Radial aplasia is one manifestation of a spectrum of anomalies known as radial ray malformations. These may occur unilaterally or bilaterally, and either as isolated malformations or in association with other birth defects (Figure 106-1). In radial aplasia, the defect occurs on the radial (thumb) side of the forearm. Skeletal findings in radial ray malformations may include absence or hypoplasia of the radius, with associated absence or hypoplasia of the scaphoid and trapezium bones of the wrist, with or without first metacarpal and thumb abnormalities (Lamb, 1972; Brons et al., 1990).

Radial aplasia results from arrest of radial longitudinal development, which may be secondary to damage at the apical ectoderm of the limb bud occurring between 6 and 12 weeks of gestation. One hypothesis regarding its etiology is that radial aplasia may result from abnormal blood vessel development, which causes an abnormal gradient of nutrients important for the differentiation of mesenchyme into bone or muscle (Van Allen et al., 1982). In radial aplasia, bones as well as associated muscles, nerves, and joints may be affected. Other causes for the developmental arrest of the radius include maternal infectious agents and local biochemical abnormalities secondary to maternal diabetes or medication.

In many cases, a correlation exists between the specific type of radial deformity and associated hematologic abnormalities (Bay and Levine, 1988). For example, absence of the radius with presence of the thumb affects the platelets and is a characteristic finding in the thrombocytopenia-absent radius (TAR) syndrome. If both the radius and thumb are absent, the hematologic findings are more severe; aplastic anemia generally results (Bay and Levine, 1988).

The incidence of radial aplasia is approximately 1 in 30,000 livebirths (Brons et al., 1990; Sofer et al., 1983). The condition is bilateral in approximately 50% of cases (Bay and Levine, 1988).

In radial aplasia, a single fetal forearm bone is identified, with acute radial deviation of the hand (Figure 106-2). The single forearm bone can be identified as an ulna by comparison with standard tables of ulnar lengths (Ylagan and Budorick, 1994). Published standards exist for normal humeral, radial, and ulnar bone lengths at different points in gestation (Jeanty et al., 1985). Detailed examination of the fetal extremities is not listed in current American College of Obstetricians and Gynecologists or American Institute of Ultrasound in Medicine (AIUM) guidelines for standard obstetric sonography (ACOG, 2008). However, examination of the extremities is critically important in the diagnosis of many genetic syndromes. The fetal limb buds may be seen sonographically as early as 8 weeks of gestation, with the limb articulations and digits becoming visible by 11 to 12 weeks of gestation (Bromley and Benacerraf, 1995).

The diagnosis of radial aplasia may be made as early as 14 to 15 weeks of gestation by noting the absence of the radius and the hand in varus position. Prolonged observation of the fetus will reveal that malposition of the hand persists during the entire sonographic examination. When the fetus moves its arms, the hand remains flexed but not rigid (Deschamps et al., 1992). More recently, three-dimensional ultrasound examination has been used to demonstrate radial agenesis in a male fetus with trisomy 18 (Huang et al., 2004).

In one report, Meizner et al. (1986) described the prenatal diagnosis of an affected fetus with absence of the radius and thumb at 18 weeks of gestation. They noted shortening and bowing of the ulna. The fetal hands were clubbed and deviated laterally. An additional finding included crossed renal ectopy of the right kidney. The family history was notable for an affected father and sibling (Sofer et al., 1983; Meizner et al., 1986). In a retrospective study of seven affected fetuses and infants with radial aplasia diagnosed during the perinatal period at the University Hospitals of Amsterdam and Rotterdam, six affected patients were noted to have associated abnormalities of the central nervous system, gastrointestinal tract, kidneys, or heart (Brons et al., 1990). Three of these affected fetuses had trisomy 18. A high degree of perinatal lethality was noted in this report; however, there may have been bias of ascertainment. These authors recommended both longitudinal visualization and transverse scanning of the bones of the extremities to clearly delineate the ulna from the radius. They recommended that the best time for visualization and separate measurements of the radius and ulna was approximately 13 to 16 weeks of gestation. Many of the cases in this report were complicated by abnormalities in amniotic fluid, including both oligohydramnios and polyhydramnios. These authors emphasized, however, that special attention should be paid to the central nervous system, heart, kidneys, vertebral column, and monitoring of fetal growth.

The differential diagnosis for conditions associated with radial aplasia is listed in Table 106-1. The conditions comprise four different categories: single-gene disorders, multiple congenital anomaly syndromes of unknown or sporadic cause, chromosomal abnormalities, and teratogen exposures. The most common chromosomal abnormality associated with radial aplasia is trisomy 18, although radial abnormalities are generally uncommon in trisomy 18 (Sepulveda et al., 1995). More commonly, a shortened radial ray is present. However, in one case of trisomy 18, upper limb defects (bilateral radial aplasia, and absent first metacarpals and thumbs) were the only sonographic abnormalities detected (Makrydimas et al., 2003). Although many authors list trisomy 13 as potentially being associated with radial aplasia, the much more common skeletal abnormality in trisomy 13 is polydactyly (see Chapter 129). In one report, a case of mosaic trisomy 22 was associated with radial aplasia (Dulitzky et al., 1981). In another case report, a translocation occurring between chromosomes 1 and 7 was associated with the presence of Wilms tumor and bilateral radial aplasia (Hewitt et al., 1991).