The term syndactyly comes from the Greek syn, meaning together and daktylos, meaning finger. It describes an apparent fusion of the digits. Syndactyly can be osseous, which refers to fusion of the bones, or cutaneous, defined as webbing of the skin between two digits. In fetal life, the limb buds may be recognized sonographically as early as 8 weeks of gestation, but the digits become visible only at 11 to 12 weeks (Bromley and Benacerraf, 1995). After the 12th week, the hand is fully formed, but separate movements of the digits are not easily observed until 15 weeks of gestation (Deschamps, et al., 1992). Absence of digital dissociation implies a diagnosis of syndactyly.

Although mild cutaneous syndactyly, or webbing, of the toes is a common familial trait, it is generally too subtle to be appreciated on a prenatal sonogram. Syndactyly severe enough to interfere with digital movement suggests a more serious pathology, such as one of the acrocephalosyndactyly syndromes. These syndromes are generally associated with abnormalities of the skull shape due to craniosynostosis (Chapter 10).

Mild syndactyly is relatively common. It has an incidence of 1 in 1650 to 1 in 3000 livebirths (Temtamy and McKusick, 1978). Syndactyly severe enough to interfere with fetal digital movement is rare. Triploidy, which is associated with syndactyly, has an incidence of 1 in 10,000 livebirths. The incidence of Apert syndrome, which includes syndactyly as one component, is 1 in 160,000 livebirths. There is an associated high neonatal mortality rate in this condition, which results in a 1 in 2 million incidence in the general population (Hill et al., 1987; Filkins et al., 1997; Boog et al., 1999).

Syndactyly is suggested by the inability to distinguish separate digits of the fingers and toes or to demonstrate independent movement of the fingers. Syndactyly is excluded if the fetus splays or interdigitates the fingers (Ginsberg et al., 1994).

Syndactyly may be one component of a syndrome. To make a syndromic diagnosis, the sonographer needs to decide if all four limbs are involved and what associated anomalies are present. Many of the syndromes associated with syndactyly also have synostosis of the cranial sutures. This may appear as acrocephaly, a tall, peaked skull shape with a high forehead, frontal bossing, and a prominent metopic suture.

One of the better known acrocephalosyndactyly syndromes is Apert syndrome. In Apert syndrome, complete syndactyly of the second through fourth digits is present, which leads to the characteristic “mitten-like” hands and feet (Figures 105-1 and 105-2). This condition also frequently involves the fifth finger. Multiple case reports have appeared in the literature regarding the prenatal sonographic diagnosis of Apert syndrome (Kim et al., 1986; Hill et al., 1987; Narayan and Scott, 1991; Parent et al., 1994; Filkins et al., 1997; Boog et al., 1999). The hallmark of the sonographic diagnosis of this condition is that despite prolonged observation, the fetus is never noted to have distinct or separate finger movements. The hands are never seen to open in this condition (Figure 105-1A). In addition, the toes appear fused (Figure 105-2A). Additional abnormalities characteristic of Apert syndrome include polyhydramnios and an acrocephalic calvarium (Hill et al., 1987), ventriculomegaly, partial agenesis of the corpus callosum (Parent et al., 1994), hydrocephalus (Kim et al., 1986), and dysmorphic features, including a high forehead, hypertelorism, and a depressed nasal bridge (Parent et al., 1994). Prenatal sonographic diagnosis of Apert syndrome has been made in both the setting of an affected mother with a fetus at 50% risk for inheriting the condition (Narayan and Scott, 1991), as well as in the setting of a negative family history with the disorder presumably due to a spontaneous mutation (Filkins et al., 1997; Hill and Grzybek, 1994; Parent et al., 1994). Three dimensional sonography improves visualization of both cranial and extremity anatomy (Esser et al., 2005).

Prenatal sonographic diagnosis of Carpenter syndrome has been reported in a twin gestation first studied at 17 weeks (Ashby et al., 1994). In this case report, one twin was noted to have clublike fetal hands, with the fingers maintained permanently in a flexed position. At 20 weeks of gestation, an abnormal, diamond-shaped head was noted in the axial plane along with preaxial polydactyly of the feet and duplication of the great toe. The fingers were described as being “pulled together,” and they never spread normally, which was noticed easily because the other twin had normal digital movements (Ashby et al., 1994).

Synpolydactyly is a rare dominantly inherited malformation of the distal limbs (Goodman, 2002). Its severity can range from partial cutaneous syndactyly to complete duplication of the digits.