Ethnic differences in macular thickness

Original Article

Ethnic differences in macular thicknessYasser M Tariq MBBS, Haitao Li PhD, George Burlutsky MStat and Paul Mitchell FRANZCO PhDCentre for Vision Research, Department of Ophthalmology and Westmead Millennium Institute, University of Sydney, Sydney,New South Wales, Australia

ABSTRACT

Background: To determine ethnic differences in time-domain (Stratus) optical coherence tomography-measured macular thickness in 12-year-old children.

Design: Population-based cross-sectional study.

Participants: A total of 2367 children from grade 7(mean age 12.7 � 0.45 years) examined during theSydney Myopia Study during 2004–2005.

Methods: Examination included determination ofbest corrected visual acuity. Autorefraction was per-formed after cycloplegia. Axial length was measuredusing non-contact interferometry and OCT wasperformed using Stratus OCT. Ethnicity was selfreported by participants’ parents.

Main Outcome Measures: Macular thickness.

Results: The four largest ethnic groups were Cauca-sian (n = 1224), East Asian (n = 291), South Asian(n = 107) and Middle Eastern (n = 146). The greatestethnic differences were found at the central macula,which was significantly thicker in Caucasian com-pared with East Asian, South Asian and MiddleEastern children (mean differences 9.0 mm, 12.1 mmand 6.5 mm, respectively; all P < 0.0001). The aver-age inner macula was significantly thicker in Cauca-sian than East Asian and South Asian children(P = 0.005 and P < 0.0001, respectively). Theaverage outer macula was significantly thicker inCaucasian than Middle Eastern and South Asianchildren (P = 0.03 and P < 0.0001, respectively).

Conclusion: Macular parameters were found tovary by ethnicity in 12-year-old children. Caucasianchildren had the thickest macular parametersand South Asian children had the thinnest. Thegreatest differences were found in the centralmacula.

Key words: children, ethnicity, macula, optical coher-ence tomography, Sydney Myopia Study.

INTRODUCTION

Optical coherence tomography (OCT) is a widelyused examination technique in the diagnosis andmonitoring of retinal pathology.1 It is especiallyuseful in the quantitative and qualitative evaluationof macular morphology. To utilize the full potentialof this technology in determining the presence ofpathological change, an understanding of the demo-graphic variables that influence normal variationis required. It was previously shown that ethni-city impacts on macular morphology.2–5 Althoughprevious reports have evaluated differences inmacular thickness between Caucasians and African-Americans3–5 or Caucasians and East Asians,2 therehave been, to our knowledge, no studies directlycomparing other ethnic groups.

In the present study, we sought to determine meandifferences in macular parameters measured byStratus OCT between Caucasian, East Asian, SouthAsian and Middle Eastern individuals. An adolescentpopulation is ideal for this study as it is relatively freefrom ocular diseases which alter normal macularmorphology (e.g. diabetic retinopathy) or conditionsaffecting the clarity of the ocular media (e.g. cataract).

� Correspondence: Professor Paul Mitchell, Centre for Vision Research, Department of Ophthalmology, University of Sydney, Hawkesbury Road,

Westmead, NSW 2145, Australia. Email: [email protected]

Received 4 January 2011; accepted 12 April 2011.

Conflict/competing interest: None declared.

Funding sources: Supported by the Australian National Health & Medical Research Council, Canberra, Australia (Grant 253732).

Clinical and Experimental Ophthalmology 2011; 39: 893–898 doi: 10.1111/j.1442-9071.2011.02593.x

© 2011 The AuthorsClinical and Experimental Ophthalmology © 2011 Royal Australian and New Zealand College of Ophthalmologists

METHODS

The Sydney Myopia Study was a population basedsurvey of eye health forming the first part of theSydney Childhood Eye Study. Based on socioeco-nomic stratification of Sydney, 21 high schools wererandomly selected, to include a proportional mix ofpublic and private/religious schools. All Year 7 chil-dren in these schools were invited to participate. Thestudy was approved by the Human Research EthicsCommittee, University of Sydney, the Department ofEducation and Training, and the Catholic EducationOffice, New South Wales, Australia, and adhered tothe tenets of the Declaration of Helsinki. Informedwritten consent was obtained from parents beforeexamination. Examinations were conducted during2004 and 2005. The study methods have beendescribed in detail previously.6

