Discordant Type I Preaxial Polydactyly in Monozygotic Twins on the Same Hand: A Case Report.

There are 4 types of preaxial polydactyly (PPD), and type I (PPD-I), also known as thumb duplication, is the most common. This frequently encountered condition has since been further described and classified by Wassel based on the level of duplication of skeletal anatomy. Genetic studies have localized possible candidate gene(s) for PPD types II to IV to the chromosomal region 7q36 but the current literature attributes PPD-I to isolated, spontaneous mutations typically with unilateral involvement only. Recent epidemiological studies have also suggested nongenetic causes for PPD-I including social and environmental factors. Herein, we would like to present a case of discordant PPD-I expression affecting the same hand in a pair of monozygotic twins and suggest possible reasons for this presentation because previous similar reports only had involvement of 1 child.