A comparison of relevant databases in the literature describing center point thickness or central subfield thickness in subjects without diabetes or in subjects with diabetes but no retinopathy, summarized in , shows that Stratus OCT™-measured thickness of the retina in diabetic subjects without retinopathy or with very mild retinopathy in the DRCR Network is similar to Stratus OCT™ data in healthy subjects in the literature6, 8
(Fraser-Bell S, et al. IOVS 2005; ARVO E-Abstract 1542) with respect to the central subfield or center point. These results are consistent with two studies using earlier OCT models.2, 3
This, however, is in contrast to two other studies which did not use an Stratus OCT™ machine which found increased retinal thickness in subjects with diabetes but no retinopathy. Lattanzio’s10
methods differ from the DRCR.net results not only in that an Stratus OCT™ machine was not used but also because the number of women in the group with diabetes but no retinopathy is unknown. It is possible that a greater proportion of men in that study could have resulted in thicker measurements. Using a retinal thickness analyzer, not an OCT, Fritsche’s data11
also suggested that the central subfield was thicker in subjects with diabetes but no retinopathy compared with subjects without diabetes, but was based on only 9 and 10 eyes, respectively.
Relevant Databases in Literature: Mean + SD Retinal Thickness
As expected anatomically, results from the DRCR Network showed thicker nasal subfields compared with temporal subfields. In contrast, the database from Fraser-Bell et al (Fraser-Bell S, et al. IOVS 2005; ARVO E-Abstract 1542) reported similar thicknesses for the nasal and temporal subfields, both for the inner zone and outer zone ().
Retinal Thickness Measurements using Stratus OCT™ in Current Study (Diabetic Subjects with Minimal or No Retinopathy) and from Healthy Eyes in Previous Studies
There were insufficient numbers of subjects in our study who were not Caucasian to permit evaluation of the data by race. However, in an exploration of the potential effect of other factors on OCT-measured central subfield thickness (), including presence of very mild retinopathy compared with no retinopathy, duration of diabetes, type of diabetes, age and refractive error, only gender was found to have an effect, with mean central subfield thicknesses greater in men than women. These results were similar to Massin,3
and Wong 7
who used an OCT that was not an Stratus OCT™, and reported that average central subfield thicknesses were greater in men than women without diabetes, but did not describe whether there were gender differences for subjects with diabetes and no retinopathy. This was confirmed in the normative database presented by Fraser-Bell et al who found a difference in mean central subfield thickness of 9 microns, compared to 15 microns in the current study. (Fraser-Bell S, et al. IOVS 2005; ARVO E-Abstract 1542). It is unknown why this gender difference was not detected in the 37 eyes from subjects without diabetes reported by Chan and colleagues in which the mean central subfield values were similar in men and women.9
Since only about one-third of the subjects in the report by Chan and colleagues were men, one might have expected a thinner mean central subfield thickness compared with other reports.
The consistency of data across studies appears large enough to consider separating norms by gender when designing clinical trials evaluating diabetic macular edema based on OCT and determining upper limits of normal for eligibility and outcomes. For example, consider a study of diabetic macular edema with an eligibility requirement of a central subfield thickness of at least 250 microns. While this value would be more than 2 standard deviations beyond the norm for both men and women, it would be a greater amount of edema compared with norms for women than for men. Such a cutoff would mean that some women with true edema more likely would be excluded from entry than men. Further, if one chooses treatment criteria based on a central subfield thickness of 250 microns or less as a level to withhold additional treatment, one would be more likely to withhold treatment in an eye that might still have true edema in a woman than a man. This 15 micron difference between the means of the two populations
() should not be equated with a 15 micron difference in an individual patient
between one visit or measurement and a subsequent visit or measurement. A 15 micron difference between measurements in an individual
might be considered within the reproducibility of the measurement,15
whereas this difference between the means of two populations
represents groups with quite different distributions of thicknesses from which the means are derived ().
The Network study has a few potential limitations that should be considered. First, myopic refractive errors that were associated with retinal pathology were to be excluded from this study; reports have indicated that the eyes of such individuals may have altered macular thickness (Fraser-Bell S, et al. IOVS 2005; ARVO E-Abstract 1542). Second, there were few blacks or Latinos enrolled to be able to generalize the results to these groups. Third, extreme outliers were excluded from this study and subjects in this study were culled from practices in which some patients likely were referred to an ophthalmologist specializing in retinal diseases, and thus, this cohort may not be a representative sample of the population of all patients with diabetes and no or very mild retinopathy.
In conclusion, this study showed that Stratus OCT™-measured thickness of the retina in diabetic subjects without retinopathy or with very mild retinopathy was comparable to previously published central subfield Stratus OCT™ data from individuals without diabetes, which is consistent with other reports using earlier OCT models comparing similar populations. Our data suggest that the nasal inner and outer zones are thicker than the temporal inner and outer zones. This is consistent with some published reports6, 9
but in contrast to the normative data for the Stratus OCT™ software. (Fraser-Bell S, et al. IOVS 2005; ARVO E-Abstract 1542) Our study also showed that the average central subfield, but not the inner or outer zones, was thicker in men compared with women, again consistent with previously published reports in individuals without diabetes. The gender differences appear large enough to consider separating norms by gender when designing clinical trials evaluating diabetic macular edema based on OCT and determining upper limits of normal for eligibility and outcomes.