Examinations

The Stratus OCT (OCT3, software version 4.0.1; CarlZeiss Meditec, Dublin, CA, USA) obtains cross-sectional retinal tomographic scans, that have beenfound to be highly reproducible.7,8 OCT was per-formed through dilated pupils using the FastMacular Thickness Map protocol. The macular scansconsisted of six bisecting scans 6 mm in length andoffset at 30 degrees, with each line comprising 128A-scans. Centration and focus was monitored duringscanning to minimize poor scans. Scans wererepeated until adequate quality images wereobtained. The 6-mm diameter macular region wassubdivided into concentric regions of central macula(radius 0.5 mm), inner macula (radius 1.5 mm) andouter macula (radius 3 mm). The inner macula andouter macula were subdivided into four quadrants.The macular scanning protocols used have beendescribed in detail previously.2,9

The eye examination also included measurementof best corrected visual acuity, ocular motility,cycloplegic autorefraction (Canon RK-F1; Canon,Tokyo, Japan), ocular biometry (IOLMaster; CarlZeiss Meditec, Inc., Dublin, CA, USA), slit-lampbiomicroscopy and mydriatic digital fundus pho-tography. Cycloplegia was achieved by instillingcyclopentolate 1% and tropicamide 1% 2 min afteradministration of amethocaine hydrochloride 1%.Adequate mydriasis of at least 6 mm was required.

The parents of participants completed detailed193-item questionnaires on demographics, ocularand medical history and birth parameters. Ethnic-ity was determined based on self-identificationby parents by choosing from a list of ethnici-ties including Caucasian (European), East Asian,Indian/Pakistani/Sri Lankan, African, Melanesian/Polynesian, Middle Eastern, Indigenous Australian,

South American and other. A child was considered tobelong to a specific ethnic group if both parents self-identified with a common ethnicity. If the motherand father had different ethnicities the child wasclassified as having mixed ethnicity, similarly if oneor both parents were of mixed ethnicity then thechild was classified as having mixed ethnicity. In thispaper, ethnicity was categorized into Caucasian(European), East Asian, Middle Eastern and SouthAsian groups. East Asian ethnicity included childrenfrom China, Malaysia, Singapore, Indonesia, Philip-pines, Japan, Korea, Myanmar, Thailand, Laos, Cam-bodia and Vietnam. South Asian ethnicity includedchildren from India, Pakistan, Sri Lanka and Nepal.Children of mixed ethnicity and from ethnicitieswith smaller numbers including Oceanian, African,Indigenous Australian and South American wereexcluded from the current analyses.

Statistical analysis

Statistical analysis was performed using SAS, Version9.2 (SAS Institute, Cary, NC, USA). Only OCT scansof the right eye with signal strength greater than 5were used. To compare the baseline characteristics ofparticipants (age, gender, visual acuity, axial length,spherical equivalent, height, weight and body massindex) in each ethnic group we used t-tests and chi-square tests. For comparison of macular parametersbetween ethnic groups mixed linear models10 wereemployed adjusting for covariates (age, sex, heightand axial length) with the school attended includedas a random effect.

RESULTS

The study included 2367 participants and of these2068 (88%) had adequate quality macular scans.The four largest ethnic groups were Caucasian(n = 1 224), East Asian (n = 291), South Asian(n = 107) and Middle Eastern (n = 146) children.Characteristics of participants included in this studyare presented in Table 1. The average age of the par-ticipants was 12.7 � 0.45 years, and 52.4% of thegroup was male. Table 2 presents ethnic differencesbetween baseline characteristics. The main differ-ences were in visual acuity, axial length, sphericalequivalent and height. For example, Caucasian chil-dren had better presenting visual acuity (57.4 � 4.2vs. 53.0 � 8.1 letters, P < 0.0001), shorter axial length(23.24 � 0.73 vs. 23.84 � 1.00 mm, P < 0.0001), weremore hyperopic (0.81 � 0.79 vs. -0.55 � 1.69 D,P < 0.0001), were taller (156.5 � 8.1 vs. 154.5 �7.7 cm, P = 0.0001) and weighed more (49.9 � 12.5vs. 47.6 � 11.8 kg, P = 0.004) than East Asianchildren.

