Sudanese Journal of Ophthalmology

: 2020  |  Volume : 12  |  Issue : 1  |  Page : 7--11

Higher-order aberrations in keratoconus suspect

Engy Mohamed Mostafa1, Mariam Vector2, Ismail Moussa1, Mohamed Anber1,  
1 Department of Ophthalmology, Faculty of Medicine, Sohag University, Sohag, Egypt
2 Department of Ophthalmology, Hospital of Ophthalmology, Sohag, Egypt

Correspondence Address:
Prof. Engy Mohamed Mostafa
Department of Ophthalmology, Sohag University, Nasr City, Sohag 82524


Purpose: The aim of this study was to assess the pattern of higher-order aberrations (HOAs) in keratoconus suspect (KCS) eyes using Scheimpflug–Placido topography (Sirius, CSO, Italy) and also to detect the sensitivity and specificity of all parameters to discriminate subclinical keratoconus (KC), and normal eye was investigated. Patients and Methods: This was a retrospective, cross-sectional study of 100 eyes of KCS patients along with 50 normal eyes as a control group. The following parameters were evaluated: sphere, cylinder, spherical equivalent, flat and steep keratometry (K), mean K, total root mean square (RMS), RMS coma, RMS spherical aberration, and anterior and posterior corneal elevation in the most curved part of the cornea. All eyes underwent Sirius Scheimpflug–Placido topography evaluation. Results: The KCS group showed significantly higher values for cylinder, mean flat, steep, and mean keratometry (K) compared to the control group. There were statistically significant differences in all aberrometric parameters in KCS compared with the control group except for spherical aberration. The optimum cutoff point and area under receiver operating characteristic curve of both RMS and elevation (anterior and posterior) achieved larger than 90% in both sensitivity and specificity. Conclusion: HOAs and corneal elevation (anterior and posterior) of the most curved part of the cornea proved to be able to distinguish KCS from the normal control group. Higher amounts of vertical coma and larger values of coma-like RMS have been found in patients with suspect KC when compared to normal corneas.

How to cite this article:
Mostafa EM, Vector M, Moussa I, Anber M. Higher-order aberrations in keratoconus suspect.Sudanese J Ophthalmol 2020;12:7-11

How to cite this URL:
Mostafa EM, Vector M, Moussa I, Anber M. Higher-order aberrations in keratoconus suspect. Sudanese J Ophthalmol [serial online] 2020 [cited 2021 May 10 ];12:7-11
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Full Text


The detection of keratoconus (KC) is a major concern in the screening of refractive surgical patients since it is known that its presence weakens the corneal stroma and can lead to iatrogenic ectasia.[1] While clinical KC can be easily detected with corneal topography and slit-lamp examination; yet, definite parameters to detect KC in its earliest stages before the presence of slit-lamp findings are still highly debatable. Several terms have been put used to describe this condition; one of them is KC suspect (KCS). True KCS corneas are an absolute contraindication to laser in situ keratomileusis.

KCS is an unclarified entity with no specific definition and no consensual diagnostic criteria.[2] In particular, localized steepening on Placido corneal topography or slight bowing of the posterior corneal surface on elevation topography has been used to characterize KCSs.[3]

The boundary between “KCS” and “early KC” is that there is no scissoring reflex in the former contrary to the latter, but both show AB/SRAX.[4]

Eyes of KC patients have five to six times more higher-order aberrations (HOAs) than a healthy eye.[4],[5]

The Sirius system is a relatively new topographer that utilizes Scheimpflug–Placido technology to enable rapid acquisition and processing of images of the cornea and anterior chamber.[6]

Characteristically, KC demonstrates a significant coma, with the vertical coma showing more pronounced increase in subclinical KC than normal eyes.[7] However, using vertical coma alone cannot achieve high enough sensitivity and specificity to discriminate subclinical KC and normal eyes.[8]

Our study was conducted to detect the pattern of HOAs in KCS eyes that can enable objective identification of KC in its subclinical stage.

 Patients and Methods

This study is a retrospective, cross-sectional study of 100 eyes of patients who were seeking refractive surgery in Sohag ophthalmologic center and diagnosed as KCS. In addition, fifty normal controls (50 eyes) have been included as a control group. The study was approved by the ethical committee of the university and adhered to the tenants of the Declaration of Helsinki.

The term KCS was reserved for corneal topography with a normal-appearing cornea on slit-lamp biomicroscopy and at least one of the signs of KCS: steep keratometric curvature (>47.00 D), abnormal localized steepening or an asymmetric bow tie pattern, abnormal inferior minus superior keratometry (I-S) between 1.4 and 1.9, oblique cylinder >1.50 D, or clinical KC in the fellow eye.[9],[10],[11]

Exclusion criteria were any eye which was diagnosed as KC, 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 full ophthalmological examination, and images were acquired by Scheimpflug–Placido topography (Sirius CSO, Firenze, Italy) on all eyes. The scanning process acquires a series of 25 Scheimpflug images (meridians) and one 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 profile of anterior cornea, posterior cornea, anterior lens, and iris is derived. Anterior surface data from both Placido images and Scheimpflug images are merged using a proprietary method. All the other measurements for the internal structures (posterior corneal curvature, anterior lens surface, and iris) are derived solely from Scheimpflug data. Measurements were performed by a single experienced examiner. A 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: (1) sphere, (2) cylinder values, (3) spherical equivalent, (4) flat K, (5) steep K, (6) mean K, (7) total root mean square (RMS), (8) RMS coma, (9) RMS spherical aberration, (10) anterior corneal elevation, and posterior corneal elevation in the most curved part of the cornea.

