Middle East African Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2016  |  Volume : 23  |  Issue : 1  |  Page : 115--121

Comparison of ocular monochromatic Higher-order aberrations in normal refractive surgery candidates of Arab and South Asian Origin


Gaurav Prakash, Dhruv Srivastava, Sounak Choudhuri, Ruthchel Bacero 
 Department of Cornea and Refractive Surgery, NMC Eye Care, New Medical Center Specialty Hospital, Abu Dhabi, United Arab Emirates

Correspondence Address:
Gaurav Prakash
Department of Cornea and Refractive Surgery, NMC Eye Care, New Medical Center Specialty Hospital, Electra Street, P. O. Box: 6222, Abu Dhabi
United Arab Emirates

Abstract

Purpose: To compare the ocular monochromatic higher.order aberration. (HOA) profile in normal refractive surgery candidates of Arab and South Asian origin. Methods: This cross.sectional, observational, comparative study was performed in the cornea department of a specialty hospital. Normal refractive surgery candidates with no ocular morbidity except refractive error were recruited. Refractive surgery candidates underwent a preoperative evaluation, including wavefront aberrometry with the iDesign aberrometer. (AMO, Inc., Santa Ana, California, United States). The HOA from right eyes were analyzed for HOA signed, absolute, and polar Zernike coefficients. Results: Two hundred Arab participants (group 1) and 200 participants of South-Asian origin (group 2) comprised the study sample. The age and refractive status were comparable between groups. The mean of the HOA root mean square (RMS) was 0.36 ± 17 μ and 0.38 ± 18 μ for Arab and South-Asian eyes, respectively (P < 0.05, rank sum test [RST]). Of the 22 higher order signed Zernike modes, only Z3−3, Z3−1, 31, Z4−4, Z4−2, Z40, Z44, and Z5−5 were significantly different from zero (one sample t-test, P < 0.002, with a Bonferroni correction of 0.05/22). All the signed and absolute Zernike terms were comparable between groups (RST, P > 0.002 [0.05/22]). The polar coefficients for coma, trefoil, spherical aberration, and tetrafoil were comparable between groups (P > 0.05, RST). Combined RMS values of third, fourth, fifth, and sixth order also were comparable between groups (P > 0.05, RST). Conclusions: Preoperative whole eye HOA were similar for refractive surgery candidates of Arab and South.Asian origin. The values were comparable to historical data for Caucasian eyes and were lower than Asian. (Chinese) eyes. These findings may aid in refining refractive nomograms for wavefront ablations.



How to cite this article:
Prakash G, Srivastava D, Choudhuri S, Bacero R. Comparison of ocular monochromatic Higher-order aberrations in normal refractive surgery candidates of Arab and South Asian Origin.Middle East Afr J Ophthalmol 2016;23:115-121


How to cite this URL:
Prakash G, Srivastava D, Choudhuri S, Bacero R. Comparison of ocular monochromatic Higher-order aberrations in normal refractive surgery candidates of Arab and South Asian Origin. Middle East Afr J Ophthalmol [serial online] 2016 [cited 2022 Sep 25 ];23:115-121
Available from: http://www.meajo.org/text.asp?2016/23/1/115/171758


Full Text



 Introduction



Ocular aberrometry has significantly increased our understanding of the optics of the eye and objective visual quality.[1],[2],[3] Wavefront analysis has expanded beyond excimer ablation and has been applied to wavefront customized lenses, visual quality assessment after ocular and multiple ocular pathologies.[4],[5],[6],[7],[8],[9]

Wavefront analyzers measure ocular aberrations using the principles of Hartmann–Shack aberrometry, ray tracing, dynamic skiascopy, or Tscherning aberrometry.[10],[11],[12],[13] These devices measure the distortions created by the refractive error of the eye in a wavefront of light traveling through the ocular system. The total aberration profile of the eye can be decomposed into Zernike polynomials and presented as lower order aberrations and higher-order aberrations (HOA), based on the radial orders. For example, radial orders 0–2 are lower order aberrations and 3 and higher are HOA. In The American National Standards Institute double index scheme is the preferred method of denoting Zernike polynomials. In this indexing method, Zmn indicates m is the radial order and n is the angular frequency for the polynomial Z.