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Table 3 presents macular characteristics in differ-ent ethnic groups. Tables 4 and 5 present the meandifferences between ethnic groups. The centralmacula and foveal minimum thickness were signifi-cantly greater in Caucasian children than children ofEast Asian, South Asian and Middle Eastern ethnic-

ity (all P < 0.0001). The greatest differences werebetween Caucasian and South Asian children forthese central macular parameters, with Caucasianchildren having a foveal minimum 8.6 mm (P <0.0001) thicker than South Asian children and acentral macula 12.1 mm (P < 0.0001) thicker. Therewere no significant differences for the central maculaand the foveal minimum between East Asian, SouthAsian and Middle Eastern groups, apart from theMiddle Eastern children having a 5.6 mm (P = 0.02)thicker central macula than the South Asianchildren. Similarly, central macular volume was sig-nificantly thicker in Caucasian than in East Asian,South Asian and Middle Eastern children.

The inner macular parameters compared with thefoveal minimum and central macular parameters dis-played smaller differences between ethnic groups.The greatest differences were seen between Cauca-sian and South Asian children with the inner maculabeing 8.1 mm thinner in South Asians versus Cauca-sian children (P < 0.0001) (Table 4). South Asianchildren had a thinner average inner maculacompared with both East Asian and Middle Easternchildren with a difference of 5.1 mm (P = 0.002) and5.5 mm (P = 0.004), respectively (Table 5). Therewere no significant differences between the East

Table 1. Baseline characteristics (means � SD, numbers withproportions) of sample

Characteristics n (%)/mean � SD

Age (years) 12.70 � 0.45†

Male, n (%) 926 (52.40)Ethnicity, n (%)

Caucasian 1224 (63.20)East Asian 291 (16.50)South Asian 107 (6.10)Middle Eastern 146 (8.30)

Visual acuity (letters)‡ 56.40 � 5.70Axial length (mm) 23.37 � 0.82Spherical equivalent (D) 0.52 � 1.17Height (cm) 155.90 � 8.00Weight (kg) 49.70 � 12.50Body mass index (kg/m2) 20.30 � 20.30

†Age range 11–14 years. ‡logMAR presenting visual acuity(total letters read). SD, standard deviation; D, dioptres.

Table 2. Baseline characteristics of participants stratified by ethnicity

Characteristics Caucasian East Asian South Asian Middle Eastern

Mean � SD Mean � SD P† Mean � SD P‡ Mean � SD P§

Age (years) 12.70 � 0.44 12.70 � 0.44 0.2400 12.50 � 0.40 <0.0001 12.70 � 0.40 0.0700Male, n (%) 649 (53.00) 135 (46.40) 0.0400 54 (50.50) 0.6100 88 (60.30) 0.1000Visual acuity (letters)¶ 57.40 � 4.20 53.00 � 8.10 <0.0001 54.80 � 7.40 0.0006 55.90 � 5.90 0.0050Axial length (mm) 23.24 � 0.73 23.84 � 1.00 <0.0001 23.50 � 0.90 0.0010 23.40 � 0.67 0.0040Spherical equivalent (D) 0.81 � 0.79 -0.55 � 1.69 <0.0001 -0.18 � 1.41 <0.0001 0.76 � 0.96 0.6100Height (cm) 156.50 � 8.10 154.50 � 7.70 0.0001 155 � 6.80 0.0400 154.20 � 7.80 0.0010Weight (kg) 49.90 � 12.50 47.60 � 11.80 0.0040 49.50 � 10.60 0.7100 51.90 � 14.30 0.1200Body mass index (kg/m2) 20.20 � 4.00 19.80 � 3.80 0.0900 20.50 � 3.80 0.4800 21.60 � 4.90 0.0010

†East Asian versus Caucasian. ‡South Asian versus Caucasian. §Middle Eastern versus Caucasian. ¶logMAR presenting visual acuity(total letters read). D, dioptres; SD, standard deviation.

Table 3. Characteristics of the macula in different ethnic groups

Characteristics Caucasian East Asian South Asian Middle Eastern

Mean (95% CI) Mean (95% CI) Mean (95% CI) Mean (95% CI)

Thickness (mm)Foveal minimum 164.1 (162.1–166.0) 156.1 (153.1–159.1) 155.4 (151.3–159.6) 158.4 (154.6–162.2)Central macula 200.3 (198.4–202.2) 191.3 (188.5–194.1) 188.2 (184.4–192.0) 193.7 (190.2–197.2)Average inner macula 272.9 (271.5–274.3) 269.8 (267.7–271.9) 264.8 (261.8–267.7) 270.3 (267.6–272.9)Average outer macula 239.8 (238.6–240.9) 239.7 (237.8–241.5) 234.2 (231.5–236.8) 237.1 (234.7–239.4)

Volume (mm3)Total macula 6.97 (6.94–7.00) 6.94 (6.89–6.99) 6.79 (6.72–6.86) 6.89 (6.82–6.96)Central macula 0.157 (0.156–0.158) 0.150 (0.148–0.152) 0.148 (0.145–0.150) 0.152 (0.149–0.154)

Data are from mixed model adjusted for age, gender, height, axial length and cluster-sampling. Central macula, average inner maculaand average outer macula are concentric regions with radii of 0.5 mm, 1.5 mm, 3 mm, respectively. CI, confidence interval.

