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Polydactyly is a condition in which one or more extra digits are present. The extra digit can be located on the ulnar or fibular side of the extremity (postaxial), the radial or tibial side of the extremity (preaxial), or centrally.
Polydactyly is defined as ≥1 extra digits of the hands or feet that may or may not contain bone.
Ultrasound Findings
Ultrasound findings include the presence of an extra digit on the hand or the foot. When suspected, polydactyly should be confirmed in the coronal and axial plane. Syndactyly may also be present. Polydactyly can be unilateral or bilateral
(Figure 1). Because the extra digit may or may not contain bone, a diagnosis with two-dimensional ultrasonography can be difficult. When suspected, a three-dimensional ultrasound examination may provide more detailed evaluation of the digits
(Figure 2). If polydactyly is identified, a detailed ultrasound examination is recommended to determine the presence of other abnormalities. Location of the extra digit (preaxial versus postaxial) should be assessed because most nonsyndromic polydactyly is postaxial.
Not all abnormalities that are associated with a specific genetic syndrome can be seen with ultrasound imaging.
Differential Diagnosis
The differential diagnosis of polydactyly includes syndromic and nonsyndromic causes. The list of syndromes that are associated with polydactyly is extensive; the following list represents some of the more common syndromes:
: Polydactyly with syndactyly, macrocephaly, hypertelorism, autosomal dominant inheritance
Genetic Evaluation
Once polydactyly is diagnosed, it is important to determine whether it is isolated or a part of a genetic syndrome. Because isolated polydactyly often is inherited in an autosomal dominant fashion, the parents should be evaluated, and a family history should be collected. Location of the extra digit can be helpful in the classification of syndromic vs nonsyndromic polydactyly; most syndromic cases are preaxial and inherited in an autosomal recessive fashion with a recurrence risk of 25%.
Diagnostic testing (amniocentesis or chorionic villus sampling) with chromosomal microarray analysis (CMA) should be offered when polydactyly is suspected to be part of a genetic syndrome. If screening or other ultrasound features are suggestive of a common aneuploidy, it is reasonable initially to perform karyotype analysis or fluorescence in situ hybridization, with reflex to CMA if these test results are normal. If there are additional anomalies, consanguinity, or a family history of a specific condition, gene panel testing or exome sequencing may be useful because CMA does not detect single-gene (Mendelian) disorders. If exome sequencing is pursued, appropriate pretest and posttest genetic counseling by a provider who is experienced in the complexities of genomic sequencing is recommended.
International Society for Prenatal Diagnosis, Society for Maternal Fetal Medicine, Perinatal Quality Foundation Joint Position Statement from the International Society for Prenatal Diagnosis (ISPD), the Society for Maternal Fetal Medicine (SMFM), and the Perinatal Quality Foundation (PQF) on the use of genome-wide sequencing for fetal diagnosis.
After appropriate counseling, cell-free DNA screening is an option for patients who decline diagnostic evaluation if a common aneuploidy is suspected.
Pregnancy and Delivery Management
A detailed ultrasound examination should be performed once polydactyly is diagnosed. Additional imaging, such as a fetal echocardiogram, should be considered if other anomalies are present or a syndromic cause is suspected. Referrals to pediatric orthopedics or plastic surgery should be considered based on sonographic findings. Pregnancy termination is an option that should be discussed with all patients in whom a fetal anomaly is detected, although isolated polydactyly typically has an excellent prognosis. The specific finding of polydactyly does not generally affect delivery management, although delivery at a tertiary care center may be appropriate based on the presence of other anomalies.
Resection of the extra digit is possible; complexity and timing vary and depend on the degree of digit development. In the setting of a suspected genetic syndrome, the prognosis depends on the severity of associated abnormalities.
Summary
Polydactyly is a condition in which ≥1 extra digits are present. It can be preaxial (radial or tibial side), postaxial (ulnar or fibular side), or central. Although the majority of cases are isolated, more than 100 genetic syndromes are associated with polydactyly. Ultrasound findings are variable, depending on the presence of bone; three-dimensional imaging can be helpful. The presence of polydactyly should prompt a detailed ultrasound examination to look for associated abnormalities. Genetic counseling should be offered to determine the likelihood of an underlying genetic syndrome and to discuss prenatal diagnostic options. Resection of the extra digit varies in complexity, depending on the degree of digit development.
References
Bromley B.
Shipp T.D.
Benacerraf B.
Isolated polydactyly: prenatal diagnosis and perinatal outcome.
International Society for Prenatal Diagnosis, Society for Maternal Fetal Medicine, Perinatal Quality Foundation
Joint Position Statement from the International Society for Prenatal Diagnosis (ISPD), the Society for Maternal Fetal Medicine (SMFM), and the Perinatal Quality Foundation (PQF) on the use of genome-wide sequencing for fetal diagnosis.
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