|
 
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| ORIGINAL ARTICLE |
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| Year : 2020 | Volume
: 32
| Issue : 4 | Page : 355-360 |
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Corneal and ocular residual astigmatism in school-age children
Samira Heydarian1, Sara Sardari2, Zahra Heidari3, Abbas Ali Yekta4, Hadi Ostadimoghaddam5, Mehdi Khabazkhoob6
1 Department of Rehabilitation Science, School of Allied Medical Sciences, Mazandaran University of Medical Sciences, Sari, Iran
2 Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran
3 Department of Rehabilitation Science, School of Allied Medical Sciences, Mazandaran University of Medical Sciences, Sari; Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran
4 Department of Optometry, School of Paramedical Sciences, Mashhad University of Medical Sciences, Mashhad, Iran
5 Refractive Errors Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
6 Department of Medical Surgical Nursing, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran, Iran
| Date of Submission | 12-Jan-2020 |
| Date of Decision | 08-Apr-2020 |
| Date of Acceptance | 21-Apr-2020 |
| Date of Web Publication | 12-Dec-2020 |
Correspondence Address:
Mehdi Khabazkhoob
Department of Medical Surgical Nursing, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran
Iran
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/JOCO.JOCO_8_20
Purpose: To determine the distribution of residual and corneal astigmatism (CA) in children aged 6–18 years and their relationship with age, sex, spherical equivalent, and biometric parameters.
Methods: In this cross-sectional study, multi-stage stratified cluster sampling was done to select students from Dezful, a city in Southwestern Iran. Examinations included the measurement of visual acuity with and without optical correction, refraction with and without cycloplegia, and biometry using the Biograph (Lenstar, Germany). The main outcomes in this report were corneal and residual astigmatism. The CA was measured by Biograph (difference between k1 and k2), and residual astigmatism was calculated using Alpine method. The power vector method was applied to analyze the data of astigmatism.
Results: Of 864 students that were selected, 683 (79.1%) participated in the study. The mean residual and CA were -0.84 diopter (D) and -0.85 D, respectively. According to the results of J0 and J45 vectors, residual astigmatism was -0.33 D and 0.04 D, and CA was 0.38 D and 0.01 D, respectively. With-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism were seen in 3.4%, 66.8%, and 4.5% of the children with residual astigmatism and 67.94%, 1.3%, and 1.5% of the children with CA. Residual astigmatism decreased with an increase in spherical refractive error, whereas CA increased with an increase in spherical refractive error.
Conclusion: The results of the present study showed a high prevalence and amount of residual astigmatism with ATR pattern among the 6–18-year-old population and the compensatory effect of this type of astigmatism on CA that mostly followed a WTR pattern.
Keywords: Biometry, Corneal astigmatism, Power vector analysis, Residual astigmatism
How to cite this article:
Heydarian S, Sardari S, Heidari Z, Yekta AA, Ostadimoghaddam H, Khabazkhoob M. Corneal and ocular residual astigmatism in school-age children. J Curr Ophthalmol 2020;32:355-60 |
How to cite this URL:
Heydarian S, Sardari S, Heidari Z, Yekta AA, Ostadimoghaddam H, Khabazkhoob M. Corneal and ocular residual astigmatism in school-age children. J Curr Ophthalmol [serial online] 2020 [cited 2021 Jan 20];32:355-60. Available from: http://www.jcurrophthalmol.org/text.asp?2020/32/4/355/303210 |
| Introduction | |  |
Astigmatism is one of the most common refractive errors worldwide, in which the anterior corneal surface has a major role.[1],[2] However, the magnitude and orientation of refractive astigmatism may not match those of corneal astigmatism (CA), and their vectorial difference is known as residual astigmatism.[3],[4],[5] Ocular residual astigmatism (ORA) may increase aberrations in the visual system, cause blurred vision, and even amblyopia regardless of its magnitude and orientation.[6] Evaluation of the corneal and internal astigmatism in children may be helpful in several clinical conditions such as contact lens fitting,[7] evaluation of the etiology of astigmatism,[8] research into the emmetropization process,[9],[10] and the detection of posterior corneal surface abnormalities, including keratoconus.[11] For instance, it is not necessary to determine the origin of astigmatism for refractive correction with spectacles, and the cylinder power of the spectacles is equal to the amount of refractive astigmatism. However, considering the amount of CA and ORA and their relationship is very important if there is a need for contact lens prescription, especially gas permeable and toric lenses.[7]
The distribution of corneal and residual astigmatism in different age groups and populations has been investigated in previous studies.[5],[12],[13],[14],[15] Some of these studies found a decrease in the compensatory effect of ORA on CA with aging.[16],[17] Therefore, considering ORA changes and its relationship with CA and aging, knowledge of the normal distribution of this parameter in children and adolescents and its determinants, including age, sex, refractive errors, and biometric parameters, is very important for predicting ORA in this age group. The aim of the present study was to evaluate the distribution of corneal and ORA and their relationship with age, sex, refractive errors, and biometric parameters in children aged 6–18 years.