Statistical analysis

Data were analyzed using STATA intercooled version 14.2 (Stata Corp, College Station, Texas). Quantitative data were represented as mean, standard deviation, median, and range. Data were analyzed using Student's t-test to compare the means of two groups. When the data were not normally distributed, the Mann–Whitney test was used. Qualitative data were presented as number and percentage and compared using the Chi-square test. Data were analyzed by sensitivity, specificity, positive predictive value, and negative predictive value derived from the area under the receiver operating characteristic (AUROC) curve. The diagnostic accuracy of different variables was expressed as the area under the receiver operating characteristic curve. The cutoff point of parameters is the value at which the highest sensitivity and specificity is achieved. Graphs were produced using Excel or STATA program. P value was considered significant if it was <0.05.


[Table 1] shows that there were statistically significant differences in all aberrometric parameters in KCS compared with the control group except for spherical aberration (all P < 0.0001 except spherical aberration P = 0.72).{Table 1}

[Table 2] shows that the mean anterior and posterior elevation of the most curved part of the cornea was significantly higher in the KCS group compared with the control group (P < 0.0001).{Table 2}

[Table 3] shows a parameter or model with a higher AUROC with higher sensitivity and specificity to discriminate KCS from normal eyes. AUROC of RMS HOAS in predicting KCS was found to be = 0.74, while for RMS COMA in KCS = 0.77, 0.52 for Spherical aberration, 0.81 for anterior elevation and 0.83 for posterior elevation. Thus, the RMS and elevation (anterior and posterior) parameters achieve larger than 90% in both sensitivity and specificity [Figure 1].{Table 3}{Figure 1}


Detection of KC is particularly important among patients considering refractive surgery in which the presence of KC and KCS would yield unsatisfactory results and cause postoperative complications as corneal ectasia. Difficulty in recognizing KC arises with very early or preclinical stages of the ectatic disorder.[12],[13]

The main characteristic of the ocular and corneal HOAs in keratoconic eyes has been reported to be increased coma, especially vertical coma.[14]

Total ocular aberrations (global aberrometry) are difficult if not impossible to measure accurately in highly aberrated eyes.[15] The Scheimpflug–Placido imaging technology (Sirius) is a relatively new advancement that has been reported to be highly reproducible and repeatable.[16],[17]

The purpose of our study was to assess HOAs in suspected KC using the Scheimpflug topography (Sirius CSO) that helps to distinguish the normal from suspect corneas.

Several authors have reported significant amounts of HOAs in this ectatic disease, especially vertical coma and coma-like aberrations. This tendency was also observed in our series. We have found significantly larger values of coma RMS and coma-like RMS in the group of keratoconic eyes.[18],[19] Gordon-Shaag et al.[20] reported that corneal HOAs were found to be significantly higher for keratoconic than normal eyes, but for KCS, the results were mixed.

In our study, we found that all aberrations were significantly higher for suspect KC than for normal eyes except RMS spherical aberrations. Coma-like aberrations in suspect wasfound to be 0.30 ± 0.18 and in normal eyes was 0.17 ± 0.08, but in spherical aberration in KCS was 0.14 ± 0.08 and in normal eyes was 0.13 ± 0.053.

Barbero et al.[21] found similarities in corneal and total patterns for aberrations in keratoconic eyes, especially for the early stages of this ectatic pathology.

Pineroet al.[22] have established corneal aberrometry as a potential diagnostic tool for diagnosing KC, especially coma-like aberrations.

de Sanctis et al.[23] found that posterior corneal elevation measured with the Pentacam rotating Scheimpflug camera is higher in eyes with KC or subclinical KC than in normal corneas and that posterior elevation is a useful index for discriminating this disease.

In our study, we found that posterior corneal elevation measured with the Sirius CSO is higher in suspect KC (19.98 ± 8.39) compared with the normal corneas (11.82 ± 3.5).

There were significant posterior elevation differences between normal eye and the eye fulfilled either of the Rabinowitz or Klyce–Maeda KC criteria. Posterior elevation of the cornea had a higher sensitivity and specificity to discriminate keratoconic eyes from normal eyes in earlier staged KC than later staged KC, based on the Amsler–Krumeich classification staging of KC.

Adding posterior corneal elevation data to an artificial intelligence algorithm would improve the sensitivity and specificity to separate normal eyes and subclinical keratoconic eyes.[24],[25]

Jafri et al.[7] reported that higher-order wavefront aberration along with a combination of topography and wavefront variables were effective for distinguishing between early and suspected KC.

Bühren et al.[19] reported that corneal tilt and coma values can be used to distinguish the control group from KC corneas with good accuracy (90.2% and 99.6%, respectively). Moreover, the cutoff values of these aberrations between the control group and the KC group were nearly similar to our study.

In our study, the cutoff value in RMS coma was >0.21 and RMS HOAS was >0.37 and the accuracy in RMS HOAS and coma was 70.50 and 72.00, respectively.

The difference between our study and other studies may be due to racial variation or different instrument which warrants further studies. Being able to elucidate KCS as early as possible using validated parameter values would help dramatically avoid drastic complications. We hope that finding definite values to differentiate between normal parameters and KCS would make it easier for ophthalmologist to scroll all parameters easily.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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