Strabismus monozygotic and dizygotic twins

The occurrence of strabismus is of interest due to the concept of an hereditary predisposition. Twin studies are valuable in the investigation of strabismus and its cause. There are two conflicting theories on the etiology of comitant strabismus. One theory expresses the motor mechanism as the causative factor, and the opposite view expresses the sensory system as the cause. The motor theory holds that there is a basic muscle disorder or abnormal neurologic tone. The sensory school maintains that poor or no potential for single binocular vision is the underlying etiology. A combination of abnormal sensory and motor input may be important. There is a great variation in individual phenotypes. Therefore, the concordance or similarity of a trait in monozygotic twins, as opposed to dizygotic twins, is of importance. A prevalence of concordance would indicate the familial character of the strabismus or the trait being studied. Also the relative incidence of a trait in monozygotic versus dizygotic twin pairs can provide supportive evidence for a particular type of inheritance. Several studies have reported concordance rates for monozygotic and/or dizygotic twins with strabismus. There are also large scale family studies of strabismus available. As our knowledge of genetics has advanced and data has accumulated, the type of postulated inheritance in strabismus has evolved.