Normative data of optic disc topography in Thai population : a study in Ubolrat District, Khon Kaen Province

Abstract

Objectives: 1. To establish normal values of topographic optic disc parameters in Thai population. 2. To explore the relationship of age on optic disc topography. Study design: Crossectional, descriptive study Setting: Ubolrat District, Khon Kaen Province. Research methodology: Topographic measurements of the optic disc of 172 normal subjects between 30 and 60 years of age were acquired and evaluated using the Heidelberg Retina Tomograph (HRT). All subjects lived in Ubolrat District, Khon Kaen Province. From the computer database of the population in Ubolrat District, three 10-year age groups were stratified and then the subjects were selected from each age group by simple random sampling technique. The average of two eyes in the same subject was used as the unit of statistical analysis. Twelve topographic optic disc parameters computed by the HRT were evaluated for frequency distributions, normality, means, and percentiles. The 95% reference intervals (normal range) and their associated 95% confidence intervals were estimated using distribution-dependent method if the data were normally distributed, and distribution free method if the data were not normally distributed. The influence of age on topographic parameters was analyzed. Ethnic differences of the calculated optic nerve topography were compared with previous reports. Results: All 12 topographic optic nerve head parameters showed a unimodal distribution. Seven parameters were distributed normally. Their means and 95% reference intervals (in parentheses) were: disk area 2.296 mm2 (1.586, 3.006), rim area 1.854 mm2 (1.330, 2.379), height variation contour 0.395 mm (0.266, 0.525), rim volume 0.496 mm3 (0.258, 0.735), cup shape -0.241 (-0.361, -0.121), mean retinal nerve fiber layer thickness 0.256 mm (0.160, 0.353), and retinal nerve fiber layer cross section area 1.369 mm2 (0.859, 1.878). Five parameters were not distributed normally. Their medians and 95% reference intervals (in parentheses) were: cup area 0.406 mm2 (0.035, 1.092), cup/disc area ratio 0.164 (0.018, 0.379), cup volume 0.064 mm3 (0.002, 0.347), mean cup depth 0.174 mm (0.063, 0.321), and maximum cup depth 0.536 mm (0.188, 0.832). Most of the topographic optic disc parameters were independent of age. The relationships of age to some of the topographic optic nerve parameters were very weak and were not clinically relevant. The categorization of the topographic optic nerve parameters into age groups was not necessary. Ethnic differences in many topographic parameters were found among Thai and other race including Asians, Caucasians, African-Americans and Hispanics. Thai subjects had the smallest disc area, rim area and rim volume. Conclusion: Thai-specific normative data of optic disc topography including 95% reference intervals were estimated for age 30 to 60 years old. Age had no clinically relevant relationship to all of the topographic optic disc parameters. There were ethnic differences in the topographic optic disc parameters. Compared to other ethnic groups, Thai had the smallest disc area, rim area and rim volume. Ethnic-specific normative data of topographic measurements of the optic disc are needed to improve the diagnosis of glaucoma.