What is Supernumerary Digit (polydactyly)?
A supernumerary digit, also known as polydactyly, is a birth condition that affects the hands or feet. This condition usually becomes apparent just after birth. Polydactyly is the most prevalent birth abnormality affecting the hands and feet. People with this condition usually have more than the normal number of 5 fingers or toes.
Historically, at around the 16th century, Parey, a scholar, wrote about individuals who were born with extra fingers, describing them as “superfluous fingers.” Additionally, artwork found in the southwestern region of the United States, known as the “Four Corners,” shows people with 6 fingers or toes. These artworks are believed to have been created by a population that lived between AD 600 and 1280.
The classification of polydactyly is dependent on the position of the extra finger or toe. “Preaxial polydactyly” refers to an extra digit on the side of the thumb or big toe, “postaxial polydactyly” refers to an extra digit on the side of the pinky finger or toe, and “central or mesoaxial polydactyly” involves extra digits between the second and fourth fingers or toes.
This condition mainly follows a pattern called autosomal dominant, meaning if one parent has this condition, the children have a chance of inheriting it. Polydactyly can appear on its own, or it may be part of another syndrome. Therefore, it’s very important for doctors to thoroughly examine newborns with this condition to check for other possible related disorders. This piece discusses the causes and development of polydactyly, and the importance of a team of healthcare professionals in assessing and treating affected patients.
What Causes Supernumerary Digit (polydactyly)?
Polydactyly, a condition where a person is born with more than the usual number of fingers or toes, is usually passed down from parents to their children. It occurs when something goes wrong with how the fingers and toes form during the baby’s development in the womb.
Currently, we know of at least 10 specific areas on genes and 6 different genes that could contribute to polydactyly when it’s not tied to any other condition. The formation of limbs and digits happens within 4 to 8 weeks after a baby is conceived and is controlled by various genetic signaling centers.
Specific changes in two genes known as sonic hedgehog (SHH) and GLI3 have been linked to at least 6 different forms of polydactyly. Changes in SHH are most often responsible for causing an extra thumb to form, which is the most common form of polydactyly. The SHH gene controls the development of the front-to-back (radial-ulnar) layout of the limb.
In normal hand development, the length of the limb is determined by signals from a specific growth factor, and the fingers develop along a certain axis controlled by specific proteins alongside SHH. SHH is present in higher concentrations on one side of the hand compared to the other, contributing to the differentiation of digits. Mutations in a gene known as LMBR1 have been linked with polydactyly because they increase the activity of SHH during limb development.
Other forms of polydactyly are associated with changes in the GLI3 gene. In some people with a condition known as Greig cephalopolysyndactyly, there’s a specific change in the GLI3 gene, which normally works against SHH. This change can result in a boost of SHH activity, leading to polydactyly by disrupting the regular front-to-back development of the fingers and toes.
Risk Factors and Frequency for Supernumerary Digit (polydactyly)
Polydactyly, or having more than the usual number of fingers or toes, happens in about 1.6 to 10.7 out of every 1,000 births. Boys are twice as likely to have it than girls. Additionally, polydactyly of the fingers is more common in the Black community, affecting about 1 in 300 people, compared to the White community, affecting around 1 in every 3,000 people.
- Preaxial polydactyly, which affects the side of the hand or foot closer to the body, is more common in White, American Indian, and Asian populations.
- Postaxial polydactyly, which affects the side of the hand or foot further from the body, is more frequently seen in the Black population.
The prevalence of postaxial polydactyly is estimated to be between 1 and 2 cases per 1,000 live births.
On the other hand, preaxial polydactyly is less common, with reported incidences ranging from 0.08 to 1.4 cases per 1,000 to 1 in 3,000 live births.
Signs and Symptoms of Supernumerary Digit (polydactyly)
Polydactyly is a condition a person is born with where they have more than the usual number of fingers or toes. It tends to run in families, so an accurate family history can aid in diagnosing. The extra digit is usually noticed very soon after birth. Physical examination will identify this addition and further tests like radiographs can help doctors decide on the best course of treatment. There are different types of polydactyly, and they are categorized into classes based on specific characteristics.
The first type is preaxial polydactyly of the hand, where an extra finger grows on the thumb side. The classes, as per the Wassel classification system, include:
- Type I includes a split in the last bone of the finger.
- In Type II, the last bone is duplicated.
- In Type III, the next bone up is split into two.
- Type IV involves a duplicated of the next bone up.
- Type V includes a split in the metacarpal, the long bone in the hand.
- In Type VI, the metacarpals are duplicated.
- Type VII includes a hand with three phalanges or sections.
Another type is postaxial polydactyly, wherein an extra finger grows on the pinky side of the hand or the outer edge of the foot. The Stelling and Turek classification system has three types:
- Type 1 includes a duplication of soft tissue.
- Type 2 features a duplication of bone.
- Type 3 includes a replication of the entire section, including the metacarpal.
