What is the significance of polydactyly in a newborn with macrosomia (large birth weight)?

From the Guidelines

Polydactyly in a newborn with macrosomia is a significant finding that warrants further evaluation for potential genetic syndromes, particularly Simpson-Golabi-Behmel syndrome (SGBS), which is associated with an increased risk of Wilms tumor and other malignancies. According to the study by Kalish et al. 1, SGBS is characterized by pre- and postnatal macrosomia, distinctive craniofacies, and variable findings including supernumerary nipples, diastasis recti, congenital heart defects, and genitourinary defects. Hand anomalies, such as postaxial polydactyly, are also common in SGBS. The study highlights the importance of regular surveillance for children with SGBS, similar to those with Beckwith-Wiedemann syndrome, due to the high risk of Wilms tumor and other cancers.

Key features of SGBS include:

• Pre- and postnatal macrosomia
• Distinctive craniofacies, including macrocephaly, coarse facial features, and macrostomia
• Variable findings, such as supernumerary nipples, diastasis recti, and congenital heart defects
• Hand anomalies, including postaxial polydactyly
• Increased risk of Wilms tumor and other malignancies

Management of a newborn with polydactyly and macrosomia should involve a multidisciplinary approach, including:

• Genetic testing to confirm the diagnosis of SGBS or other genetic syndromes
• Comprehensive evaluation for associated anomalies, such as congenital heart defects and genitourinary defects
• Regular surveillance for Wilms tumor and other cancers, as recommended by the AACR Workshop Committee 1
• Surgical correction of the extra digits, typically between 9-12 months of age
• Endocrine assessment to evaluate for potential hypothalamic involvement and its effects on growth and metabolism
• Genetic counseling for the family, as SGBS is caused by mutations in the glypican genes, GPC3 or GPC4, and has an X-linked inheritance pattern 1

From the Research

Significance of Polydactyly in Newborns with Macrosomia

The significance of polydactyly in a newborn with macrosomia (large birth weight) can be understood by considering the following points:

• Polydactyly is a common congenital limb defect that can present with various morphologic phenotypes 2
• It can be an isolated condition or part of a congenital syndrome, and its frequency varies widely among populations 3
• Polydactyly can be classified into three major groups: preaxial, central, and postaxial, and its treatment options depend on the underlying feature 3, 4
• The condition is usually inherited as an autosomal dominant trait, but it can also be caused by defects in the anterior-posterior patterning of limb development, and at least 10 loci and six genes have been identified as causing non-syndromic polydactyly 4, 5
• Polydactyly is the most frequently observed congenital hand malformation, with a prevalence between 5 and 19 per 10000 live births, and it can occur as an isolated disorder or as part of a syndrome 5
• The genetic bases of syndromic and non-syndromic polydactylies are complex and not fully understood, and there is no single comprehensive published summary of all syndromic and non-syndromic polydactyly presentations 6

Polydactyly Subtypes and Their Significance

The different subtypes of polydactyly have distinct characteristics and significance:

• Preaxial polydactyly is associated with triphalangism of the thumb and is caused by a single genetic locus, but there is genetic heterogeneity for preaxial polydactyly associated with duplications of biphalangeal thumbs 5
• Central polydactyly is a rare form of polydactyly that can occur as an isolated condition or as part of a syndrome 4
• Postaxial polydactyly is the most common form of polydactyly and can occur as an isolated condition or as part of a syndrome 3, 4

Clinical Implications of Polydactyly

The clinical implications of polydactyly in newborns with macrosomia are:

• Polydactyly can be an indication of an underlying syndrome or genetic disorder, and careful clinical and radiographic evaluation is necessary to determine the underlying cause and develop an appropriate treatment plan 2, 3
• Treatment options for polydactyly depend on the underlying feature and can range from simple soft-tissue procedures to complex surgical reconstruction 3, 4
• Early diagnosis and treatment of polydactyly can improve functional and cosmetic outcomes, and genetic counseling can help families understand the risks and implications of polydactyly 4, 6