J.A. Elledge, M.D. Davis, Larry D. Hubbard, James L. Reimers, C.A. Fink, D.G. Hafford, R.A. Susman

Abstract

Purpose: To compare classic and modern assumptions regarding disc dimensions, as documented in fundus photos, and explore their impact on linear and area measurements of abnormalities. To investigate distance from disc center to macular center (D-M) as a potential calibration factor.

Methods: Long-standing clinical convention assumed that the disc is 1.5mm/1500μm. Historical studies accepted this figure, and pragmatically established nearly identical standardized disc diameters (DD) for application to slides from the Zeiss FF2-4 fundus camera: ETDRS/AREDS, 4.7 mm; MPS, 4.5 mm. Although the standard DD as an established unit remains unchanged by our measurement of a more recent sample, careful observation by other studies (e.g., Jonas JB et al., 1988; Mansour AM, 1990) indicated 1 DD = 1.8-1.9 mm would be a more accurate equivalence in vivo. As modern technology required a more accurate equivalence, various modern digital camera systems have now adopted 1DD=1.8mm as their equivalence.

Results: Measurement of 174 eyes (87 subjects) on Zeiss photos yielded tentative results of 1 DD=4.6 mm and average D-M=2.54 DD. Across subjects, measured D-M was half as variable as measured DD proportionately. Having refined our technique, we are re-measuring the sample and will present final results. Following are in vivo dimensions and areas for various features under different assumptions:
Assumption Equivalence 125μm druse 1 DA zone of DME
“Classic” 1 DD= 1.5mm 125 μm 1.77 mm2
“Modern” 1 DD= 1.8mm 150 μm 2.54 mm2
Best Estimate 1 DD ~ 1.9mm ~158 μm ~2.84 mm2

Conclusions: Although the standardized DD remains an absolute and reliable measure, its equivalence in mm/μm varies according to classic, modern, or best estimate assumptions. Confusion about underlying assumptions can result in uncertainty and perhaps unintentional differences – e.g., the Stratus OCT 6 mm macular grid appears to be actually that, while the ETDRS macular grid with 2DD diameter is 7.2 mm in some digital camera systems.