Lower order aberrations correlate with conventional refraction. HOA are not measurable by conventional refraction, and thus are the major topic of interest in wavefront analysis. Measurement of the HOA profile can be affected by multiple factors including pupil size, cycloplegia, age, ethnicity, ocular surface status, and the device used.[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28]

Previous studies that document HOA for different demographic subgroups groups have reported interesting results. The HOA profiles of Caucasian, Asian Chinese, Asian Indian, and Brazilian eyes have been documented in the literature.[22],[23],[24],[25],[26],[27],[28] However, other than a study measuring corneal spherical aberration, there are no studies in the literature to evaluate the total ocular wavefront profile in Arab eyes.[29] In this study, we evaluate the ocular (whole eye) wavefront profile of refractive surgery candidates of Arab origin and compare it to candidates of South Asian (Indian sub-continent) origin. To the best of our knowledge, this is the first study evaluating and comparing the HOA profile of refractive surgery candidates of Arab origin.

 Methods



Study population

This cross-sectional, comparative, observational study was performed at a specialized hospital. All the candidates gave informed consent for their participation. The study had the approval of the Institutional Review Board and followed the tenets of the Declaration of Helsinki. The candidate pool consisted of patients interested in refractive surgery. Subjects who wore soft contact lenses were advised not to wear them from 2 weeks prior to the tests. None of the subjects in our study wore hard contact lenses. All candidates underwent detailed history taking, slit lamp evaluation and Scheimpflug topographic analysis (Sirius, Costruzione Strumenti Oftalmici, Italy), pupillometry, tear volume, and film stability assessment, dilated anterior segment and indirect ophthalmoscopic retinal evaluation as a part of the initial assessment. Exclusion factors were dry eyes, any previous surgical intervention in the eye or corneal scars/irregularity, cataract, or any other ocular morbidity including corneal ectasia or scarring.

Wavefront aberrometer and measurements

The iDesign advanced Wavescan studio (Abbott Medical Optics, Santa Ana, CA, USA) was used for wavefront measurement in all cases. This device is a Hartmann–Shack based aberrometer that is a new, upgraded version of the Wavescan aberrometer (Abbott Medical Optics, Santa Ana, CA, USA). It has 5 times greater resolution, encompassing over 1250 data point for a 7.0-mm pupil compared to 240 points for the Wavescan. In addition to the higher and lower order aberration data, this device also provides a Hartmann–Shack based topography, white-to-white corneal diameter, and scotopic pupil diameter. The range of measurement is from −16 D to +12 D of sphere, up to 8 D of cylinder, and up to 8 μm of root mean square (RMS) HOA.[30] The wavefront outcomes and the repeatability of the keratometric function have been found to be satisfactory in two recent studies.[31],[32]

Wavefront acquisition was performed after dark adaptating the pupil for 30 min. No dilating drops or lubricants were instilled prior to wavefront measurement. The wavefront acquisition was carried out in dark room conditions. After properly focusing the eye, the candidates were advised to blink a few times before the actual wavefront capture. The candidates were advised to close the eyes between measurements for 1 min. The software then analyzed the images for iris registration, wavefront data, and corneal topography data. The review screen showed a green icon for all three when the measurements were of appropriate quality for treatment or analysis. Cases were included in the study only if the wavefront capture was ≥6 mm. Three consecutive “good” measurements were taken. The total attempts were limited to a maximum of five, i.e., if useful data were not captured for three out of five attempts; the subject was excluded from the study. The machine automatically selected the wavefront scan with the best quality for the treatment. This scan was also chosen for analysis in the study. The data were noted and analyzed for a 6-mm pupil diameter. For HOA, the machine used Fourier' transforms for analysis and presented Zernike polar coefficients and standard (signed) Zernike coefficients RMS values [Figure 1]. The complete HOA values RMS (HOARMS) value in microns was used for analysis as well as the data of individual polynomials up to sixth order. Two hundred consecutive cases were included in both groups (group 1: Arab origin; group 2: South Asian (Indian sub-continental) origin.{Figure 1}

Statistical analysis

All the data were exported as JPEG images and manually entered into an MS Excel (Microsoft Corp., Richmond, VA, USA) sheet. The data was then transferred to SPSS 16.0 (IBM Corp., Armonk, New York, USA) for analysis. Only the right eyes were used for analysis. The rank sum test (RST) was used to analyze the difference between means. Bar charts were used to illustrate the spread of Zernike polynomials/modes. The data was analyzed for polar as well as signed coefficient. Polar coefficients were computed as the RMS values of signed coefficients. Coma was computed as RMS of Z1−1 and Z1[1]; trefoil as RMS of Z1−3 and Z1[3]; secondary astigmatism as RMS Z4−2 and Z4[2]; tetrafoil as RMS Z4−4 and Z4[4]. RMS function is always positive denoting only the magnitude but not sign, hence, spherical aberration was converted to its absolute value when comparing to other polar modes.

