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F. Carbonaro, D. A. Mackey, T. Spector, C. J. Hammond; The Heritability of Intra-Ocular Pressure and Corneal Hysteresis: A Classical Twin Study. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2885.
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Corneal biomechanical properties influence the results and outcomes of intraocular pressure (IOP) measurements. Central corneal thickness (CCT) has become an important factor in the evaluation of glaucoma. Other corneal tissue properties such as Corneal Hysteresis (CH), the result of viscous damping in the corneal tissue, and a measure of the overall "resistance" of the cornea, the corneal resistance factor (CRF), can be measured, and used to calculate corneal-corrected IOP (IOPcc). This IOP, which is affected less by corneal properties than other measuring techniques, may be more accurate in glaucoma assessment. The purpose of this study was to assess the relative importance of genes and environment on these corneal factors.
Monozygotic (MZ) and Dizygotic (DZ) twin pairs from the TwinsUK Adult Twin Registry, were examined using the Reichert Ocular Response AnalyserÂ® (ORA). Corneal Hysteresis (CH), Central Corneal Thickness (CCT), Corneal Resistance Factor (CRF), Goldmann-correlated IOP (IOPg), and Corneal-Compensated Intraocular Pressure (IOPcc) were recorded. Heritability of the parameters, the amount of variation explained by genetic factors (h2), was calculated using a structural equation modelling program (Mx).
86 pairs of twins (41 MZ, 45 DZ) were examined, with a mean age 50.5 years (range 16-76). Most were female (90%) and all were Caucasian. Mean IOPg was 15.4mmHG (SD 3.24), and mean CH was 10.39 mmHg (SD 1.55); this was inversely related to age (r=-0.36, p<0.001). Within-pair correlations for MZ pairs (rMZ) were higher than DZ pairs (rDZ), suggesting genetic influences. Modeling suggested CCT was highly heritable (rMZ 0.91, rDZ 0.51, h2 0.92), as were IOPg (rMZ 0.80, rDZ 0.48, h2 0.77) and CRF (rMZ 0.81, rDZ 0.49, h2 0.82). However, CH was less heritable, with a significant "shared environment" (c2) effect (rMZ 0.79, rDZ 0.59, h2 0.28, c2 0.37). This may, in part, be explained by the strong age effect, as part of the twins' shared environment is the sharing of age. Further modeling with larger numbers of twins will be able to explore this.
Genetic factors are important in determining IOP and corneal properties in this cohort of British twins. While central corneal thickness and the corneal resistance factor measured by ORA are highly heritable, corneal hysteresis seems less so, which may be explained by its strong inverse relationship with age. Further research is required to explore this relationship, and factors influencing hysteresis.
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