There’s also a version called central polydactyly, which affects the index, middle, or ring finger or the second, third, or fourth toe. This is more rare and often shows up alongside other types or along with fused toes or fingers. The different classes include:
- Type 1 includes a duplication with no attachment to an adjacent finger.
- Type 2 includes shared joints, split metacarpals, or phalanges with an adjacent finger.
- Type 3 includes a fully formed separate digit, including the metacarpal.
The last type is foot polydactyly, classified by the Venn-Watson system into six subtypes:
- Class 1 includes duplication of the end part of the toe with a normal metatarsal.
- Class 2 features a block metatarsal, which is an abnormal shape of the bone.
- Class 3 includes a Y-shaped metatarsal.
- Class 4 has a T-shaped metatarsal.
- Class 5 includes a normal metatarsal shaft with a widened head.
- Class 6 includes a complete duplication, including the metatarsal.
Testing for Supernumerary Digit (polydactyly)
Polydactyly, a condition where a person is born with extra fingers or toes, is usually found at birth or through prenatal ultrasound. The best time to examine and classify the specific type of polydactyly is shortly after the baby is born and before any problems become apparent. After the baby undergoes a thorough physical exam and X-ray, doctors can plan if and how a surgery could help restore the usual look, size, and function of the affected digit.
Another important aspect to consider is whether the polydactyly is part of a larger syndrome. Some of these related syndromes are:
Bardet-Biedl Syndrome: This is a hereditary disorder that primarily affects the individual’s vision, often resulting in loss of night and peripheral vision due to a condition called retinitis pigmentosa. Additional symptoms can include obesity, learning difficulties, kidney disease, fused digits (syndactyly), and polydactyly. This syndrome is confirmed through genetic testing.
Greig Cephalopolysyndactyly Syndrome: This genetic disorder is characterized by unusual facial features and extra digits. Symptoms include an enlarged forehead, wide-set eyes, an abnormally broad nasal bridge, and additional or webbed digits.
Short-Rib Polydactyly Syndrome: This is a severe hereditary disorder that affects the skeleton and vital organs, leading to underdeveloped ribs, polydactyly, and severe organ abnormalities. The diagnosis is often made prenatally through an ultrasound scan.
Smith-Lemli-Opitz Syndrome: This hereditary disorder is characterized by a small head size (microcephaly), learning difficulties, behavioral problems, fused second and third toes, polydactyly, and malformations in the lungs, kidneys, genitalia, and digestive system. It is caused by a mutation in a specific gene that prevents cells from producing enough cholesterol, causing an accumulation of cholesterol byproducts.
McKusick-Kaufman Syndrome: This syndrome is characterized by extra fingers or toes, congenital heart disease, and genital abnormalities. In females, accumulations of secretions in the uterus or vagina (hydrometrocolpos) can cause a noticeable large mass in the abdomen. The diagnosis can be initially established through clinical evaluation and subsequently confirmed through genetic testing.
Pallister-Hall Syndrome: This hereditary disorder is characterized by polydactyly, abnormal epiglottis (a flap in the throat that keeps food from going into the windpipe), decreased function of the pituitary gland, an imperforate anus (an anus that is sealed off), and a hypothalamic hamartoma (a non-cancerous tumor in the brain).
Fanconi Anemia: This is a genetic disorder that affects DNA repair. Those affected are at risk of developing aplastic anemia or acute myeloid leukemia and face an elevated risk of solid tumors. Symptoms can include short stature, skin pigmentation abnormalities, and small eyes (microphthalmia). Thumb abnormalities are also common.
Holt-Oram Syndrome: Also known as heart-hand syndrome, this is characterized by upper arm anomalies and heart defects, often a hole in the heart. All individuals with this syndrome will have additional abnormalities like missing or extra thumbs.
Down Syndrome: This is a genetic condition that often results in cognitive delays, congenital heart defects, low thyroid hormone levels, early onset dementia, hearing loss, distinctive facial features like epicanthal folds (a skin fold of the upper eyelid), and a noticeable gap between the first and second toes. Polydactyly can also occur.
Treatment Options for Supernumerary Digit (polydactyly)
When someone has more fingers than normal, known as polydactyly, doctors want to improve the look and function of the hand. The decision to operate or not depends on whether the extra finger will affect hand function and if it has bones. If surgery is needed, the goal is to keep as much healthy skin as possible, make sure the finger is stable, and plan carefully to ensure a strong grip.
Preaxial polydactyly is when the extra finger is on the thumb side of the hand. Doctors typically operate on children with this condition between 12 and 18 months old. Common operations include removing the extra finger, fusing two smaller thumbs into one larger one, or moving the tip of the thumb to a more useful position. The goal is to create a stable, functional thumb, reducing the need for more surgery in the future.
