What Is Polydactyly?

Polydactyly is a condition in which a person is born with extra fingers or toes. The term comes from the Greek words for “many” (“poly”) and “digits” (“dactylos”).

There are several types of polydactyly. Most often the extra digit grows next to the fifth finger or toe.

Polydactyly tends to run in families. It may also result from genetic mutations or environmental causes.

The usual treatment is surgery to remove the extra digit.

Polydactyly causes a person to have extra fingers or toes on one or both of their hands or feet.

The extra digit or digits may be:

• complete and fully functional
• partially formed, with some bone
• just a small mass of soft tissue (called a nubbin)

Polydactyly most often occurs without other abnormalities. This is called isolated or nonsyndromic polydactyly.

Sometimes it is associated with another genetic condition. This is called syndromic polydactyly.

Isolated polydactyly

There are three types of isolated polydactyly, classified by the position of the extra digit:

• Postaxial polydactyly occurs on the outside of the hand or foot, where the fifth digit is. In the hand, this is called the ulnar side. This is the most common type of polydactyly.
• Preaxial polydactyly occurs on the inside of the hand or foot, where the thumb or big toe is. In the hand, this is called the radial side.
• Central polydactyly occurs in the middle digits of the hand or foot. It’s the least common type.

As genetic technologies have advanced, researchers have classified subtypes based on variations in the malformation and the genes involved.

Syndromic polydactyly

Syndromic polydactyly is much less common than isolated polydactyly. A 1998 study of 5,927 people with polydactyly found that only 14.6 percent of them had been born with an associated genetic disorder.

There are many rare syndromes associated with polydactyly, including cognitive and developmental abnormalities and head and face malformations. A 2010 study of polydactyly classification found 290 associated conditions.

Advances in genetic techniques have made it possible to identify specific genes and mutations that contribute to many of these disorders. The 2010 study identified mutations in 99 genes associated with some of these conditions.

With this detailed understanding of the genetics, doctors are better able to look for and treat other conditions in children born with polydactyly.

Syndromes associated with polydactyly

Here are a few of the genetic syndromes associated with polydactyly:

• Down syndrome is strongly associated with double thumbs.
• Syndactyly involves digits that are fused or weblike. A 2017 study estimated that this occurs in 1 in 2,000 to 3,000 births.
• Acrocephalosyndactyly involves early fusion of skull bones and syndactyly.
• Greig syndrome involves abnormal development of the limbs, head, and face. Fused digits or extra digits or an abnormally wide thumb or big toe also occur.
• Carpenter syndrome involves a pointed head with polydactyly occurring in the fifth finger or the first or second toe.
• Saethre-Chotzen syndrome involves a double first toe and syndactyly between the second and third fingers.
• Bardet-Beidl syndrome is associated with polydactyly and syndactyly in the fingers and toes.
• McKusick-Kaufman syndrome involves heart defects, genital abnormalities, and polydactyly.
• Cornelia de Lange syndrome involves developmental disorders.
• Pallister-Hall syndrome is associated with developmental defects and syndactyly.
• Short-rib polydactyly involves a narrow thorax and preaxial polydactyly. Varieties of this syndrome include Jeune syndrome, Ellis van Creveld syndrome, Saldino-Noonan syndrome, and Majewski syndrome.
• Triphalangeal thumb-polydactyly syndrome involves hand and foot malformation with both preaxial and postaxial polydactyly.

Isolated or nonsyndromic polydactyly

Isolated polydactyly is most often passed from a parent to a child through genes. This is known as autosomal dominant inheritance.

Six of the specific genes involved have been identified:

• GLI3
• GLI1
• ZNF141
• MIPOL1
• PITX1
• IQCE

The chromosome locations of these genes have also been identified.

A 2018 literature review of polydactyly genetics suggests that inherited mutations in these genes and their signaling pathways affect the embryo’s growing limbs during weeks 4 through 8.

Nonfamilial cases of isolated polydactyly

There’s some evidence that environmental factors play a role in isolated polydactyly cases that aren’t familial. A 2013 study of 459 children in Poland with isolated preaxial polydactyly that wasn’t familial found that it occurred more frequently in:

• children of women with diabetes
• children with lower birth weights
• children of lower birth order (e.g., first- or second-born)
• children whose fathers had lower educational levels
• children whose mothers had upper respiratory tract infections in the first three months of pregnancy
• children whose mothers had a history of epilepsy
• children who were exposed in embryo to thalidomide

Syndromic polydactyly

As genetic technologies have advanced, researchers have identified more of the genes and mechanisms involved in polydactyly and its associated syndromes.

The syndromes are thought to be caused by genetic mutations that affect the signaling pathways during an embryo’s development. Limb malformations are often associated with problems in other organs.

As they learn more about these genes, researchers hope to shed light on the mechanisms involved in limb development.

Treatment for polydactyly depends on how and where the extra digit is connected to the hand or foot. In most cases, the extra digit is removed in the child’s first two years. This gives the child typical use of their hand and allows their feet to fit into shoes.

Sometimes adults will have the surgery to improve the appearance or function of their hand or foot.

Surgery is usually outpatient, with local or topical anesthesia. Different surgical techniques are a subject of ongoing research.

Fifth digit

Removal of an extra little finger or toe is usually a simple procedure.

In the past, nubbins were usually just tied off, but this often left a bump. Now surgery is preferred.

The child will receive stitches to close the wound. The stitches dissolve within two to four weeks.

Thumb or big toe

Removal of an extra thumb may be complex. The remaining thumb must have an optimal angle and shape to be functional. This may require some remodeling of the thumb, involving the soft tissues, tendons, joints, and ligaments.

Central fingers or toes

This surgery is usually more complex and requires remodeling of the hand to ensure it’s fully functional. It may require more than one surgery, and the child may need to wear a cast for a few weeks after surgery.

Sometimes a pin will be inserted to hold the bones together while they heal.

The doctor may prescribe physical therapy to reduce scarring and help improve function.

Ultrasound sonographs can show polydactyly in the embryo in the first three months of development. The polydactyly may be isolated, or it may be associated with another genetic syndrome.

The doctor will ask whether there is a family history of polydactyly. They may also do genetic testing to check for anomalies in the chromosomes that could indicate other conditions.

If other genetic conditions are involved, the doctor and possibly a medical geneticist will discuss the outlook for the child.

Once the child is born, polydactyly can be diagnosed on sight. If the doctor suspects that the child has other genetic conditions, they will do more testing on the child’s chromosomes.

The doctor may also order an X-ray of the digit or digits involved to see how they are attached to the other digits and whether they have bones.

Polydactyly is a fairly common condition. It’s depicted in ancient art going back almost 10,000 years and has been passed down through generations.

If polydactyly is not associated with other genetic syndromes, the extra digit can usually be removed with routine surgery. Most cases are in this category. The child’s doctor may continue to monitor the functioning of the hand or foot involved.

Surgery is also possible for adults to improve the appearance or functionality of the hand or foot.

Syndromic polydactyly can cause impairment in other parts of the body. It may also involve impaired development and cognitive disability, so the outlook will depend on the underlying syndrome.