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KEY POINTS
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Key Points

  • Defect occurs on the radial (formerly preaxial) side of the forearm. Findings may include absence or hypoplasia of the radius, absence or hypoplasia of the scaphoid and trapezium bones of the wrist, or abnormalities of the thumb and the first metacarpal.

  • Radial deformities may be associated with hematologic abnormalities.

  • Incidence is 1 in 30,000. Occurs bilaterally in 50% of cases.

  • May be diagnosed as early as 14-15 weeks. Sonographically, a single forearm bone is seen with radial deviation of the hand.

  • Differential diagnosis includes chromosome abnormalities, single-gene (dominant or recessive) disorders, teratogen exposure, and multiple congenital anomaly syndromes.

  • Should exclude trisomy 18 by performing a karyotype.

  • Delivery is recommended at a tertiary center to permit consultation with genetics, radiology, and orthopedics.

  • Recurrence risk depends on underlying condition.

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CONDITION
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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).

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Figure 106-1

Postnatal photograph of a newborn infant with bilateral radial aplasia and absentthumbs.

Graphic Jump Location
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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.

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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).

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INCIDENCE
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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 ...

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