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 Table of Contents  
ORIGINAL ARTICLE
Year : 2020  |  Volume : 21  |  Issue : 4  |  Page : 236-241

Analysis of corneal asphericity (Q value) and its related factors in adult Egyptians: a cross-sectional study


1 Department of Ophthalmology, Faculty of Medicine, Benha University, Benha, Egypt
2 Department of Ophthalmology, Zagazig Eye Hospital, Zagazig, Egypt

Date of Submission29-Apr-2020
Date of Decision12-Jun-2020
Date of Acceptance26-Jul-2020
Date of Web Publication28-Dec-2020

Correspondence Address:
MBBCh Islam Saeed
Department of Ophthalmology, Zagazig Eye Hospital, Almousallameyah Primary School Street, Almousallameyah, Zagazig 44839
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/DJO.DJO_39_20

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  Abstract 


Purpose The aim of this study was to determine the corneal Q value and its related factors in adult Egyptians and to compare the Q value of Egyptians with other ethnic groups.
Design This was a cross-sectional study.
Patients and methods The study included 500 eyes of 250 Egyptian patients. There were 130 female and 120 male patients, with a mean age of 29.96±9.31 years (range=20–60 years). The corneal mean Q value at 6.0- and 8.0-mm diameter together with Q value from each quadrant (nasal, temporal, superior, and inferior) and two meridians (horizontal and vertical) at 8.0-mm diameter were measured using Pentacam HR. Eyes were stratified by age, sex, and spherical equivalent (SE) refractive error. The mean radius of curvature (Rm) and maximum keratometry reading (K-max) of the anterior corneal surface, corneal thinnest location, and anterior chamber depth were measured.
Results The average Q values of the anterior surface were −0.27 (±0.125) and −0.37 (±0.141) at 6.0 and 8.0 mm diameter, respectively. The average Q value of each quadrant (nasal, temporal, inferior, and superior) and horizontal and vertical meridians was −0.55 (±0.245), −0.244 (±0.130), −0.33 (±0.189), −0.35 (±0.27), −0.40 (±0.144), and −0.34 (±0.178) at the 8-mm diameter, respectively. The average Q values of the anterior surface were significantly correlated with age, sex, SE refractive errors, maximum keratometry reading (K-max), mean radius of curvature (front), and anterior chamber depth. There was no significant correlation between the average Q values and corneal thickness at the thinnest location. The adult Egyptian Q values were close to African Americans but different from other ethnic groups.
Conclusion There was a significant correlation between Q value and age, sex, refractive error, K-max, mean radius of curvature (Rm), and anterior chamber depth. Corneal Q values of adult Egyptians were different from previous studies involving Europeans, white Americans, and Asian populations and was close to African Americans.

Keywords: corneal asphericity, Pentacam HR, Q value


How to cite this article:
Elsayed MR, Saeed I, Hamed AM, Fayk HM, Attia TN. Analysis of corneal asphericity (Q value) and its related factors in adult Egyptians: a cross-sectional study. Delta J Ophthalmol 2020;21:236-41

How to cite this URL:
Elsayed MR, Saeed I, Hamed AM, Fayk HM, Attia TN. Analysis of corneal asphericity (Q value) and its related factors in adult Egyptians: a cross-sectional study. Delta J Ophthalmol [serial online] 2020 [cited 2021 Apr 19];21:236-41. Available from: http://www.djo.eg.net/text.asp?2020/21/4/236/304943




  Introduction Top


The cornea is the main refractive component in a human eye, forming generally 70% of the total refractive power. Previous studies found that the cornea could be described as a quadric surface with surface asphericity [1]. The Q value, a quantified indicator of the aspherical degree, is described as the radial change from the center to the periphery of the quadric surface. As a key parameter of the mathematical model of the cornea, the Q value reflects the corneal shape and optical properties including spherical aberration, refractive power, etc. [2].

More recently, research has concentrated on the study of the corneal Q value and its distributions, as well as its influence on optical properties of the human eye [3]. The corneal Q value in the elderly population is an important factor in the design of intraocular lens and for the treatment of refractive errors [4].

Previous studies have demonstrated that the mean Q values range from −0.03 to −0.33. Values varied widely in part because of sample size, selection bias, limitations in technology accuracy, measurements confined to the horizontal meridian, and choice of equation to attain a best fit curve [5].

