Concordance of strabismic phenotypes in monozygotic versus multizygotic twins and other multiple births

Purpose: The concordance of strabismic phenotypes was examined in monozygotic versus multizygotic twins and other multiple births to study the role of genetic background in different types of comitant strabismus. Methods: Medical charts of 45 consecutive pairs of twins (16 monozygotic and 18 dizygotic twins, and 11 with unknown zygosity), 3 sets of triplets (one monozygotic and 2 trizygotic triplets), and one set of quadruzygotic quadruplets examined at 6 institutions between 1973 and 1999 were reviewed retrospectively. The concordance was defined as both or all members having either esotropia or exotropia in common. Results: The concordance of strabismic phenotypes was noted in 33 of 49 pairs or sets (67.3%): 14 of 17 monozygotic twins or triplets (82.4%), 10 of 21 multizygotic twins, triplets, or quadruplets (47.6%), and 9 of 11 twins with unknown zygosity (81.8%). The concordance rate was significantly higher in monozygosity than in multizygosity (P = .043, Fisher exact probability test). The predominant concordant phenotypes in monozygosity were accommodative esotropia and intermittent exotropia. Conclusion: A high concordance rate of strabismic phenotypes, predominantly of accommodative esotropia and intermittent exotropia in monozygosity, suggests the genetic background for these types of strabismus.