The twins and non- twins comparison studies are the best way for determination of genetic factors of ocular disorders. Refractive mirror image is one of the strong genetic similarities of twins in addition to their identical phenotype and sex. These studies can also detect environmental factors more obviously.
In the present study, epidemiologic and ocular findings of twins were compared with non- twins who were age and sex matched. There were 30% monozygotic and 70% dizygotic twins by considering their phenotype and sex similarities. Thirty- six (66%) of monozygotic twins and 72 (57%) of dizygotic twins were female (108 out of 184). Female to male ratio was approximately 2 folds in monozygotic and 1.3 folds in dizygotic twins, whereas this ratio was near 1 (female, 46.2%) in non- twins children. Other reviews did not stress on gender of their study subjects.
Five (5%) twins showed refractive mirror image, of them 3 (60%) were monozygotic and 3 (60%) were amblyopic, but no one was anisometropic.
Monozygotic twin prevalence is 2/1000 of the general population and only 25% of monozygotic twins shows mirror image (5/100000 of the general population) 1,3,4. Therefore, few case reports of mirror image were found in the literature which most of them had anisometropic refraction and were amblyopic 5 as ours. Some of other case reports presented strabismic mirror image by defining right esotropia in one child and left esotropia in the other child of the same twins 1, 6, but we did not see any of them in our study. The reason could be due to different genetic factors in various societies and also due to our less sample size.
Although ocular anomaly, physical delay, seizure and macular hypoplasia were more in twins compared to non- twins, the difference was not statistically significant in our study.
Yu et al 7 reported 38 out of 1000 birth anomaly on 20803 twins in California with more prevalence in monozygotic twins. Odds ratios were 5.91, 2.52 and 3.48 for clubfoot, strabismus and spina bifida, respectively.
In literature, parental smoking had significant association with strabismus as an environmental factor 7. In our study, there were 2.36 folds more history of strabismus surgery of twins compared to non- twins with 4.5% attributed to being twins.
Yu et al concluded that birth abnormalities are multifactorial and both genetic and environmental factors could be effective together 7.
Although there was no significant difference between twins and non- twins regarding their spherical equivalent and anisometropia, amblyopia was higher in twins. Its reason may be related to more premature cases in twins, (less gestational age (n = 57) and low birth weight (n = 22)), and history of strabismus surgery (n = 28) as the results of the study by Bornstein et al 15 that reported more retinopathy of prematurity, myopia and strabismus in prematurity in twin pregnancies.
Although being female, low birth weight, having neonatal jaundice, physical delay, amblyopia and seizure were found to have significant association with less gestational age in the univariate analysis, only being female, having low birth weight and seizure showed significant association with less gestational age in multivariate analysis.
30% of our twins were phenotypically very similar, while phenotypic correlation were reported 68% and 40% among monozygotic and dizygotic twins in the study by Zhang et al 10, the reason could be related to the determination of the degree of appearance similarity which is a reliable matter. We did not study the similarity levels of central corneal thickness 11, optic nerve characteristics 8, based on the laboratory tests between twin and non- twins as Toh et al 11 and He et al 8 studies.
The reason was the younger age of our cases for performing these tests in the most cases (central corneal thickness, optic nerve tomography), however, this is one of the limitations of our study.
In conclusion, female sex, less gestational age, low birth weight and amblyopia were significantly higher in twins. In multivariate analysis, female sex, low birth weight and seizure showed significant correlation with less gestational age. In addition, we suggested looking for refractive mirror image and its accompanying anisometropia and amblyopia to prevent their further problems.