 Results



Demographic, refractive error, and total higher-order aberration root mean square

The mean age, sphere, cylinder, spherical equivalent, and total HOARMS were comparable (P > 0.05, rank rum test), [Table 1]. The gender distribution was also comparable (female: male = 112: 88 (group 1) and 126: 74 (group 2); χ[2] = 2.03, P = 0.15, Chi-square test).{Table 1}

Distribution of individual Zernike modes

Of the 22 higher order signed Zernike, only Z3−3, Z3−1, Z3[1], Z4−4, Z4−2, Z4[0], Z4[4], and Z5−5 were significantly different from zero (one sample t-test, P < 0.002 after Bonferroni correction [0.05/22]). Therefore, significantly higher terms from the third and fourth order were different from zero (7/9 terms) compared to only 1/13 terms from the fifth and sixth order (P = 0.001, Fischer's exact test). However, all the absolute Zernike modes were significantly different from zero (one sample t-test, P < 0.002 after Bonferroni correction [0.05/22]).

The mean and standard values for signed and absolute Zernike modes are presented in [Table 2]. All the signed and absolute Zernike terms were comparable between groups (Wilcoxon RST, P > 0.002 after Bonferroni correction (0.05/22), [Table 2].{Table 2}

Polar coefficients and root mean square of entire orders

The polar coefficients of coma, trefoil, spherical aberration, and tetrafoil were comparable between groups (P > 0.05, RST), [Table 3]. The frequency distribution patterns of coma, tetrafoil, and spherical aberration are presented in [Figure 2]. The combined RMS values of third, fourth, fifth and sixth order were comparable (P > 0.05, RST), [Table 3]. The frequency distribution pattern of the four orders (3rd to 6th) is presented in [Figure 3].{Table 3}{Figure 2}{Figure 3}

Within group analysis indicated the mean RMS values of individual orders was seen in the following sequence from largest to smallest: 3rd, 4th, 5th followed by 6th order (Kruskal–Wallis test, P < 0.001 for both group 1 and group 2). Furthermore, 3rd order was significantly higher than the 4th order (RST, P < 0.001 for both groups 1 and 2). Coma, trefoil, and spherical aberration were the three major Zernike modes in both the groups. Coma was significantly higher than trefoil and spherical aberration in both groups (RST, P < 0.001 for both groups 1 and 2 for comparisons between coma and spherical aberration, and between coma and trefoil).

 Discussion



In this study, we have evaluated the total ocular monochromatic wavefront profile of normal candidates from Arab origin for the 1st time in published literature. The data was compared to a South Asian cohort in the same setting. The selection of South Asian origin for comparison was based on the population distribution profile in our cornea and refractive surgery services, where these two groups constitute the largest majority of patients.

We found that there was no significant difference in the age, gender, and refractive error between groups. This observation indicates the comparability of these groups. The Zernike modes such as signed coefficients, absolute coefficients, and polar modes were compared separately. Salmon and Van de Pol have previously explained the rationale behind evaluating signed, absolute, and polar coefficients separately.[33] We found that the total HOARMS value, signed and absolute values of Zernike modes were comparable between Arab and South Asian eyes. Our result were similar to previous reports that in normal, unoperated eyes, the magnitude of fifth order, and higher HOA are very small and not significantly different from zero.[22],[34] This pattern occurs dues to variation of signed Zernike coefficients around zero and is seen typically in normal or unoperated eyes, which are typically less distorted than eyes that have undergone surgery or pathological eyes. When only absolute values were considered, all coefficients were significantly different. This justifies using all the Zernike modes in computing the RMS of all Zernike coefficients.

Ethnic variations in HOA have been noted previously.[22],[23],[24],[25],[26],[27],[28] However, there are very few studies comparing two different ethnic populations in the same setting. Using the same setting can control for variability of factors such as type and make of aberrometer, population cohort, and age. We found that the mean HOARMS value in our study was 0.36 ± 17 µ and 0.38 ± 18 µ for Arab and South Asian eyes, respectively. There was no difference in mean HOARMS value in the two groups. Nakano et al. compared Asian (Chinese) eyes with non-Asian eyes in a Brazilian setting.[26] They found that in spite of the higher refractive error in the Asian eyes, the HOARMS values for various orders and the total HOARMS were comparable between groups. They did not comment on individual Zernike polynomials. Cervino et al. found that the HOARMS values for Caucasian and British Asians were similar.[25] With the exception of Z3[3] and Z40, all other Zernike modes in their study were comparable.[26] The only previous study specifically for South Asian origin was from Northern India and was published 6 years ago.[28] The authors showed that the HOA profile of the South Asian (Indian) eyes was similar to the literature for Caucasian eyes compared to those with Chinese eyes.[28] The mean HOARMS for Asian Indian eyes (0.36 ± 0.26 µ) from this previous study is similar to that seen our study. Furthermore, the mean HOARMS in both groups were also comparable to that seen the largest study on normative data of wavefront aberration by Salmon and van de Pol.[33] They evaluated 2560 eyes of mixed origin from multiple centers with multiple aberrometers and found the mean HOARMS was 0.33 ± 0.13 µ at 6 mm pupil diameter).[34] In the current study, the mean HOARMS was lower than that seen in Chinese Asian eyes reported in 2 studies (0.49 ± 0.16 µ for Wei et al.,[27] 0.51 ± 0.71 µ for Nakano et al.[26]) for a 6 mm pupil.