This study aimed at determining the corneal Q value of adult Egyptians and its distribution using Pentacam HR device and to compare the Q value of Egyptians with other ethnic groups. This may help in creating a more accurate model for the Egyptian eye and intraocular lens.


  Patients and methods Top


The study used a cross-sectional design. The protocol adhered to the tenets of the Declaration of Helsinki and was approved by the Institutional Review Board (IRB) of the Faculty of Medicine, Benha University, Benha, Egypt. A written informed consent was signed by each patient for participation in the study and for publication of data before being enrolled in the study.

A total of 250 patients (500 eyes) were sequentially selected from patients presenting for examination at Tebah Eye Center, Giza, Egypt. The study included males and females aged 20 years and older. Exclusion criteria were previous corneal disease or surgery.

Slit-lamp biomicroscopy and dilated fundus ophthalmoscopy were done for all participants to document clear corneas and absence of significant media opacities that can hinder fixation. Corneal topography was obtained using the Pentacam HR (Oculus, Wetzlar, Germany), which combines a slit illumination system and a rotating Scheimpflug camera. Corneal topography measurement was repeated twice for each eye of the 250 patients with good quality.

Topographic analysis was obtained for each eye. The mean Q values at 6.0 and 8.0 mm diameter together with the Q value from each quadrant (nasal, temporal, superior, and inferior) and horizontal and vertical meridians were measured at the 8 mm diameter. The mean radius of curvature (front), K-max (front), thinnest location, and anterior chamber depth were also measured.

The patients were divided into six groups based on their manifest refractive errors. The manifest refractive errors were classified based on the magnitude of spherical equivalent (SE) as follows:
  1. High hyperopia: SE more than or equal to +4.0 diopters (D).
  2. Moderate hyperopia: SE less than +4.0 D to more than or equal to +2.0 D.
  3. Low hyperopia: SE less than +2.0 D to more than or equal to +0.25 D.
  4. Low myopia: SE more than −3.0 D.
  5. Moderate myopia: SE less than −3.0 D to more than −6.0 D.
  6. High myopia SE less than or equal to −6.0 D.


Statistical analysis

The results were analyzed using the statistical package for the social sciences (SPSS 25 Software; IBM Corporation, Armonk, New York, USA). Normally distributed continuous data were expressed as mean±SD, and not normally distributed continuous data were expressed as median and range.

Categorical data were expressed as numbers and percentage. t test was used to compare between two groups, whereas one-way analysis of variance with its post-hoc analysis was used to compare between more than two groups. Not normally distributed variables were analyzed using Mann–Whitney U test. χ2 test was used to compare between the qualitative data. Spearman’s rank correlation coefficient was used to determine the association between the continuous variables. Results were considered statistically significant at a P value of less than 0.05.


  Results Top


The study included 500 eyes of 250 Egyptian patients with a mean age of 29.96±9.31 years (range=20–60 years). Age groups are presented in [Table 1]. Male patients represented 48%, whereas female patients represented 52% of the study population.
Table 1 Age distribution and mean Q values in different age groups

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The mean radius of curvature (Rm) of the anterior corneal surface was 7.77±0.263 mm. The mean value of the maximum keratometry reading (K-max) of the anterior corneal surface was 44.82±1.593 D. The mean value of the corneal thickness at the thinnest location was 534.9±30.8 μm, whereas the mean value of the anterior chamber depth was 3.13±0.315 mm, measured from the interior surface of the cornea.

The mean Q value of the anterior surface at 6 mm diameter was −0.27 (±0.125, range=−0.281 to −0.259, 95% confidence interval, [Figure 1]), whereas at 8-mm diameter, it was −0.37 (±0.141, range=−0.382 to −0.358, 95% confidence interval, [Figure 2]). The mean corneal Q values at the 8-mm diameter for each quadrant (nasal, temporal, inferior, and superior) and at horizontal and vertical meridians were −0.55 (±0.245), −0.244 (±0.130), −0.33 (±0.189), −0.35 (±0.27), −0.40 (±0.144), and −0.34 (±0.178), respectively ([Table 2]).
Figure 1 Mean Q value distribution among the population at 6-mm diameter (cornea front).

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Figure 2 Mean Q value distribution among the population at 8-mm diameter (cornea front).

