Background: Monozygotic (MZ) or "identical" twins arise from a single fertilized egg, which divides into two embryos at an early stage of development. As a result, MZ twins have identical genomes and are always of the same sex.
Methods: A case of optic nerve hypoplasia and anisometropia, in association with mirror-image presentation in a set of 12-year-old identical twins, is reported. The monozygotic twinning event responsible for identical twins--as well as the rare phenomenon of mirror imaging--is described.
Results: The combined occurrence of anisometropia and optic nerve hypoplasia in mirror-image presentation in a set of monozygotic twins provides a unique opportunity to study the genetic versus environmental influences on the development of optic nerve hypoplasia.
Conclusions: Although the cause of optic nerve hypoplasia remains unclear, its associated mirror-image presentation in this case suggests a possible genetic predisposition.