The most prominent order of Zernike polynomials in our study was the 3rd order (0.28 ± 14 µ for Arab eyes, 0.29 ± 16 µ for South Asian eyes). This was comparable to the value reported by Salmon and van de Pol (0.25 ± 0.12 µ), Wang and Koch (0.22 ± 0.09 µ) and Prakash et al. (0.23 ± 0.15 µ) at 6 mm pupil for mixed population of, Caucasian and South Asian patients respectively.[28],[33] The average value for third order was higher in Chinese Asian eyes (0.37 ± 0.16 µ) compared to our values for both groups.[27]

Coma, trefoil, and spherical aberration were the largest aberrations in both groups. This trend was comparable to previous studies, which report a similar predominance of these three aberrations in normal nonsurgical eyes.26-28,33

There are drawbacks to our study. The sample size is relatively small. Second, even though this aberrometer is a newer, more advanced version of an already established aberrometer (Wavescan, AMO, CA), which has been shown to have good repeatability,[35] a new study of repeatability would be helpful.

To conclude, we found that the HOA profile similar in normal, unoperated eyes of Arab and South Asian origin. Our data concur with a previous study on eyes of South Asian patients. The values were comparable to historical data for Caucasian eyes and were lower than that seen for Chinese Asian eyes. These findings can have useful implications for nomogram creation and adjustments for refractive surgery candidates of Arab origin undergoing wavefront-guided LASIK.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1Mello GR, Rocha KM, Santhiago MR, Smadja D, Krueger RR. Applications of wavefront technology. J Cataract Refract Surg 2012;38:1671-83.
2Maeda N. Clinical applications of wavefront aberrometry – A review. Clin Experiment Ophthalmol 2009;37:118-29.
3Bruce AS, Catania LJ. Clinical applications of wavefront refraction. Optom Vis Sci 2014;91:1278-86.
4Salvetat ML, Brusini P, Pedrotti E, Zeppieri M, Miani F, Marcigaglia M, et al. Higher order aberrations after keratoplasty for keratoconus. Optom Vis Sci 2013;90:293-301.
5Rudolph M, Laaser K, Bachmann BO, Cursiefen C, Epstein D, Kruse FE. Corneal higher-order aberrations after Descemet's membrane endothelial keratoplasty. Ophthalmology 2012;119:528-35.
6Razmjoo H, Vaezi MH, Peyman A, Koosha N, Mohammadi Z, Alavirad M. The effect of pterygium surgery on wavefront analysis. Adv Biomed Res 2014;3:196.
7Huelle JO, Katz T, Druchkiv V, Pahlitzsch M, Steinberg J, Richard G, et al. First clinicial results on the feasibility, quality and reproducibility of aberrometry-based intraoperative refraction during cataract surgery. Br J Ophthalmol 2014;98:1484-91.
8Yagci R, Uzun F, Acer S, Hepsen IF. Comparison of visual quality between aspheric and spherical IOLs. Eur J Ophthalmol 2014;24:688-92.
9Sabesan R, Johns L, Tomashevskaya O, Jacobs DS, Rosenthal P, Yoon G. Wavefront-guided scleral lens prosthetic device for keratoconus. Optom Vis Sci 2013;90:314-23.
10Molebny VV, Panagopoulou SI, Molebny SV, Wakil YS, Pallikaris IG. Principles of ray tracing aberrometry. J Refract Surg 2000;16:S572-5.
11Mrochen M, Kaemmerer M, Mierdel P, Krinke HE, Seiler T. Principles of Tscherning aberrometry. J Refract Surg 2000;16:S570-1.
12Thibos LN. Principles of Hartmann-Shack aberrometry. J Refract Surg 2000;16:S563-5.
13Rozema JJ, Van Dyck DE, Tassignon MJ. Clinical comparison of 6 aberrometers. Part 1: Technical specifications. J Cataract Refract Surg 2005;31:1114-27.
14Iseli HP, Bueeler M, Hafezi F, Seiler T, Mrochen M. Dependence of wave front refraction on pupil size due to the presence of higher order aberrations. Eur J Ophthalmol 2005;15:680-7.