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Table 2 Mean Q values in the four quadrants and at horizontal and vertical meridians at 8-mm and Q values at 6-mm diameter annulus (cornea front)

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The mean Q value of the anterior surface at 8.0-mm diameter was statistically significantly different (P<0.05) between male and female patients, with females having more prolate corneas than males ([Table 3]).
Table 3 Mean Q values for sex groups

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The distribution of the Q value in different age groups is summarized in [Table 1]. The mean Q values of the anterior surface at 6.0- and 8.0-mm diameters were statistically significantly different (P<0.05) across the different age groups. The corneal Q value of the anterior surface tended toward being less negative with increasing age (less prolate), which is more evident between the first two age groups (20–29 and 30–39 years), which represented the biggest number of patients included in the study (84.8%).

The distribution of Q value based on SE refractive error is shown in [Table 4]. The mean Q values of anterior corneal surface at 8-mm diameter were statistically significantly different among the different refractive groups where the cornea tends to be more oblate (less negative) with increasing hyperopia and to be more prolate (more negative) with increasing myopia.
Table 4 Mean Q values for different refractive error groups

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The mean corneal Q value at 6.0-mm diameter showed a weak positive correlation with K-max (cornea front) and a weak negative correlation with the mean radius of curvature (Rm) (cornea front), whereas the anterior chamber depth showed a weak positive correlation with the mean Q value at both 6.0- and 8.0-mm diameters. There was no statistically significant correlation between the Q value and corneal thickness at the thinnest location ([Table 5]).
Table 5 Mean Q value correlations

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  Discussion Top


In the present study, the corneal Q values of the anterior corneal surface at 6.0- and 8.0-mm diameters were −0.27 (±0.125) and −0.37 (±0.141), respectively. Previous studies has reported the corneal Q value at 6-mm diameter to be −0.29 in Chinese [6], −0.19 in Portuguese [7], −0.24 in Iranian [8], −0.22 in Germans [9], −0.20 in white Americans and −0.26 in African Americans [3]. The results of the current study were close to that of the study by Fuller and Alperin [3] in African Americans (−0.26). The difference in Q value of the present study from other previous studies may be owing to differences in testing equipment, sample size, patient age, and race.

The Pentacam HR system was the testing equipment in the present study. Only Fuller and Alperin [3] used the same instrument. The Bausch and Lomb Orbscan IIz system was used in a Chinese study by Xiong et al. [6]. Amorim-de-Sousa et al. [7] used the corneal video-keratoscope MODI 2.0 (CSO, Italy). All other studies used different methods.

The current study had a sample size of 500 eyes, which is the same as in the study by Yazdani et al. [8]. The study by Xiong et al. [6] had a sample size of more than 1000 eyes, whereas the study by Fuller and Alperin [3] enrolled 320 eyes, confirming the variations in sample size between studies.

In the present study, the mean age was 29.96 years, with 84.8% of cases being between 20 and 39 years. Thus, this study is more representative of the middle age adults. Similarly, the study by Yazdani et al. [8] had a mean age of 29.51 years, which is almost the same as ours [8]. On the contrary, the mean age was 53.64 years in the study by Xiong et al. [6], with 69.8% of the patients being older than 50 years. In the study by Fuller and Alperin [3], the mean age in the African-American group was 41.3 years and in the white group was 40.4 years.

The purpose of the present study was to determine the Q value of the Egyptian population. Amorim-de-Sousa et al. [7] reported the Q value of the Portuguese people and Yazdani et al. [8] studied the Iranian’s Q value. Fuller and Alperin [3] reported the Q values of white Americans and African Americans. Xiong et al. [6] studied the Q values of the Chinese population. The present study found that the mean Q value of the anterior surface was close to that of the study by Fuller and Alperin [3], being −0.26 in African Americans, which used the same device and at the same diameter of 6.0 mm.

The corneal Q value of the anterior surface tended toward being less negative with increasing age (less prolate), which was more evident between the first two age groups (20–29 and 30–39 years), having the biggest number of patients included in the study (84.8%). Fuller and Alperin [3] found a very weak correlation between age and the mean Q value, only reaching significance in the white group (P=0.01). On the contrary, Amorim-de-Sousa et al. [7] and Yazdani et al. [8] studies showed no significant correlation between age and corneal asphericity.

