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.
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