| Methods | | |
This cross-sectional study was conducted in October 2012.[18] This study was part of another study whose main findings were on ocular biometry, corneal hysteresis, and resistance factor of students in Dezful, a city in southwestern Iran. Details of the methodology of this study have been reported in a previous report, and here we present a brief review. Multi-stage stratified cluster sampling was used to select students from Dezful schools. In this study, from boys' and girls' schools in Dezful, one primary school, one secondary school, and one high school were randomly selected (6 schools in total). Grade and sex were considered the strata, and 6 strata were evaluated in this study. In each stratum, from each grade (cluster), one class was selected randomly, and all of its students were examined. Only students whose parents signed the informed consent form were included in the study. On the examination day, after selecting a site with proper illumination, the students who had their parents' consent entered the study. In schools with more than one class for each grade (cluster), one class was randomly selected, and all of its students were examined. The students were first interviewed, and then, examinations started.
Examinations
The students underwent visual acuity measurement with and without correction, biometry, and refraction with and without cycloplegia. Biometry (mean of three measurements) was done using biograph (LENSTAR/Biograph, WaveLight AG, Erlangen, Germany) by an optometrist, and the data of each eye were checked for errors before recording. If there were errors, artificial tears were instilled, and biometry was repeated after 5 min. After biometry, all students underwent objective refraction without cycloplegia using the Topcon KR 8000 autorefractor (Topcon Corporation, Tokyo, Japan). Three refractive measurements were performed, and the mean of the three measurements was considered in the analysis. Finally, cycloplegic refraction was performed in all students. Autorefractometry was repeated 30 min after instilling three drops of cyclopentolate 1.0% at 5 min apart. CA was calculated based on keratometry values obtained with biograph. Simple calculations of the mathematical difference between refractive and CA cannot be used to calculate residual astigmatism because the orientation of the refractive and CA may not be similar in many cases. Therefore, vector analysis should be used to calculate the difference.[19] The Alpins method was used to calculate residual astigmatism in the present study.[20] The astigmatism axis was categorized as with-the-rule (WTR) (0° ± 30°), against-the-rule (ATR) (90° ± 30°), and oblique (other values).[13]
The power vector method proposed by Thibos and Horner was applied to analyze the data of astigmatism. In this method, all spherocylindrical refractive errors (sphere [S], cylinder [C], axis [α]) are geometrically represented by three dioptric components, including spherical equivalent (SE), J0, and J45. Astigmatism is defined as a Jackson crossed cylinder (JCC) with power j at axis α. This JCC can be further resolved into the sum of 2 other JCC vectors, one with power J0 at axis α = 90° = 180° and the other with power J45 at axis α = 45° = 135°. Positive and negative values of J0 are considered WTR and ATR astigmatism, respectively, and J45 values are considered oblique astigmatism. J0 and J45 are calculated using the following formulas, in which C is negative cylindrical power and α is the cylindrical axis.[21]
J0 = -C/2 × cos (2 α)
J45 = -C/2 × sin (2 α)
Data analysis
SPSS statistical software package, version 22.0 (IBM Corporation, Armonk, NY, USA) was used for data analysis. Due to the high correlation between the results of the two eyes (r = 0.741), we only considered information related to the right eye in our analyses.
We have used cycloplegic SE in our analysis. The mean and standard deviation of different types of astigmatism were determined according to age and sex. Simple and multiple linear regression were used to investigate the effect of each variable on the ORA and CA.
If the relationship was not linear and the data were u-shaped, one group was considered the reference group against which other groups were compared.
