Analysis of Optic Discs From the Adult Twin Eye Study in Tasmania

Abstract

Abstract: : Purpose: To investigate the degree of heritability in optic disc morphology, by comparing cup-disc areas in a pilot study of mono(MZ) and dizygotic(DZ) twins. Methods: 36 MZ and 19 DZ twin pairs were studied from the Australian NHMRC Twin Registry. MZ twins ranged from 16 – 51 years of age (mean 34) for and DZ twins ranged from 15-49 years of age (mean 29). All participants underwent a comprehensive ophthalmic examination which included autorefraction, keratometry, and simultaneous stereo disc photography with a Nidek 3Dx Stereo Fundus Camera. Images were digitized at high resolution and viewed stereoscopically (16-bit true color) using a Z-screen (Stereographics Corp, USA). Custom software was used to plot, stereoscopically, both the neuroretinal and scleral rim for each optic disc image. Cup, neuroretinal and disc area were calculated at five degree segments around the disc. Structural Twin heritability was analysed using the structural equation modeling package Mx. Results: The mean cup and disc areas did not differ significantly between MZ and DZ twin pairs and there was no significant difference in variance between twin types. The pattern of correlations suggested genetic influences for both cup and disc area (MZ : DZ correlation ratio; 0.639: 0.345 for cup area and 0.581: 0.328 for disc area). The results of genetic model fitting supported this, but could not distinguish significantly between genetic and shared environmental effects. However, for both variables, Akaike’s Information Criterion (AIC), suggested that a model including genetic and non-shared environment only would provide the best explanation of the data. Conclusions: The data suggest genetic and shared environmental influences on both cup and disc areas. Although the sample size is insufficient to distinguish significantly between these two influences as a source of twin similarity, the best fitting model, based on the most AIC value suggests that genetic influences are more important.

Keywords: optic disc • genetics • imaging/image analysis: clinical