Depending on the exact nature of the condition, surgeons may use different techniques. If the child has extra thumbs that are equally small and underdeveloped, doctors might merge the two into a single, more functional thumb. If one thumb is larger and more practical than the other, doctors might remove the smaller thumb and rebuild the larger one to make it more useful.
Sometimes the thumb has three bones instead of the usual two. If the extra bone forms a wedge, the thumb is still usable, and doctors will try to correct the angle, length, and opposition of the thumb. For longer thumbs that lie flat like the other fingers, the best solution is often to reshape it to function more like a thumb.
If the extra finger is on the little finger side of the hand, known as postaxial polydactyly, treatment usually involves surgery to remove the extra finger, especially if it has bones. Like with preaxial polydactyly, doctors aim to create a sturdy, functional finger.
When it comes to polydactyly in the middle of the hand or on the foot, treatment usually happens around 12 months old with the main goal of improving the look and performance of the hand or foot. Common treatments typically include removing or tying off the extra fingers. One principle, based on previous studies, guides doctors to prioritize making a useful thumb and three functional fingers over trying to make a normal-looking hand with five fingers. For foot polydactyly, the main goal of surgery is to restore a normal foot shape while making sure the growth plate alignment is preserved and the soft tissues around the toe joints are properly balanced.
What else can Supernumerary Digit (polydactyly) be?
Typically, polydactyly, which is the presence of extra fingers or toes, is easy to diagnose, especially if the extra digit has both bone and soft tissue. However, if the extra digit is less developed and only has soft tissue, there might be a different condition, called infantile digital fibromatosis, at play.
Infantile digital fibromatosis is rare, harmless, and recurrent fibrous growth that occurs on the top area of the fingers or toes. It often affects the last four fingers or toes but usually not the thumb and big toe. The growths appear as firm, reddish-pink, and slow-growing bumps, typically reaching about 2 cm in size. In most instances, these bumps naturally disappear after about a year. However, treatment options are available if needed.
What to expect with Supernumerary Digit (polydactyly)
Polydactyly, which is the condition of having more than the usual number of fingers or toes, can affect both looks and physical activities. The main worries when it comes to polydactyly are its impact on the development of precise, coordinated movements, and its overall effect on the person’s ability to perform tasks.
But with the right treatment at the right time, most people with polydactyly can expect to use their hands or feet effectively, leading to a generally good outcome.
Possible Complications When Diagnosed with Supernumerary Digit (polydactyly)
Not treating a condition called ‘polydactyly’ in the hand can limit hand movement and affect the development of fine motor skills. ‘Polydactyly’ refers to having more than five fingers or toes per hand or foot. When it occurs in the foot, one common problem is that the big toe may move inward, causing joint pain and making it difficult to wear shoes.
Surgery to treat polydactyly in either the hands or feet can lead to different complications. Some include:
- Risk of developing a painful nerve tissue growth or a cyst, often due to the type of stitch method used.
- Possible complications reported are painful scarring, infection, unstable joints, remaining deformity, angled growth, growth stoppage, stiff joints, and nail bed deformities.
- For example, one study reported that 19% of patients with polydactyly in their thumbs or big toes needed follow-up treatment, usually due to problems such as pain or instability.
Overall, addressing polydactyly is important for hand and foot functionality, but treatment options should be considered carefully due to the potential for complications.
Recovery from Supernumerary Digit (polydactyly)
If you’ve undergone a less complex surgery that did not involve the reshaping of bone (bony osteotomy) or reconstruction of a ligament, you can usually start active exercises to help regain the movement in your joints after about 10 to 14 days post-surgery. However, if your surgery was more comprehensive and involved more reconstruction, it could take about 4 to 6 weeks before you start with a trained hand therapist for gentler joint movement exercises.
If you had Kirshner wires (thin metal rods to hold bone in place) used during your surgery, they can usually be taken out about 6 to 8 weeks after the operation. After about 6 to 8 weeks post surgery, you can gradually increase using your hand while wearing a protective forearm-based thumb spica splint (a special type of cast that includes the joint of your thumb) for the sake of protection.
Preventing Supernumerary Digit (polydactyly)
Polydactyly, or the presence of extra fingers or toes, is a condition that babies can be born with. It’s often passed down in families. What this means is that if one parent has the gene that causes this condition, there’s a chance their children could also have it.
It is important to quickly recognize and address this condition before the child starts experiencing problems with their hand or foot. Acting swiftly may lead to better function of the limb and an improved appearance. Though polydactyly usually occurs on its own, it can sometimes be a part of a larger syndrome.
To get a better understanding of the condition, X-rays are often taken. These X-rays help doctors understand the condition better and plan any necessary surgeries. While complications from surgery are rare, it’s important to be mindful that issues can arise, like painful scarring, abnormal growth, or the development of neuromas, which are painful growths of nerve tissue.
Working together with healthcare professionals is key to ensure the best care for patients. By intervening early and examining the condition thoroughly, doctors can effectively manage polydactyly, which can greatly improve the quality of life for those affected by the condition.
