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Racial Differences in Macular Thickness in Healthy Eyes
ASEFZADEH, BAHARAK OD; CAVALLERANO, ANTHONY A. OD, FAAO; FISCH, BARRY M. OD
VA Boston Healthcare System, Jamaica Plain Campus, Jamaica Plain, Massachusetts, and New England College of Optometry, Boston, Massachusetts
The authors hold no commercial, financial, or proprietary interests related to this article or Optical Coherence Tomography (Carl Zeiss Meditec, Dublin, CA).
Received September 22, 2006; accepted June 8, 2007.
Purpose. The relationship between race and macular thickness remains unknown. This relationship may be important for early and accurate diagnosis of macular disease and glaucoma, and may also provide insight into disease mechanisms. In this study, we compared macular thickness in healthy eyes of black and white subjects using optical coherence tomography (Stratus OCT).
Methods. This study used a matched, cross-sectional design. Subjects underwent OCT macular thickness map scanning in each eye, four-field, 45-degree digital retinal imaging in each eye, and blood pressure measurement. Retinal images were evaluated for absence of posterior pole disorders, including macular and optic nerve disease. Retinal thickness was evaluated in the central fovea, and in rings placed at 1, 3, and 6 mm from fixation.
Results. Compared with whites (n = 7), blacks (n = 7) had significantly thinner total foveal thickness (TFT, retinal thickness in the central 1 mm diameter area; OD: p < 0.03; OS: p < 0.02; OU average: p < 0.02), and thinner total macular thickness (TMT, retinal thickness in 6mm diameter area excluding central foveal thickness; OS: p < 0.02; OU average: p < 0.03). There was a trend for central foveal thickness (retinal thickness at fixation) to be thinner in blacks than whites (OD: p = 0.12; OS: p = 0.08). There was no significant difference in macular thickness between right and left eyes.
Conclusions. Retinal thickness as measured by Stratus OCT in the fovea and macula is significantly thinner in blacks compared with age-matched whites. Larger multiracial prospective studies are needed to confirm these results and to evaluate the need for race-specific normative values.
The relationship between race and normal macular thickness remains poorly characterized. Diagnosis of macular abnormalities by optical coherence tomography (OCT) often depends on comparison with normative values. Although the normative database of the Stratus OCT includes subjects of several racial backgrounds, most of the subjects were whites (personal communication, Carl Zeiss Meditec). If they exist, race-dependent differences in normal retinal thickness should be taken into account for accurate diagnosis of macular disease.
Glaucoma and diabetic macular edema are more prevalent in blacks than in whites,1–3 and can affect macular thickness. In glaucoma, loss of retinal nerve fibers leads to decreased total macular thickness (TMT),4–7 whereas in diabetic macular edema, accumulation of intraretinal fluid causes an increase in TMT.8,9 Identification of racial differences in macular thickness in healthy eyes may be useful to predict early or subclinical macular edema or glaucoma, and may also provide insights into disease mechanisms. As a first step towards this goal, we compared mean macular, foveal, and central foveal thickness (CFT) in healthy eyes of blacks and whites.
This was a matched, cross-sectional study. Subjects were recruited as they presented for a routine eye examination at the Jamaica Plain Veteran’s Affairs Medical Center. Sample size was determined prospectively for adequate statistical power to detect a difference between the means in blacks and whites. The study protocol followed the tenets of the Declaration of Helsinki and was approved by the VA Boston Institutional Review Board and Research and Development Committee. All subjects provided informed consent. Black and white subjects were matched for age and gender, and similar refractive error and mean arterial blood pressure (MAP). White subjects were matched to black subjects without regard for retinal thickness data.
Subjects were excluded from the study if they met any one of the following criteria: (1) best-corrected visual acuity worse than 20/70 in either eye due to media opacity; (2) any macular disorder, including diabetic macular edema and drusen; (3) abnormal Amsler grid testing; (4) previous retinal laser treatment (e.g., focal or scatter photocoagulation); (5) central visual field defect preventing fixation on OCT; (6) poor quality fundus or OCT images; (7) diagnosis of diabetes; (8) diagnosis of impaired glucose tolerance or impaired fasting glucose; (9) intraocular inflammation; (10) diagnosis of glaucoma; (11) previous intraocular surgery apart from cataract extraction; (12) cataract extraction within the past 12 months; (13) refractive error greater than or equal to 6 D, or (14) inability or unwillingness to provide informed consent.
We obtained self-report of race and ethnicity. Medical and ocular histories were obtained from the VA computerized patient record system (CPRS). All subjects underwent pupillary dilation. Optical coherence tomography (Stratus OCT, Zeiss Meditec, USA) macular thickness mapping was acquired in each eye, in addition to four-field, 45-degree digital retinal imaging (Canon, USA) in each eye obtained according to VA digital imaging standards.10,11 All subjects also underwent blood pressure measurement with the Omron IC Intellisense Digital Blood Pressure Monitor after imaging was completed. Snellen visual acuity, Amsler Grid, refractive error, and intraocular pressure were recorded from the CPRS comprehensive eye examination notes from the same day as the study.
Macular thickness was determined at 37 points along six, 6-mm line scans (Fig. 1). Each data point was automatically calculated by entering predetermined axial scan numbers along each of the six lines using the retinal thickness analysis protocol (Fig. 2). Mean CFT for each subject was calculated as the average of measurements from the midpoint of each individual line scan (data point 1 in Fig. 1b). Mean total foveal thickness (TFT) was calculated as the average of measurements at 12 points along a ring at 1000 μm from fixation and the average CFT from the midpoint of each individual line scan (data points 2–13 in Fig. 1b). Mean TMT was calculated as the average of all values within the 6000-μm ring with the exclusion of central foveal measurements (data points 2–37 in Fig. 1b). Average macular retinal nerve fiber layer (RNFL) thickness was measured using the RNFL thickness analysis protocol.