Ethical issues
The Ethics Committee of Tehran University of Medical Sciences approved the study protocol, which was conducted in accordance with the tenets of the Declaration of Helsinki. All participants signed a written informed consent. Institutional Review Board approval was obtained (No: 91.s.130.1358).
| Results | | |
Of 864 students that were selected, 683 participated in the study (response rate = 79.1%). Two cases were removed due to corneal scarring, one due to congenital cataracts, one due to a history of sharp trauma, and two due to a history of ocular surgery.
Male students comprised 55.1% of the students (n = 373). The mean age of the participants was 12 ± 3.4 years (6–18 years).
[Table 1] presents the mean value of different types of astigmatism according to age and sex. The mean residual astigmatism was -0.84 D, and the mean CA was -0.85 D in all participants. According to the results of linear regression analysis, residual astigmatism had no significant correlation with sex (P = 0.173). The trend of the residual astigmatism changes with age was not significant, either. There was no significant difference in CA between male and female students; however, it decreased with an increase in age. | Table 1: Mean and 95% confidence interval of ocular residual astigmatism and corneal astigmatism
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The results of J0 and J45, according to residual and CA are presented in [Table 2]. | Table 2: Mean and 95% confidence interval of J0 and J45 of ocular residual and corneal astigmatism
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According to [Figure 1], 25.3% and 27.4% of the students had corneal and residual astigmatism of 0.5 D or less, while >40% had corneal and residual astigmatism of 0.5–1 D. Moreover, about 2% and 4% of the participants had residual and CA >2 D, respectively.
The results of the present study showed that 67.94% of the students had WTR, 1.3% had ATR, and 1.5% had oblique CA. Moreover, 3.4% had WTR residual, 66.8% had ATR residual, and 4.5% had oblique residual astigmatism.
According to [Figure 2] and [Figure 3], residual astigmatism decreased with an increase in spherical refractive error. In contrast, the opposite relationship was found for CA (CA increased with an increase in spherical refractive error). | Figure 3: The mean and 95% of ocular residual astigmatism by spherical error
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[Table 3] shows the association of corneal and residual astigmatism with the studied variables. After analyzing these variables in the multiple models, it was found that both astigmatisms only have a significant relationship with mean keratometry. According to the table, a higher amount of ORA and CA was found in subjects with the steeper cornea. The relationship between corneal and residual astigmatism components with age and sex were also evaluated. | Table 3: Simple linear regressions between ocular residual and corneal astigmatism and the studied variables
Click here to view |
| Discussion | | |
This study is one of the few population-based investigations of corneal and residual astigmatism and their associated factors in a 6–18-year-old population. The results showed that the mean corneal and residual astigmatism had the same value but in opposite directions. In other words, residual astigmatism had an ATR pattern with a neutralizing effect on CA that mostly followed a WTR pattern. In general, the results showed no significant correlation between sex and type of astigmatism (corneal or residual). Although there was no significant relationship between age and type of astigmatism, CA decreased non-significantly with aging. Vector analysis also showed a small but significant decrease in J0 in both types of astigmatism with age. In other words, with an increase in age, the prevalence of WTR in CA and ATR in residual astigmatism decreased. No significant relationship was found between SE and type of astigmatism. U-shaped analysis, however, showed that CA increased and residual astigmatism decreased with an increase in refractive errors, especially myopia. Among biometric components, only anterior corneal curvature had a significant correlation with residual and CA, such that an increase in the anterior corneal curvature caused an increase in the corneal and residual astigmatism.
An advantage of the present study was its large sample size and population-based design, compared to other similar studies, and the evaluation of some probable predictors of ORA and CA. To the best of our knowledge, few studies have investigated residual and CA and their relationship with age, sex, and refractive errors in children and adolescents,[22],[23],[24],[2] and no study has evaluated the relationship between corneal and residual astigmatism and biometric components.
The mean corneal and residual astigmatism was -0.84 D and -0.85 D in this study, respectively. Harvey et al.[22] conducted a longitudinal study to evaluate the changes of refractive astigmatism components in Native American children (Tohono O'odham) from 3 to 19 years old. In this study, on the baseline visit, the mean CA was 1.79 D WTR, and the mean residual astigmatism was -0.6 D ATR. Liu et al.[24] studied astigmatism components in Chinese myopic subjects aged 6–16 years (mean age: 11 years) and reported a mean CA of 1.12 D and a mean ORA of -0.63 D. Huynh et al.[23] studied astigmatism components in children aged 5–8 years (mean age: 6 years) and reported different values of CA (-0.82 D) and ORA (-0.76). In other words, the results of the previous studies indicate the effect of ethnicity and age on the prevalence and amount of astigmatism. In addition, this difference can be due to differences in the refraction method (with or without cycloplegia) and the use of different devices for measurement of corneal and residual astigmatism as well as differences in the demographic characteristics and refractive status of the participants.
