Study shows that amblyopia, strabismus, female sex, lower gestational age and lower birth weight are significantly higher in twins. Photo: Tim Bish/Unsplash.

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This story was originally published June 5, 2023.

No. 26 biggest news story of 2023:

Studies have shown that genetic and environmental factors have substantial roles in child development and manifestation of various ocular features, as well as congenital anomalies, particularly in monozygotic twins. In this study, researchers aimed to compare the epidemiological and ocular findings of twin children in comparison with non-twin age-matched individuals as controls and found that amblyopia, strabismus and female sex are significantly more common in twins.

A total of 90 twins (180 cases) were compared with 182 non-twin-matched children. Among the twins, 27 (30%) were monozygotic (identical) and 63 (70%) were dizygotic (non-identical). Thirty-six (66.7%) of the monozygotic and 72 (57.1%) of the dizygotic twins were female. All participants underwent a comprehensive ophthalmic examination including measurement of BCVA, cycloplegic refraction, ocular deviation and strabismus, as well as anterior and posterior ophthalmic examinations. Mirror-image twin (MIT) was defined according to the laterality of symmetrical ocular characteristics of twins.

Refractive form of MIT was seen in five twins (2.8%). The spherical refractive error was more hyperopic in twins compared with non-twins. BCVA in the twin group was significantly worse than non-twins with a higher percentage amblyopic (37.2% vs. 10.4%). Twins and controls had strabismus in 17.2% and 1.6% of cases, respectively. Regarding the comparison between monozygotic and dizygotic twins, a more significant percentage of monozygotic twins had amblyopia and strabismus.

Amblyopia was higher in twins possibly due to their more prevalent hyperopia, and a higher percentage of amblyopia was also seen in monozygotic twins compared with dizygotic ones, although there was no significant difference between spherical equivalent values. This may be related to more premature cases in twins, lower gestational age (n=56), lower birth weight (n=22) and history of strabismus surgery (n=28) as the results of a previous study reported more retinopathy of prematurity, myopia and strabismus in prematurity in twin pregnancies. Five twins showed refractive mirror image; of them, three were monozygotic and three were amblyopic, but none were anisometropic.

Previous studies defined strabismic mirror image as right esotropia in one child and left esotropia in the other child of the same set of twins, but the authors of this study did not see strabismic mirror image.

“The reason could be due to different genetic factors in various societies and also due to our less sample size,” the authors explained. “In our study, there was 2.36-fold more history of strabismus surgery of twins compared with non-twins with 4.5% attributed to being twins. In addition, a higher percentage of strabismus was found in twins compared with non-twins, especially in monozygotic individuals.”

Lastly, female sex, low birth weight and seizures showed significant association with lower gestational age in multivariate analysis.

“In conclusion, female sex, less gestational age, low birth weight, amblyopia and strabismus were significantly higher in twins,” the authors concluded. “Therefore, it is important to check their refractive error, amblyopia and strabismus to prevent their further complications.”