficult using corneal topography and biomicroscopic, retinoscopic, and pachymetric findings . Several indices have been proposed to help in the diagnosis of keratoconus and subclinical keratoconus with different topography systems [5-13]. They include quantitative descriptors such as the KISA% index proposed by Rabinowitz and Rasheed , the Keratoconus Prediction Index and Keratoconus Index proposed by Maeda et al. . Smolek and Klyce , developed a neural network classification based on corneal topography indices. Other detection schemes based on Zernike decomposition of the anterior corneal surface have been described by Schwiegerling et al. , and Langenbucher et al. . With the Scheimpflug imaging system, corneal elevation either anterior or posterior started to be more investigated and researches debated which of the corneal surfaces had higher sensitivity in detecting keratoconus [11,17-20]. Incorporating corneal thickness, corneal volume and corneal curvature using a Scheimpflug camera have been evaluated in several other articles [21,22].
The Sirius system is a relatively new Scheimpflug-placido topographer that enables rapid acquisition and processing of the cornea and anterior chamber .
Our study was conducted to define changes in the anterior segment of keratoconus eyes at different stages of the disease in a sample of the southern Egyptian population with special emphasis on detecting parameters that can enable objective identification of early keratoconus.
Patients and Methods
This study is a retrospective study of 500 patients (500 eyes) who were seeking refractive surgery in Sohag refractive center and diagnosed as clinical keratoconus between August 2014 and August 2015. In addition, fifty normal volunteer subjects (50 eyes) have been included as a control group. This study adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee, Sohag University Hospital, Egypt.
For analysis, keratoconus eyes have been classified into 4 subgroups according to Amsler-Krumeich classification . The term keratoconus suspect was coined for corneal topography with abnormal localized steepening or an asymmetric bow-tie pattern, a normal-appearing cornea on slitlamp biomicroscopy, and at least 1 of the following signs: steep keratometric curvature (>47.00 D), oblique cylinder greater than 1.50 D, central corneal thickness less than 500 mm, or clinical keratoconus in the fellow eye [12,13,17,25,26].
Eyes were considered normal if they had no ocular pathology, no previous ocular surgery, no significant refractive error, and no irregular corneal pattern. In this group, only 1 eye of each patient was evaluated (random sampling).
Exclusion criteria were previous ocular surgery, corneal scarring, trauma, pregnancy or lactation, glaucoma and causes of ocular astigmatism other than corneal i.e. lenticular astigmatism such as early cataract, lens subluxation or lenticonus.
All patients have been subjected to comprehensive eye examination and images were acquired by Scheimpflug placido topography (Sirius, Firenze, Italy) on all eyes with software version used 18.104.22.168 (Phoenix, Costruzione Strumenti Oftalmici). The scanning process acquires a series of 25 Scheimpflug images (meridians) and 1 Placido top view image. The ring edges are detected on the Placido image so that height, slope, and curvature data are calculated using the arc-step method with conic curves. From the Scheimpflug images, the profiles of anterior cornea, posterior cornea, anterior lens, and iris are derived. Anterior surface data from both Placido images and Scheimpflug images are merged using a proprietary method. All the other measurements for internal structures (posterior corneal curvature, anterior lens surface, and iris) are derived solely from Scheimpflug data. Previous studies have reported that the system’s pachymetric and shape measurements (curvature, eccentricity, elevation) have good repeatability [23,27]. Measurements were performed by a single experienced examiner (EM). Patient’s eye was aligned along the visual axis using a central fixation light. Patients were instructed to blink between shots to keep eyes moist. The examination which met excellent quality of the topographic and tomographic image, alignment and anterior and posterior coverage was saved. Eyes with scans not attainable, artificial tears were added to allow better acquisition.
In this study, the following parameters were evaluated: corneal thickness (CT) at the corneal apex, the thinnest corneal thickness (TCT), anterior chamber depth (ACD) defined as the distance from the corneal endothelium to the anterior surface of the lens capsule, corneal volume in a diameter of 9 mm, corneal volume (CV), keratometry (K) (both anterior and posterior as well as steep and flat keratometry), corneal asphericity (Q), corneal elevation (both anterior and posterior) with aspherotoric surface as a reference, total root mean square (RMS), RMS Coma, RMS spherical aberration (SA), RMS Astigmatism, Baiocchi Calossi Versaci front index (BCVf) and BCV back index (BCVb).
Schwiegerling et al.  and Li et al. , have shown that the following coefficients are the most relevant for keratoconus detection: vertical trefoil c3-3, vertical coma c3-1, horizontal coma c3+1, primary spherical aberration c40, and second order vertical coma c5-1. The BCVf, which is expressed in micrometers, was obtained by properly combining these coefficients (from the anterior corneal surface) and weighting them by a function of the coma axis. Likewise, a linear combination of c3-3, c3-1, c3+1, c4º, and c5-1 and information about the coma axis on the posterior Zernike decomposition were used to define the BCVb.
Data was analyzed using STATA intercooled version 9.2. Quantitative data was analyzed using ANOVA and post hoc Bonferroni test for comparison of the means of the five groups. When the data was not normally distributed Kruskal – Wallis test and Mann-Whitney test was used. Correlation analysis was performed by using Pearson’s correlation test. Qualitative data was compared using Chi square test. Comparisons were made between all groups individually to try to deduct which factors were significant between groups G1 compared normal vs. suspect , G2 compared normal vs. mild KC, G3 compared normal vs. moderate KC, G4 compared normal vs. severe KC, G5 compared suspect vs. mild KC, G6 compared suspect vs. moderate KC, G7 compared suspect vs. severe KC, G8 compared mild KC vs. moderate KC , G9 compared mild KC vs. severe KC, G10 compared moderate KC vs. severe KC. P value was considered significant if it was ≤ 0.05.
Demographic features of patients in normal eyes as control group along with the suspect KC and the other three groups of KC are stated in Table 1.
Table 2 and 3 shows anterior segment characteristics and corneal aberrations respectively. All parameters showed statistically significant differences between all groups (p<0.0001, one way ANOVA).
As expected the thinnest corneal thickness and corneal thickness at apex were significantly different in all comparison groups. Comparing suspect KC parameters against the mild KC parameters most of the values did not show any significance except for the anterior corneal power reading values (p=0.001).
Corneal volume was proved to be significant in comparing normal vs suspect KC group as well as in normal vs mild KC and normal vs moderate KC. Yet there was no correlation between steep keratometry and CV. ACD values showed inconsistent differences between groups. Posterior corneal elevation showed differences between all groups (Table 4).
Total RMS, RMS coma and BCV f and BCV b were significant for most groups except for the group 5 (Table 5).
Correlation of steep anterior keratometry showed significance with all parameters except corneal volume. Steep K revealed negative correlation with thinnest corneal point and thickness at apex which to be expected (Table 6).
There was strong positive correlation between anterior steep keratometry and all corneal aberrations (p=0.0001). The BCV parameter showed a positive correlation as well (Figure 1).
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