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Asian and Middle Eastern children for inner macularparameters. The ethnic differences seen in the fourquadrants of the inner macula were similar to thedifferences in the average inner macula (data notshown).

Caucasian children had an average outer macula5.6 mm (P < 0.0001) thicker than South Asian chil-dren and 2.7 mm (P = 0.03) thicker than MiddleEastern children (Table 4). East Asian children hadan average outer macula 5.5 mm (P = 0.0002) thickerthan South Asian children (Table 5). The Caucasianchildren had a larger total macular volume thanSouth Asian children (P < 0.0001) and MiddleEastern children (P = 0.02). East Asian children hada larger total macular volume than South Asian chil-dren (P = 0.0002).

DISCUSSION

In this population based sample of predominantly12-year-old children there were significant differ-ences between ethnic groups for OCT-measuredmacular parameters. These differences were greatestbetween Caucasian and South Asian groups. Thecentral macular region had the greatest differences

with Caucasian children having a significantlythicker central macula compared with the other threeethnicities.

In a previous report we compared macular param-eters between East Asian and Caucasian children atage 6 years.2 The foveal minimum was 8.2 mmthicker, the central macula was 9.3 mm thicker andthe average inner macula was 2.8 mm thicker (allP � 0.0005) in Caucasian children. The mean differ-ences seen in this younger population were remark-ably similar to those seen in our 12-year-old group,suggesting that ethnic variability in macular param-eters does not change during these childhood years.A study by El Ashry and associates11 found in theiradult British population that mean minimum fovealthickness in East Asians was 150.3 � 4 mm com-pared with 173.4 � 21 mm for Caucasians which isin agreement with our finding of thicker centralmacular parameters in Caucasians compared withEast Asians.

The mean minimal foveal thickness reported byEl Ashry and associates11 in their South Asian popu-lation (denoted as Indian) was 176.3 � 16 mm,which was very similar to their value for Caucasians(173.4 � 21 mm) and thicker than their value for East

Table 4. Mean differences in macular parameters between Caucasian and other ethnic groups

Characteristics Caucasian versus East Asian Caucasian versus South Asian Caucasian versus Middle Eastern

Difference (95% CI) P Difference (95% CI) P Difference (95% CI) P

Thickness (mm)Foveal minimum 8.0 (5.0–10.9) <0.0001 8.6 (4.5–12.7) <0.0001 5.7 (1.9–9.4) 0.0030Central macula 9.0 (6.3–11.7) <0.0001 12.1 (8.4–15.8) <0.0001 6.5 (3.1–10.0) 0.0002Average inner macula 3.0 (0.9–5.2) 0.0050 8.1 (5.2–11.0) <0.0001 2.6 (-0.1–5.3) 0.0500Average outer macula 0.1 (-1.8–2.0) 0.9300 5.6 (2.9–8.2) <0.0001 2.7 (0.2–5.1) 0.0300

Volume (mm3)Total macula 0.03 (-0.03–0.08) 0.3000 0.18 (0.11–0.25) <0.0001 0.08 (0.01–0.15) 0.0200Central macula 0.007 (0.005–0.009) <0.0001 0.010 (0.007–0.012) <0.0001 0.005 (0.002–0.008) 0.0002


Table 5. Mean differences in macular parameters between East Asian, South Asian and Middle Eastern groups

Characteristics East Asian versus South Asian East Asian versus Middle Eastern Middle Eastern versus South Asian

Difference (95% CI) P Difference (95% CI) P Difference (95% CI) P

Thickness (mm)Foveal minimum 0.64 (-3.8–5.1) 0.7700 -2.3 (-6.5–1.9) 0.2800 3.0 (-2.2–8.1) 0.2600Central macula 3.13 (-0.9–7.1) 0.1300 -2.4 (-6.3–1.4) 0.2100 5.6 (0.9–10.3) 0.0200Average inner macula 5.1 (1.9–8.3) 0.0020 -0.4 (-3.4–2.6) 0.7900 5.5 (1.8–9.2) 0.0040Average outer macula 5.5 (2.6–8.4) 0.0002 2.6 (-0.1–5.3) 0.0600 2.9 (-0.4–6.3) 0.0900

