Overview

If you’ve just learned that someone you know has Carpenter syndrome and wants to understand what causes extra fingers and toes to appear, you’re in the right place. This article will help you understand the causes of it.

Polydactyly in Carpenter syndrome refers to the presence of extra fingers or toes, a common physical feature of this rare genetic condition. Most children with Carpenter syndrome are born with one or more additional digits. Those are often formed next to the big toe or pinky finger. These extra digits are part of the broader skeletal differences caused by mutations in the RAB23 or MEGF8 genes, which affect how the body develops in early pregnancy. Polydactyly is usually not harmful on its own and is often treated surgically for functional or cosmetic reasons.¹

Keep reading our article to find out how Carpenter syndrome develops, how doctors diagnose it and what kinds of treatments for people affected by polydactyly are out there right now.

What is Carpenter syndrome?

Let’s start from the beginning and try to get a better understanding of the cause. Polydactyly is a common symptom of Carpenter syndrome. Carpenter syndrome is a very uncommon hereditary condition that changes how bones grow and fuse. Children born with this condition often have:

• Abnormally shaped skull (craniosynostosis)
• Extra fingers or toes (polydactyly)
• Fused fingers or toes (syndactyly)
• Distinctive facial features (flat nasal bridge, wide-set eyes)
• Possible heart defects, hearing loss, or developmental delays

This syndrome is a part of a group of disorders called acrocephalopolysyndactyly syndromes. Very complicated name, I know. Interestingly, only around 70–100 cases have been reported globally, which makes this syndrome very, very rare^1,2

What is polydactyly? 

Just to go over this again, polydactyly means being born with more than five fingers or toes. The extra digit may be:

• Fully formed with bones and joints^1,3
• Small and underdeveloped (a "nubbin")⁴
• Located on the outer (postaxial) or inner (preaxial) side of the limb^1,3

Carpenter syndrome usually causes extra toes, while extra fingers are less common. Sometimes, the fingers or toes may also be joined together by skin.^1,3

How is Carpenter syndrome connected to polydactyly?

In Carpenter syndrome, polydactyly is not just a random extra digit. It’s part of a pattern of physical differences caused by a genetic mutation. The same genes that affect skull development also affect limb patterning, leading to extra or fused digits. These changes happen very early in pregnancy while the baby is forming in the womb.⁵

What causes Carpenter syndrome and polydactyly?

Carpenter syndrome is caused by mutations in the RAB23 or MEGF8 genes. These genes are responsible for directing how cells grow and organise themselves during early development.

This syndrome is inherited in an autosomal recessive way.⁵ This means that a child must inherit two copies of the Carpenter syndrome gene, one from each parent. Parents are usually carriers with no symptoms, so they don’t even suspect it. And then if both parents are carriers:

• 25% chance of an affected child
• 50% chance of a carrier child
• 25% chance of a child who inherits neither faulty gene⁵

How is Carpenter syndrome diagnosed?

Diagnosis usually begins after birth when signs like polydactyly or an unusually shaped head are noticed. NHS Doctors may use:

• Physical exams to identify obvious features
• X-rays or CT scans are used to examine bones
• Genetic testing to confirm RAB23 or MEGF8 mutations

If the parents are known carriers or if prenatal tests reveal abnormalities, a prenatal diagnosis may occasionally be possible.¹

Treatment options and management for polydactyly

In the NHS for Carpenter syndrome, treatment for hand and foot differences often focuses on syndactyly, where the fingers or toes are joined together by skin or soft tissue, as described earlier. When separating joined fingers surgically, the index and middle fingers are usually separated first, and (if necessary) the remaining digits are separated by further surgeries.

Although extra fingers or toes (polydactyly) are a common feature of Carpenter syndrome, surgery to remove them is not always required. In particular, fused or duplicated toes are less likely to interfere with daily activities and therefore may not need surgical correction unless they cause specific functional issues.

Surgical outcomes and recovery for polydactyly

Surgery to remove extra fingers or toes in children with Carpenter syndrome often leads to very good results, especially when done early. In cases where the extra thumb is more developed, children typically recover well after surgery and regain strong use of their hands. Most kids have a smooth recovery without early complications, and even years later, they still have good hand function. A small number may need follow-up surgery later on, but this is usually to correct things like finger angle or discomfort.^6,7

For smaller, less developed extra digits, doctors can use a quick and gentle method involving electrocautery. Under local anaesthesia, this method removes the additional digit using heat techniques. It’s done while the baby is still very young, and most parents stay with their child during the short procedure. Recovery is quite fast, and most children go home the same day. In nearly all cases, the thumb looks normal after healing, and parents are highly satisfied with how it turns out.^6,7

Summary

Carpenter syndrome is a rare genetic condition that affects how the skull, face, hands and feet are formed. It is caused by mutations in the RAB23 or MEGF8 genes, which play an important role in how cells grow and develop during early pregnancy. These genetic changes disrupt normal bone formation and result in a variety of physical differences. The most common ones are craniosynostosis (early fusion of skull bones), syndactyly (fused digits), and polydactyly (extra fingers or toes).

You may have already realised that polydactyly is one of the hallmark signs of Carpenter syndrome. Such a characteristic is formed while the child is still in the womb and often involves extra digits on both the hands and feet. In many cases, the additional fingers or toes are fully formed and may function normally, although surgical removal is sometimes recommended to improve mobility, function or appearance. Fused digits may also be surgically separated in stages to support proper hand development.

Carpenter syndrome is passed down in an autosomal recessive way. That means a child must receive one copy of the faulty gene from each parent to be affected. Parents are usually carriers without symptoms. It’s possible to reveal your child has Carpenter syndrome before birth using prenatal imaging and genetic testing, but most cases are identified shortly after delivery based on physical features. Diagnosis typically involves clinical exams, X-rays or CT scans and, like we just said, confirmation through genetic testing.

While there is no cure for Carpenter syndrome, early treatment can make a significant difference. Polydactyly can be surgically corrected by separating fused fingers or toes. With appropriate care from a multidisciplinary team, many children affected by polydactyly caused by Carpenter syndrome can grow up to lead happy and fulfilling lives.

FAQs

Is polydactyly always present in Carpenter syndrome?

Polydactyly (having extra fingers or toes) is one of the main signs doctors look for when diagnosing Carpenter syndrome. Most people affected by this syndrome have additional fingers, and everyone has extra toes. It’s such a common feature that it helps set Carpenter syndrome apart from other conditions. The number and shape of the extra digits can vary, but they’re nearly always there. Seeing polydactyly along with other signs like fused fingers or an unusual head shape often leads doctors to do more tests. So yes, polydactyly is almost always present in people with Carpenter syndrome.⁸

Will my child have other difficulties?

Carpenter syndrome is a rare condition that can impact several body regions. Early skull bone fusion is one of the primary problems, which can result in an odd head shape and, in certain situations, elevated intracranial pressure. This may also affect the face, resulting in features such as a flat nasal bridge, small jaws, and downward-slanting eyes. Some children have learning difficulties, while others have normal development. Growth may be slower than average, and childhood obesity is common. Additional characteristics may include heart disorders, eye issues and hearing impairments. Rarely, organs such as the heart may grow on the other side of the body. Joint and bone problems are also seen, such as hips or knees growing at unusual angles and a curved spine. Boys are often born with undescended testicles, and some children may go through puberty earlier than usual. Symptoms can vary widely.²

Why is it called Carpenter syndrome? 

NHS mentions that Carpenter syndrome is a type of craniosynostosis named after the doctor who first described the condition.