Polydactyly, derived from Greek words meaning “many digits,” is a congenital variation where a baby is born with one or more extra fingers or toes. This occurrence is relatively common, affecting between 0.37 and 1.2 per 1,000 live births globally. It represents a disruption in the complex process of limb formation during early fetal development. The causes are primarily genetic, ranging from the inheritance of a single gene to being a feature of a much larger, complex syndrome. Determining the specific cause requires careful assessment of the extra digit’s structure and any accompanying health issues.
Anatomical Classification of Extra Digits
The first step in understanding the cause of polydactyly involves classifying the extra digit based on its anatomical location on the foot. This classification system helps medical providers determine the likely genetic mechanism and any potential association with a syndrome.
Postaxial and Preaxial Types
The first is postaxial polydactyly, where the extra toe is located on the outside of the foot, next to the fifth (little) toe. This type is generally the most frequent and often occurs in isolation, without other associated health conditions. The second type is preaxial polydactyly, involving the inside of the foot, where the extra toe appears next to the first (big) toe. This variation is less common than the postaxial form and often involves a more complex duplication of the bone structure.
Central Polydactyly
Finally, central polydactyly is the rarest presentation, where the extra digit is located among the second, third, or fourth toes. The complexity of the extra digit can range significantly, from a small, soft tissue bump, sometimes called a nubbin, to a fully formed digit with all the necessary bones and joints.
Isolated Genetic Causes
When polydactyly occurs without any other associated health concerns, it is classified as non-syndromic or isolated polydactyly. In these instances, the cause is typically a mutation in a single gene that controls the patterning of the developing limb bud.
Many isolated cases follow an autosomal dominant inheritance pattern, meaning that a child needs to inherit the gene variant from only one parent to express the trait. This inheritance often demonstrates variable expression, meaning that family members carrying the same gene mutation can present with different physical outcomes. For example, one parent might have a small, soft-tissue nubbin on one foot, while their child might have a fully duplicated toe on both feet.
Specific genes, such as GLI3 and ZNF141, have been linked to these non-syndromic forms. Their normal function in anterior-posterior limb development is disrupted, indicating an error in the signaling pathways that tell the developing limb how many digits to form.
Polydactyly as Part of a Syndrome
Polydactyly is sometimes a recognizable feature of a larger, underlying genetic condition, referred to as syndromic polydactyly. This occurs in a smaller percentage of cases, but it is often associated with more significant health concerns involving multiple body systems. The presence of physical or developmental differences suggests that the cause is a complex genetic syndrome rather than a simple, isolated gene mutation.
One example is Bardet-Biedl syndrome, characterized by postaxial polydactyly of the hands and feet, alongside features like retinal degeneration and kidney abnormalities. This condition is typically inherited in an autosomal recessive manner, requiring two copies of the mutated gene for the syndrome to manifest.
Another example is Ellis-van Creveld syndrome, which features short-limbed dwarfism, heart defects, and postaxial polydactyly, often involving the hands. The genetic basis for syndromic polydactyly is far more complex than isolated cases, often involving genes that have broad roles in the development of various organs.
For instance, the EVC gene, associated with Ellis-van Creveld syndrome, is involved in the hedgehog signaling pathway, which is critical for patterning multiple structures during embryogenesis. The resulting extra digit is one visible symptom of a systemic developmental issue. Identifying the specific syndrome is important for managing the related health issues that are not visible at birth.
Determining the Underlying Cause
Once polydactyly is observed at birth, the medical assessment focuses on determining whether the cause is isolated or syndromic. The diagnostic process begins with a comprehensive physical examination, where the physician looks for any other subtle physical differences or anomalies. A detailed family history is also crucial, as a pattern of extra digits in close relatives suggests an isolated, inherited cause.
Medical imaging, specifically X-rays, is utilized to visualize the underlying bony structure of the extra digit and its connection to the rest of the foot. The X-ray helps to classify the type of polydactyly and assess its complexity, which guides both the diagnostic and treatment approach.
If the physical exam and family history suggest the possibility of a syndrome, genetic testing may be employed to identify a specific gene mutation. This comprehensive evaluation is necessary to provide an accurate diagnosis and ensure any associated health issues are addressed.