The relationship between corneal and residual astigmatism is even more prominent in components of power vectors. In this study, the mean J0 was 0.35 in CA and -0.30 in residual astigmatism, and the distribution of J0 values was mostly positive and WTR in CA and mostly negative and ATR in ORA, which was consistent with other studies.[22],[26],[27],[28] [Table 4] summarizes some information provided in previous studies on astigmatism and its components among children. | Table 4: Summary of some previous studies on corneal and ocular residual astigmatism and their components among children
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Moreover, similar to previous studies, the distribution of ORA_J45 was around 0 D.[22],[24],[27] Several studies have investigated the interrelationship between corneal aberrations and intraocular optical components in different age groups. The results of these studies confirm the findings of the present study and indicate the compensatory effect of intraocular components on corneal aberrations in younger subjects[27] and the augmentation effect of ORA on CA in older subjects.[29]
In the present study, there was no significant difference in the mean ORA and CA between different age groups while vector analysis showed significant differences in the mean J0 of CA and ORA between different age groups (P < 0.05). The mean J0 was significantly lower in older age groups. In comparison with CA_J0, ORA_J0 changed less with age, which was similar to the results of a study by Liu on an adult Chinese population.[14] Similarly, the results of a study conducted by Sanfilippo et al.[17] showed a decrease of about 0.034 D in CA_J0 with each 10-year increase in age. On the other hand, similar to previous studies, evaluation of the changes of astigmatism orientation with age showed that with aging, the prevalence of WTR in CA[12],[14],[16] and ATR in ORA decreased.[30] A review of the literature suggests a marked correlation between age and orientation of refractive astigmatism, indicating that refractive astigmatism shifts from WTR to ATR with age. According to different reports, this shift occurs after the age of 50 years, and refractive astigmatism has more stable conditions before this age. A study in Germany showed that the shift from WTR to ATR occurred in the age range 35–74 years.[28] Moreover, this study found that CA shifted from WTR to ATR with aging and that changes of refractive astigmatism with aging mostly originated from changes in the orientation and magnitude of CA. Similarly, in a Korean study of the total astigmatism (TA), CA, and ORA changes with age in a 3–83-year-old population, CA showed a shift from WTR to ATR, and residual astigmatism showed a pattern of decreasing ATR with ageing. The authors concluded that the pattern of TA with ageing seemed to be influenced by CA.[31]
In the present study, the mean ORA and CA had no significant correlation with sex. There are contradictory reports of the correlation of sex with refractive astigmatism and its components, and there is no consensus in this regard. Some studies found a marked correlation between sex and different types of astigmatism[24],[28] and reported a lower prevalence of ORA in boys versus girls,[22],[23],[24] while most studies reported no relationship between sex and astigmatism.[25],[27] Some studies in older age groups reported a significant correlation between male gender and decreased odds of CA because the cornea of adult males is flatter compared to their female counterparts.[17],[32],[33] However, this difference between different age groups was not prominent in the present study because most gender-related corneal changes occur at older ages. More studies are needed in this regard.
In this study, the results of multiple regression analysis showed no correlation between corneal and residual astigmatism and SE. However, U-shaped analysis showed an increase in CA and a decrease in ORA with an increase in refractive errors (myopia and hyperopia). In other words, with an increase in the amount of myopia and hyperopia, the compensatory effect of ORA on CA fades and considering the higher CA and less compensatory effect of ORA, refractive astigmatism will increase. Previous studies found an increase in refractive astigmatism with an increase in myopia or hyperopia. Hashemi et al.[13] studied the correlation of astigmatism components with refractive errors. They reported that in high refractive errors, the pattern of refractive and residual astigmatism was WTR and ATR, respectively, which confirms the results of the present study and indicates the more important role of CA in determining the pattern of refractive astigmatism in higher amounts of refractive errors.