Digital fundus images were evaluated by optometrists who were trained and certified in digital retinal image reading for diabetic retinopathy.
Continuous data were analyzed by two-tailed, unpaired Student’s t-test. Pearson’s correlation coefficient was used to assess the relationship between macular thickness and age, and macular thickness and MAP.
Seven non-Hispanic black subjects and seven age-matched non-Hispanic white subjects were evaluated. Table 1 summarizes subject characteristics. All subjects were male. No significant differences in mean systolic and diastolic arterial blood pressure, spherical equivalent refraction, and intraocular pressure were found between blacks and whites. Table 2 summarizes individual subject characteristics and retinal thickness values. No subject was myopic. Spherical equivalent refractions ranged from plano to +2.75 D. Average MAP was 100 ± 13 and 99 ± 10 mm Hg for blacks and whites, respectively (p = 0.85). Additionally, there were no significant differences in CFT, TFT, or TMT between right and left eyes in either group.
Table 3 summarizes the retinal thickness data. Mean TFT and mean TMT were significantly thinner in blacks than in whites (TFT OD: p < 0.03; TFT OS: p < 0.02; TFT OU average: p < 0.02; TMT OS: p < 0.02; TMT OU average: p < 0.03). There was a trend for CFT to be thinner in blacks compared with whites (p = 0.12 OD; p = 0.08 OS). Average macular RNFL thickness in blacks was 27 ± 2 μm OD, 26 ± 3 μm OS compared with 32 ± 6 μm OD and 30 ± 6 μm OS in whites (p = 0.09 and p = 0.12, respectively). The numbers of subjects with detached, partially attached, and attached vitreous were similar in both groups. No correlation between average retinal thickness OU and MAP was found in either group.
As retinal quantification becomes more prevalent in clinical decision making, it is important to investigate racial differences in normative data. In the present study, we found that blacks have significantly thinner foveal and macular thickness compared with whites.
Previous studies of healthy eyes demonstrate that mean peripapillary RNFL is similar in blacks compared with whites,12 whereas other studies show that it is significantly thinner.13,14 A potential hypothesis regarding why TMT was thinner in blacks compared with whites in our study is thinner RNFL. Peripapillary RNFL correlates with TMT.15,16 Retinal thickness in the macula as measured by OCT includes the RNFL. Mean macular RNFL thickness has been reported as 30.44 ± 4.11 μm in normal Chinese eyes17 and 44.8 ± 14.8 μm in a normal Latino population.18 In the present study, there was a trend for average macular RNFL thickness to be thinner in blacks compared with whites (p = 0.09 OD and p = 0.12 OS). However, this corresponded to only an approximately 5-μm difference. It therefore does not seem likely that thinner RNFL alone fully accounts for the approximately 20 to 30 μm difference we observed in TFT and TMT in blacks compared with whites, especially because no subject had glaucoma or other disease affecting the RNFL. Furthermore, we found a trend for thinner retinal thickness at fixation (CFT), a measurement that does not include the RNFL, in blacks compared with whites (p = 0.12 OD; p = 0.08 OS). Although these approximately 24 to 27 μm differences approached but did not meet statistical significance, they may suggest race-specific outer retinal thickness variation. Further studies with a larger sample size are required to address these issues.
Axial length and refractive error may cause differences in OCT macular thickness measurements. Increasing myopia is associated with reduced macular thickness.19 However, there is no correlation between retinal thickness and amount of myopia when high myopes are excluded.20 We did not measure axial length in our study, but did record refractive error for each subject. Important features of our sample are that no eyes were myopic, and all spherical equivalent refractions ranged from plano to +2.75 D. There was no significant difference in refractive error between blacks and whites. Therefore, it is unlikely that refractive error played a role in the differences we observed between groups.
It is not known if systemic blood pressure affects macular thickness in normal eyes. Our sample had a relatively narrow range of blood pressure measurement, because all subjects with systemic arterial hypertension were well-controlled medically. We found no correlation between MAP and macular thickness in blacks and whites.
Further study of larger samples will help to confirm the observations of this study. It will also be important to study these phenomena in women and children. Although a previous study found significantly greater average macular thickness in the central 1000 μm diameter area in men compared with women,21 more recent studies demonstrate no difference in macular thickness between men and women.22,23 Although we were able to evaluate blacks and whites, our population did not include any Asian, Native American, Mexican, or Indian subjects. Examination of macular thickness in normal eyes of these populations may also be of great value due to their higher risk for diabetes24 and subsequent diabetic eye complications.25
In conclusion, this study found that foveal thickness and macular thickness are significantly thinner in normal eyes of non-Hispanic blacks compared with non-Hispanic whites. Race- specific differences in macular thickness are of relevance in the assessment of a variety of disorders affecting retinal thickness, ranging from diabetic macular edema to glaucoma. Thinner baseline macular thickness must be taken into consideration when interpreting Stratus OCT macular scans in black patients, especially in cases of early macular edema in which subtle thickening may be masked. Additional multiracial studies with larger sample sizes are needed to confirm these results and to evaluate the need for race-specific normative values.
The authors thank Dr. Todd Otani of Carl Zeiss Meditec for information regarding the Stratus OCT Macular Thickness Normative Database.
The VA Boston Ocular TeleHealth Center
VA Boston Health Care System, Eye Clinic
8th Floor, 150 South Huntington Avenue
Jamaica, Plain, MA 02130
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macular thickness; optical coherence tomography; retinal thickness; race
© 2007 American Academy of Optometry
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