In the present study, a correlation between the corneal Q value and sex was found. Similar findings were observed by Xiong et al. [6] and Scholz et al. [9]. In the current study, the female group was found to have a higher negative mean Q value than that of the male group at 8.0-mm diameter, indicating that females have a more prolate anterior corneal surface than males. However, Amorim-de-Sousa et al. [7], Yazdani et al. [8], and Fuller and Alperin [3] found that the corneal Q values were not correlated with sex.

The mean Q values at 8.0-mm diameter were statistically significantly different among the different refractive groups, where the cornea tends to be more oblate (less negative) with increasing hyperopia and to be more prolate (more negative) with increasing myopia. Some studies indicated a lack of correlation with refractive errors [3],[10], whereas others indicated a decrease in Q value from myopia to hyperopia [10].

The present study found a weak correlation between corneal asphericity and K-max (anterior), mean radius of curvature (Rm) (anterior), and anterior chamber depth at 6.0-mm diameter.

There are some limitations in the current study. First, we did not include the patients who were younger than 20 years. Therefore, there were no analyses conducted on the differences of the Q value among children, or whether the Q value changes at the age of puberty while it is quite stable in adults. Second, the sample size was of a moderate size compared with previous studies, and further larger sample studies are recommended for high-evidence results. In addition, this epidemiological study is hampered by the fact that Egyptians are multi-ethnic, including whites (Arabs or Eurasians), Berbers (of North African descent), and Africans [11]. So, ethnic differences and comparison with other ethnic diversities may be biased.


  Conclusion Top


There was a significant correlation between asphericity and age, sex, refractive error, K-max, mean radius of curvature (Rm), and anterior chamber depth. This may provide a useful reference for designing visual optical products and future studies of the optical properties of the human eye. In addition, it was found that the corneal Q values of adult Egyptians were different from previous studies involving Europeans, White Americans, and Asian populations, whereas it was close to African Americans.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Zheng S, Ying J, Wang B, Xie Z, Huang X, Shi M. Three-dimensional model for human anterior corneal surface. J Biomed Opt 2013; 18:165–169.  Back to cited text no. 1
    
2.
Queirós A, Villa-Collar C, Jorge J, Gutiérrez ÁR, González-Méijome JM. Multi-aspheric description of the myopic cornea after different refractive treatments and its correlation with corneal higher order aberrations. J Optom 2012; 5:171–181.  Back to cited text no. 2
    
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Fuller DG, Alperin D. Variations in corneal asphericity (Q value) between African-Americans and whites. Optom Vis Sci 2013; 90:667–673.  Back to cited text no. 3
    
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Queirós A, Villa-Collar C, Gutiérrez ÁR, Jorge J, Ribeiro-Queirós MS, Peixoto-de-Matos SC et al. Anterior and posterior corneal elevation after orthokeratology and standard and customized LASIK surgery. Eye Contact Lens 2011; 37:354–358.  Back to cited text no. 4
    
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Amorim-de-Sousa A, Vieira AC, González-Méijome JM, Queirós A. Age-related variations in corneal asphericity and long-term changes. Eye Contact Lens 2019; 45:99–104.  Back to cited text no. 7
    
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Yazdani N, Shahkarami L, OstadiMoghaddam H, Ehsaei A. Topographic determination of corneal asphericity as a function of age, gender, and refractive error. Int Ophthalmol 2017; 37:807–812.  Back to cited text no. 8
    
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Scholz K, Messner A, Eppig T, Bruenner H, Langenbucher A. Topography-based assessment of anterior corneal curvature and asphericity as a function of age, sex, and refractive status. J Cataract Refract Surg 2009; 35:1046–1054.  Back to cited text no. 9
    
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Nieto-Bona A, Lorente-Velázquez A, Mòntes-Micó R. Relationship between anterior corneal asphericity and refractive variables. Graefes Arch Clin Exp Ophthalmol 2009; 247:815–820.  Back to cited text no. 10
    
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Manni F, Leonardi P, Barakat A, Rouba H, Heyer E, Klintschar M et al. Y-chromosome analysis in Egypt suggests a genetic regional continuity in Northeastern Africa. Hum Biol 2002; 74:645–658.  Back to cited text no. 11
    


    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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