As for the relationship between biometric parameters and the amounts of CA and ORA, after modifying the effect of other variables in the multiple regression model, the results showed a marked relationship only between the anterior corneal curvature (mean keratometry) and CA and ORA. In other words, ORA and CA increased with an increase in the anterior corneal curvature. Previous studies also confirmed higher ORA and CA in participants with steeper corneas.[24],[28],[34]
The limitation of our study was that although refractive errors were studied with and without cycloplegia and the results of cycloplegic refraction were used for analysis, ocular biometry was done before cyclorefraction. However, although some previous studies, considering lack of significant differences in J0 and J45 with and without cycloplegia, were done without cycloplegia,[23],[25] there were some studies that evaluate corneal and residual astigmatism in children under cycloplegia.[22],[24] This difference in the measurement method should be considered in comparison with the results of different studies.
The results of the present study showed a higher prevalence and amount of ORA among a 6–18-year-old population and the compensatory effect of this type of astigmatism on CA. Therefore, the possibility of the presence of ORA and its relationship with CA should be considered before making any decisions to select appropriate contact lens design for the correction of refractive errors or refractive surgery. In other words, if clinicians only consider CA in the selection of contact lens type or in refractive surgery and ignore internal astigmatism, the desired refractive outcomes will not be achieved.
Financial support and sponsorship
This project was supported by Tehran University of Medical Sciences.
Conflicts of interest
There are no conflicts of interest.
| References | | |
| 1. | Wolfram C, Höhn R, Kottler U, Wild P, Blettner M, Bühren J, et al. Prevalence of refractive errors in the European adult population: The Gutenberg Health Study (GHS). Br J Ophthalmol 2014;98:857-61.  |
| 2. | Xiao X, Liu WM, Ye YJ, Huang JZ, Luo WQ, Liu HT, et al. Prevalence of high astigmatism in children aged 3 to 6 years in Guangxi, China. Optom Vis Sci 2014;91:390-6. |
| 3. | Alpins N. Astigmatism analysis by the Alpins method. J Cataract Refract Surg 2001;27:31-49. |
| 4. | Alpins NA. New method of targeting vectors to treat astigmatism. J Cataract Refract Surg 1997;23:65-75. |
| 5. | Remón L, Benlloch J, Furlan WD. Corneal and refractive astigmatism in adults: A power vectors analysis. Optom Vis Sci 2009;86:1182-6. |
| 6. | Chou YS, Tai MC, Chen PL, Lu DW, Chien KH. Impact of cylinder axis on the treatment for astigmatic amblyopia. Am J Ophthalmol 2014;157: -140. 908 |
| 7. | Wiggins NP, Daum KM, Snyder CA. Effects of residual astigmatism in contact lens wear on visual discomfort in VDT use. J Am Optom Assoc 1992;63:177-81. |
| 8. | Read SA, Collins MJ, Carney LG. A review of astigmatism and its possible genesis. Clin Exp Optom 2007;90:5-19. |
| 9. | Gwiazda J, Grice K, Held R, McLellan J, Thorn F. Astigmatism and the development of myopia in children. Vision Res 2000;40:1019-26. |
| 10. | Kee CS. Astigmatism and its role in emmetropization. Exp Eye Res 2013;114:89-95. |
| 11. | Naderan M, Rajabi MT, Zarrinbakhsh P. Distribution of anterior and posterior corneal astigmatism in eyes with keratoconus. Am J Ophthalmol 2016;167:79-87. |
| 12. | Asano K, Nomura H, Iwano M, Ando F, Niino N, Shimokata H, et al. Relationship between astigmatism and aging in middle-aged and elderly Japanese. Jpn J Ophthalmol 2005;49:127-33. |
| 13. | Hashemi H, Khabazkhoob M, Peyman A, Miraftab M, Jafarzadehpur E, Emamian MH, et al. The association between residual astigmatism and refractive errors in a population-based study. J Refract Surg 2013;29:624-8. |
