Having an extra finger or toe is called polydactyly. The extra digit may range from a small, raised bump to a complete, working finger or toe. Most of the time, it’s smaller than the other digits and not well-formed.
Sometimes the extra digit is only skin (a nubbin), and it connects to the hand or foot with only a narrow stalk of tissue. If the digit is better formed, it may have all the normal tissues, such as bone, muscles, blood vessels, and nerves. In this case, it connects to the hand or foot deep inside.
Polydactyly is also called supernumerary digit. “Supernumerary” means “more than the normal number.”
Polydactyly, also known as hyperdactyly or hexadactyly is the most common hereditary limb anomaly characterized by extra fingers or toes, with various associated morphologic phenotypes as part of a syndrome (syndromic polydactyly) or may occur as a separate event (non-syndromic polydactyly). Broadly, the non-syndromic polydactyly has been classified into three types, i.e.; preaxial polydactyly (radial), central polydactyly (axial), and postaxial polydactyly (ulnar). Mostly inherited as an autosomal dominant entity with variable penetrance and caused by defects that occur in the anterior-posterior patterning of limb development. In humans, to-date at least 10 loci and six genes causing non-syndromic polydactyly have been identified, including the ZNF141, GLI3, MIPOL1, IQCE, PITX1, and the GLI1. In the present review, clinical, genetic, and molecular characterization of the polydactyly types has been presented including the recent genes and loci identified for non-syndromic polydactyly. This review provides an overview of the complex genetic mechanism underlie polydactyly and might help in genetic counseling and quick molecular diagnosis.
Postaxial Polydactyly (PAP)
Postaxial polydactyly has one or more than one extra fibular or ulnar digit along with the fifth finger. Its prevalence as live births is 1–2/1000, with some ethnic group differences (Zhou et al., 2004). PAP is 75% more common than PPD, about 8% of bilateral PAP cases with lower and upper limbs are associated with many other congenital syndromic defects. Specifically for the PAP, two distinct categories have been recognized, i.e., postaxial type A, having extra digit (fully developed digit), and postaxial type B (incomplete digit), both these types differ in severity, penetrance estimates, and inheritance pattern (Temtamy and McKusick, 1978; Watson and Hennrikus, 1997). PAP-type A is further classified into eight genetic types: PAP-type A1-A7 and PAP-type 8 with or without Ellis–van Creveld syndrome (EVC) phenotypes. In humans, six potential disease-causing genes have been reported to cause different types of non-syndromic polydactylies. This includes a GLI family zinc finger 3 gene (GLI3, MIM 165240), the IQ Domain-Containing Protein E (IQCE; MIM 617631), a zinc finger protein 141 gene (ZNF141, MIM 194648), a mirror image polydactyly gene (MIPOL1, MIM 606850), a paired-like homeodomain 1 gene (PITX1, MIM 602149), and GLI family zinc finger 1 gene (GLI; 165220).
Children with polydactyly are born with 1 or more extra fingers or toes.
The extra digit may be:
A small raised bump or a piece of skin that looks like a small finger or toe that isn’t fully formed (nubbin)
A complete, working finger or toe
For some children, polydactyly is only 1 feature of a more complex genetic condition or syndrome. These children will have other signs and symptoms.
If your child is born with an extra finger or toe, the doctor will examine your child carefully. During the exam, the doctor will check for other signs to tell whether your child has a more complex condition.
Your child may need an X-ray to see if there are bones in the extra digit and to see how the extra digit connects to the rest of the hand or foot. If the doctor thinks your child might have other genetic problems, your child might have tests to check their chromosomes.
Treatment Options for Polydactyly
In most cases, doctors remove an extra finger or toe in early childhood. The goal of treatment is to give your child a hand or foot that works well and looks typical. There are also practical concerns, such as removing an extra toe so your child’s foot fits well into shoes.
The method for removing an extra digit depends on how it connects to the hand or foot. An extra digit may connect with only a narrow stalk of tissue, or it may connect more deeply and share bones, muscles, and other tissues with the hand or foot.