Purpose
To assess the influence of genetic and environmental factors on optic structure, comparing correlations between monozygotic (MZA) and dizygotic (DZA) twins separated at birth.
Methods
This is a retrospective review of twin pairs from the Minnesota Study of Twins Reared Apart (MISTRA). Medical records and optic disc stereo photographs from twins that had been separated at birth were reviewed. Only patients without glaucoma or optic nerve diseases were included. Vertical disc diameter (VD), vertical cup (VC), vertical cup to disc ratio (VCDR), superior rim (SR), inferior rim (IR), horizontal diameter (HD), horizontal cup (HC), nasal rim (NR), temporal rim (TR) and horizontal cup to disc ratio (HCDR) were measured with computer software by the same masked experienced ophthalmologist and values were correlated in monozygotic (MZ) and dizygotic (DZ) twins.
Results
A total of 280 eyes from 70 twin pairs (44 MZA and 26 DZA pairs) were included. Mean MZA age was 38.45 ± 12.93 and mean DZA age was 42.38 ± 12.78 years old (p=0.014). Visual acuity was 1.08 ± 0.24 in MZA and 1.13 ± 0.21 in DZA twin pairs (p=0.147) and cup to disc ratio, as documented in the clinical chart, was 0.26 ± 0.16 mm in MZA and 0.23 ± 0.18 in DZA pairs (p=0.129). There were no statistically significant differences between digital optic nerve measurements in the MZA and DZA twins. The measures for MZA and DZA twin pairs, respectively, were 1.82 ± 0.21 mm and 1.80 ± 0.17 mm (p=0.364), VC 0.49 ± 0.31 and 0.45 ± 0.32 (p=0.363), VCDR 0.26 ± 0.18 and 0.25 ± 0.16 (p=0.542), SR 0.64 ±0.12 and 0.64 ± 0.14 (p=0.753), IR 0.71 ± 0.12 and 0.72 ± 0.13 (p=0.472), HD 1.60 ± 0.2 and 1.59 ±0.21 (p=0.730), HC 0.42 ± 0.29 and 0.42 ± 0.31 (p=0.813), NR 0.58 ± 0.11 and 0.57 ±0.08 (p=0.241), TR 0.52 ± 0.13 and 0.55 ± 0.17 (p=0.069) and HCDR 0.25 ±0.17 and 0.25 ±0.17 (p=0.845). MZA twin pairs had statistically significant correlations for all measured disc parameters: VD 0.844, VC 0.790, VCDR 0.750, SR 0.639, IR 0.599, HD 0.746, HC 0.824, NR 0.511, TR 0.647 and HCDR 0.733. In contrast, DZS twins had statistically significant correlations for the following measures VD, HD and TR (correlation coefficients: VD 0.673, VC 0.222, VCDR 0.227, SR 0.164, IR 0.366, HD 0.570, HC 0.315, NR 0.153, TR 0.683 and HCDR 0.337).
Conclusions
There is a high correlation between optic disc structural parameters in twins reared apart that it is stronger in MZA than DZA twin pairs.