Volume (mm3)Total macula 0.15 (0.07–0.23) 0.0002 0.05 (-0.03–0.13) 0.1900 0.10 (0.01–0.19) 0.0300Central macula 0.002 (-0.001–0.006) 0.1300 -0.002 (-0.005–0.001) 0.2100 0.004 (0.001–0.008) 0.0200


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Asians (150.3 � 4 mm). The El Ashry findings con-trast with our finding of a significantly thinner fovealminimum in South Asian compared with Caucasianchildren, and that of no significant differencebetween South Asian and East Asian children. Thisdiscrepancy with our results may be due to the adultpopulation with a wide age range (21 to 81 years),and a lack of adjustment for confounding variablesand comparatively small sample size (n = 100) in theEl Ashry study. In another report of macular thick-ness in a South Asian population, Tewari and asso-ciates12 report a mean foveal minimum thickness of149.2 � 21 mm, similar to our value.

Many studies have documented normative valuesfor macular parameters in Caucasian13–15 and EastAsian16–18 populations. However, few studies areavailable for younger populations. In an Americanstudy, El Dairi and associates5 in a Caucasian popu-lation aged between 3 to 17 years (n = 154), reporteda Stratus OCT-measured mean central macular thick-ness of 198 mm (95% confidence interval 160–237mm), which is very similar to our value of 200.3 mm(95% confidence interval 198.4–202.2 mm). In anEast Asian population aged 11 to 12 years, Luo andassociates19 reported a Stratus OCT-measured meanminimum foveal thickness of 157.0 mm (95%confidence interval 119.4–194.6 mm), which isremarkably similar to our East Asians cohort of156.1 mm (95% confidence interval 153.1–159.1 mm).The similar values for central macular parametersfound in these studies add weight to our finding ofdifferences in central macular thickness betweenthese ethnic groups.

It is not clear why macular thickness measured byOCT varies by ethnicity. It has been speculated thatthe differences in melanin in the retinal pigmentepithelium in different ethnic groups could alter theOCT low-coherence laser light signal,3,20 as melaninscatters and absorbs light. This attenuated signalmay result in a decreased thickness measurementin more darkly pigmented individuals. Furtherresearch, including histological studies and studieswith higher resolution spectral-domain OCT, isneeded to determine how ethnicity impacts onretinal thickness and which retinal layers are par-ticularly affected.

Two recent papers have utilized the newer genera-tion spectral-domain OCT to examine ethnic differ-ences in macular morphology. Grover and associates21

and Wagner-Schuman and associates22 have bothreported a reduction in central macular thicknessin African-Americans compared with Caucasians,consistent with Stratus OCT reports. Grover andassociates,21 using the Spectralis OCT (HeidelbergEngineering, Vista, CA, USA), also examined Asiansubjects (ethnicity not further specified) and found athicker central macula (279.5 � 16.9 mm) compared

with their Caucasian group (272.7 � 20.8 mm), afinding that is in contrast with the present study.However, they had a small number of Asian subjects(n = 11) which decreases the validity of their findings.Also the use of the Spectralis OCT system by Groverand associates may mean comparability with StratusOCT studies may be invalid because of the differentboundary detection definitions used, with the inclu-sion of the outer segment retinal pigment epithelium-Bruch’s membrane complex by Spectralis OCT.23

The use of a large population based sample withstandardized measurement protocol is a majorstrength of this study. This predominantly 12-year-old sample is also largely free of ocular diseaseallowing unhindered measurement of true baselineretinal morphology. The disadvantage of using achildhood sample is the question of whether resultsare applicable to older age groups. Another limita-tion is that ethnicity was determined by self-report,which could result in a cultural rather than a bio-logical identification of individuals’ ethnicity.

In conclusion, we found that macular parametersvaried by ethnicity in 12-year-old children. Cauca-sian children had the thickest macular parametersand South Asian children had the thinnest. Thelargest differences existed in the central maculawhich was significantly thicker in Caucasian com-pared with East Asian, South Asian and MiddleEastern children.

ACKNOWLEDGEMENT

Supported by the Australian National Health &Medical Research Council, Canberra, Australia(Grant 253732).

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Ethnic differences in macular thickness