The concordance of strabismus in monozygotic twins was examined in order to study the role of genetics in the different types of strabismus.

The medical charts of 39 pairs of monozygotic twins (63 of 78 subjects had strabismus) dated between May 1985 and December 2005 were reviewed retrospectively. We analyzed each case by refraction, type of strabismus, age of onset, amount of deviation, and stereopsis.

Twenty-three of 39 pairs of twins (59%) showed phenotypic concordance; 15 of 21 pairs showed intermittent exotropia (71%), five of nine pairs showed infantile esotropia (55%), and three of four pairs showed partially accommodative esotropia (75%). All pairs with accommodative esotropia showed discordance. One discordant pair showed infantile esotropia, one expressed infantile esotropia, and another expressed sensory exotropia due to congenital cataract.

The concordance rate of monozygotic twins was 59% in this study. Partially accommodative esotropia and intermittent exotropia had high concordance rates of strabismic phenotypes in monozygotic twins. Based on the results of this study, it is suggested that there may be a strong genetic component regarding these types of strabismus. Concordant pairs of monozygotic twins showed similarity in onset, deviation angle, postoperative result, and recurrence.