| 14. | Liu YC, Chou P, Wojciechowski R, Lin PY, Liu CJ, Chen SJ, et al. Power vector analysis of refractive, corneal, and internal astigmatism in an elderly Chinese population: The Shihpai Eye Study. Invest Ophthalmol Vis Sci 2011;52:9651-7. |
| 15. | Tejedor J, Guirao A. Agreement between refractive and corneal astigmatism in pseudophakic eyes. Cornea 2013;32:783-90. |
| 16. | Leung TW, Lam AK, Deng L, Kee CS. Characteristics of astigmatism as a function of age in a Hong Kong clinical population. Optom Vis Sci 2012;89:984-92. |
| 17. | Sanfilippo PG, Yazar S, Kearns L, Sherwin JC, Hewitt AW, Mackey DA. Distribution of astigmatism as a function of age in an Australian population. Acta Ophthalmol 2015;93:e377-85. |
| 18. | Hashemi H, Jafarzadehpur E, Mehravaran S, Yekta A, Ostadimoghaddam H, Norouzirad R, et al. Corneal resistance factor and corneal hysteresis in a 6- to 18-year-old population. J Cataract Refract Surg 2014;40:1446-53. |
| 19. | Alpins N, Ong JK, Stamatelatos G. New method of quantifying corneal topographic astigmatism that corresponds with manifest refractive cylinder. J Cataract Refract Surg 2012;38:1978-88. |
| 20. | Alpins NA, Goggin M. Practical astigmatism analysis for refractive outcomes in cataract and refractive surgery. Surv Ophthalmol 2004;49:109-22. |
| 21. | Thibos LN, Horner D. Power vector analysis of the optical outcome of refractive surgery. J Cataract Refract Surg 2001;27:80-5. |
| 22. | Harvey EM, Miller JM, Twelker JD, Sherrill DL. Longitudinal change and stability of refractive, keratometric, and internal astigmatism in childhood. Invest Ophthalmol Vis Sci 2014;56:190-8. |
| 23. | Huynh SC, Kifley A, Rose KA, Morgan I, Heller GZ, Mitchell P. Astigmatism and its components in 6-year-old children. Invest Ophthalmol Vis Sci 2006;47:55-64. |
| 24. | Liu Y, Cheng Y, Zhang Y, Zhang L, Zhao M, Wang K. Evaluating internal and ocular residual astigmatism in Chinese myopic children. Jpn J Ophthalmol 2017;61:494-504. |
| 25. | Chen Z, Liu L, Pan C, Li X, Pan L, Lan W, et al. Ocular residual and corneal astigmatism in a clinical population of high school students. PLoS One 2018;13:e0194513. |
| 26. | Eom Y, Kang SY, Kim HM, Song JS. The effect of posterior corneal flat meridian and astigmatism amount on the total corneal astigmatism estimated from anterior corneal measurements. Graefes Arch Clin Exp Ophthalmol 2014;252:1769-77. |
| 27. | Park CY, Oh JH, Chuck RS. Predicting ocular residual astigmatism using corneal and refractive parameters: A myopic eye study. Curr Eye Res 2013;38:851-61. |
| 28. | Schuster AK, Pfeiffer N, Schulz A, Hoehn R, Ponto KA, Wild PS, et al. Refractive, corneal and ocular residual astigmatism: Distribution in a German population and age-dependency – The Gutenberg health study. Graefes Arch Clin Exp Ophthalmol 2017;255:2493-501. |
| 29. | Piñero DP, Ruiz-Fortes P, Pérez-Cambrodí RJ, Mateo V, Artola A. Ocular residual astigmatism and topographic disparity vector indexes in normal healthy eyes. Cont Lens Anterior Eye 2014;37:49-54. |
| 30. | KhabazKhoob M, Hashemi H, Yazdani K, Mehravaran S, Yekta A, Fotouhi A. Keratometry measurements, corneal astigmatism and irregularity in a normal population: The Tehran eye study. Ophthalmic Physiol Opt 2010;30:800-5. |
| 31. | Kim CS, Ryu JW, Kim HS, Lee YC. Distribution and change of total astigmatism, corneal astigmatism and residual astigmatism with age in patient with emmetropia. Taehan An'gwa Hakhoe chi 2005;46:485-93. |
| 32. | Goto T, Klyce SD, Zheng X, Maeda N, Kuroda T, Ide C. Gender- and age-related differences in corneal topography. Cornea 2001;20:270-6. |
| 33. | Hoffmann PC, Hütz WW. Analysis of biometry and prevalence data for corneal astigmatism in 23,239 eyes. J Cataract Refract Surg 2010;36:1479-85. |
| 34. | Muftuoglu O, Erdem U. Evaluation of internal refraction with the optical path difference scan. Ophthalmology 2008;115:57-66. |
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]
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