15Ginis HS, Plainis S, Pallikaris A. Variability of wavefront aberration measurements in small pupil sizes using a clinical Shack-Hartmann aberrometer. BMC Ophthalmol 2004;4:1.
16Hiraoka T, Miyata K, Nakamura Y, Miyai T, Ogata M, Okamoto F, et al. Influences of cycloplegia with topical atropine on ocular higher-order aberrations. Ophthalmology 2013;120:8-13.
17Wei S, Song H, Tang X. Correlation of anterior corneal higher-order aberrations with age: A comprehensive investigation. Cornea 2014;33:490-6.
18McGinnigle S, Eperjesi F, Naroo SA. A preliminary investigation into the effects of ocular lubricants on higher order aberrations in normal and dry eye subjects. Cont Lens Anterior Eye 2014;37:106-10.
19Won JB, Kim SW, Kim EK, Ha BJ, Kim TI. Comparison of internal and total optical aberrations for 2 aberrometers: ITrace and OPD scan. Korean J Ophthalmol 2008;22:210-3.
20Kim DS, Narváez J, Krassin J, Bahjri K. Comparison of the VISX wavescan and NIDEK OPD-scan aberrometers. J Refract Surg 2009;25:429-34.
21Rozema JJ, Van Dyck DE, Tassignon MJ. Clinical comparison of 6 aberrometers. Part 2: Statistical comparison in a test group. J Cataract Refract Surg 2006;32:33-44.
22Wan XH, Li SM, Xiong Y, Liang YB, Li J, Wang FH, et al. Ocular monochromatic aberrations in a rural Chinese adult population. Optom Vis Sci 2014;91:68-75.
23Shimozono M, Uemura A, Hirami Y, Ishida K, Kurimoto Y. Corneal spherical aberration of eyes with cataract in a Japanese population. J Refract Surg 2010;26:457-9.
24Lim KL, Fam HB. Ethnic differences in higher-order aberrations: Spherical aberration in the South East Asian Chinese eye. J Cataract Refract Surg 2009;35:2144-8.
25Cerviño A, Hosking SL, Ferrer-Blasco T, Montes-Mico R, Gonzalez-Meijome JM. A pilot study on the differences in wavefront aberrations between two ethnic groups of young generally myopic subjects. Ophthalmic Physiol Opt 2008;28:532-7.
26Nakano EM, Bains H, Nakano K, Nakano C, Portellinha W, Oliveira M, et al. Wavefront analysis in Asian-Brazilians. J Refract Surg 2006;22 9 Suppl: S1024-6.
27Wei RH, Lim L, Chan WK, Tan DT. Higher order ocular aberrations in eyes with myopia in a Chinese population. J Refract Surg 2006;22:695-702.
28Prakash G, Sharma N, Choudhary V, Titiyal JS. Higher-order aberrations in young refractive surgery candidates in India: Establishment of normal values and comparison with white and Chinese Asian populations. J Cataract Refract Surg 2008;34:1306-11.
29Al-Sayyari TM, Fawzy SM, Al-Saleh AA. Corneal spherical aberration in Saudi population. Saudi J Ophthalmol 2014;28:207-13.
30Coleman SC. High-Definition Wavefront: 5X Resolution. ASCRS Eye World; April, 2008. Available from: http://www.eyeworld.org/ewsupplementarticle.php?id=281. [Last accessed on 2015 Apr 11].
31Prakash G, Srivastava D, Choudhuri S. A novel Hartman Shack-based topography system: repeatability and agreement for corneal power with Scheimpflug+Placido topographer and rotating prism auto-keratorefractor. Int Ophthalmol. 2015;35:869-80.
32Smadja D, De Castro T, Tellouck L, Tellouck J, Lecomte F, Touboul D, et al. Wavefront analysis after wavefront-guided myopic LASIK using a new generation aberrometer. J Refract Surg 2014;30:610-5.
33Salmon TO, van de Pol C. Normal-eye Zernike coefficients and root-mean-square wavefront errors. J Cataract Refract Surg 2006;32:2064-74.
34Wang L, Koch DD. Ocular higher-order aberrations in individuals screened for refractive surgery. J Cataract Refract Surg 2003;29:1896-903.
35Liang CL, Juo SH, Chang CJ. Comparison of higher-order wavefront aberrations with 3 aberrometers. J Cataract Refract Surg 2005;31:2153-6.