Polydactyly is the presence of a hand or a foot with more than five fingers or toes. Although the medical term polydactyly was first ascribed to the 17^th century Amsterdam physician Theodor Kerckring (Blauth and Olason, 1988), the presence of polydactyly was mentioned in the Bible (Nicolai and Schoch, 1986) and is even evident on the left leg of St. Joseph in Raphael’s famous 16^th century painting of “The Marriage of the Virgin” (Mimouni et al., 2000). Polydactyly is the most frequent congenital anomaly of the human hand (Tsukurov et al., 1994). Polydactyly affecting the thumb or great toe is now classified as “radial/tibial” (formerly preaxial), which refers to a duplication of digits on the radial side of the hand and the tibial side of the foot, or “ulnar/fibular” (formerly postaxial), in which the extra digits occur on the ulnar and fibular sides of the hand and foot, respectively (Talamillo et al., 2005). Cases in which the three central digits are affected are referred to as “central polydactyly” and are the least common of the three types. In general, radial/tibial polydactyly is more common than ulnar/fibular polydactyly (Temtamy and McKusick, 1978). Polydactyly has been further subclassified on a descriptive basis. In ulnar/fibular polydactyly, type A denotes fully developed extra digits. Type B describes rudimentary extra digits or a pedunculated skin tag that is usually found on the hands and only rarely on the feet. This type is very common among blacks. In radial/tibial polydactyly, when the thumb is polydactylous it is known as type I, when a triphalangeal thumb is present it is known as type II, when the index finger is polydactylous it is type III, and when polysyndactyly occurs it is known as type IV.

Morphogenetically, the hand and foot malformations can be considered as a process of bifurcation of one or several finger or toe rays in the longitudinal axis progressing from peripheral to central. Segment polarity genes initially identified in Drosophila, such as sonic hedgehog (SHH), play a critical role in this process.

Notable differences exist in the incidence of polydactyly according to race and gender. In a 1963 study performed in New York City, the incidence of ulnar/fibular polydactyly in blacks was 10.7 in 1000 livebirths, whereas in a comparable white population, the incidence was 1.6 in 1000 livebirths. In this study, polydactyly was noted to be twice as common in males as in females (Mellin, 1963). In general, there is a 1%> incidence of ulnar/fibular polydactyly of the hand in black individuals. This is inherited as an autosomal dominant trait. There is also a high frequency of ulnar/fibular polydactyly in some Latino populations (Orioli, 1995). In one Japanese study, radial/fibular polydactyly was demonstrated in the hand and ulnar/fibular polydactyly in the foot (Masada et al., 1986). Affected females predominated when the foot was involved and males predominated when the hand was involved. In this study of 523 cases, associated anomalies were seen in 5% of patients (Masada et al., 1986). In another Japanese study of 330 feet in 265 patients affected with foot polydactyly, a 10% incidence of a positive family history was noted (Watanabe et al., 1992). More recently, in a sonographic study of 17,760 consecutive Israeli women, Zimmer and Bronshtein (2000) detected 26 fetuses with polydactyly. This equated to an incidence of 1 in 698 women. Most of the examinations in this study were performed at 14-16 weeks of gestation.

Maternal smoking increases the chance of fetal limb defects (Hampton, 2006; Man and Chang, 2006).

The phalanges are identifiable as early as the 13th week of gestation by transabdominal sonographic examination (Figures 104-1 and 104-2) (Deschamps et al., 1992), and by 11 weeks of gestation by transvaginal scan (Hobbins et al., 1994). The major consideration in the prenatal sonographic examination is to determine whether the polydactyly is isolated. As polydactyly is a component of many genetic syndromes, it is especially important to perform a thorough anatomic survey to look for associated anomalies. Specific associations that should be ruled out include skeletal dysplasia, polycystic kidneys, and encephalocele. Once polydactyly is identified it is especially important to note the length of the long bones (Bromley and Benacerraf, 1995). Polydactyly of the hand is easier to diagnose than that of the foot (Figures 104-3 and 104-4). An ulnar/fibular skin tag can be easily over-looked, whereas a normally formed extra digit will be more easily visualized. Triphalangeal thumbs are also considered within the diagnostic spectrum of polydactyly. Three dimen-sional (3D) sonography may provide improved resolution of the fingers and toes, especially when using transparent mode reconstruction (Kos et al., 2002).

The majority of antenatally diagnosed cases of polydactyly will be isolated and associated with a normal prognosis (Bromley et al., 2000). The major consideration in the differential diagnosis is to determine whether the condition is isolated and familial or due to a syndrome. Isolated polydactyly is also considered to be a manifestation of the heterozygote (carrier) state for Ellis–van Creveld and Meckel–Gruber syndromes (Goldblatt et al., 1992; Nelson et al., 1994; Wright et al., 1994). Trisomy 13, which manifests as polydactyly in 75% of affected patients (Tsukurov et al., 1994) should be ruled out by prenatal karyotyping if additional sonographic abnormalities are present (see Chapter 129). Fetuses with trisomy 13 have polydactyly, cardiac defects, central nervous system defects including holoprosencephaly, and other major organ abnormalities. If the chromosomes are normal but the fetus is noted to have polydactyly and holoprosencephaly, a possible diagnosis is pseudotrisomy 13, which is inherited as an autosomal recessive condition (Verloes et al., 1991). For patients to receive this diagnosis, the karyotype must be normal and holoprosencephaly must be present with polydactyly or other anomalies.

Alternatively, polydactyly with other serious central nervous system defects can be present (Lurie and Wulfsberg, 1993). The constellation of brain abnormalities in the set-ting of polydactyly suggests Meckel–Gruber syndrome, which is also inherited as an autosomal recessive trait (Gershoni-Baruch et al., 1992). Fetuses with Meckel–Gruber syndrome characteristically have an occipital encephalocele, polycystic kidneys, and polydactyly. This condition was first described in 1822 (Ueda et al., 1987). Isolated polydactyly has been questioned in carriers of the Meckel–Gruber syndrome. Nelson et al. (1994) described a family in which three boys were affected. Their father and paternal first cousin had bilateral ulnar/fibular polydactyly, which raised the question of whether they were manifesting heterozygotes. One hundred percent of patients affected with Meckel–Gruber syndrome have cystic renal dysplasia and 55% of these patients have polydactyly.