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Radial Ray Deficiency


Introduction

Ultrasound (US) evaluation of a fetus with a radial ray defect is a difficult diagnostic dilemma because the defect may be isolated, but can also result from chromosomal aneuploidy, teratogenic exposure, and more than 200 distinct genetic syndromes, including skeletal dysplasias, particularly those associated with mesomelia. Radial aplasia or hypoplasia is a rare abnormality occurring in 2 : 10,000 liveborns, but more common than isolated ulnar abnormalities. Frequently, when the radius is abnormal, the ulna is as well, as are the digits of the hands. Unilateral radial abnormalities are less likely to have an underlying genetic etiology compared to bilaterally affected limbs. Among the genetic etiologies, one of the more common disorders with radial ray anomalies is Cornelia de Lange syndrome (CDLS) ( Chapter 128 ).

Disorder

Definition

Radial ray defects comprise a large spectrum of upper limb anomalies that can range from partial (radial hypoplasia) to complete (radial aplasia) deficiency of the radius with or without accompanying deficiency of the thumb and other concomitant anomalies of the upper limbs. Some authors have classified them into four types based on their radiologic severity, with type I as the mildest and type IV showing complete absence of the radius, and there are other classification schemes which do not necessarily help identify the underlying genetic disorder.

Prevalence and Epidemiology

Isolated radial ray defects affect 2 : 10,000 newborns, though the incidence of more complicated upper limb deficiencies is approximately 5.25 : 10,000. However, if the defect is part of a genetic syndrome whether associated with aneuploidy or genetic disorders, then the prevalence is based on the incidence of the individual disorder. It is important to note that two-thirds of infants with congenital limb deficiencies have other congenital abnormalities and increased perinatal mortality and morbidity.

Etiology, Pathophysiology, and Embryology

The radius develops similarly to other appendicular bones through the process of endochondral ossification and develops from lateral plate mesoderm. Development of the limbs is under exquisite genetic control, and the genes that regulate the process are highly conserved through evolution. Radial ray deficiency is also seen in aneuploidies, including trisomy 18, 13, mosaic trisomy 16, and chromosomal microdeletion syndromes. Teratogens, specifically valproic acid, thalidomide, and aminopterin are established to cause disruption to the development of the upper limbs. Table 53.1 lists many of the disorders associated with radial ray deficiency, frequently associated with other congenital or morphologic abnormalities. Both bilaterally affected upper limbs and/or lower limbs are more likely to be associated with an underlying genetic mechanism.

TABLE 53.1
DISORDERS THAT INCLUDE RADIAL RAY DEFECTS

  • Aase syndrome

  • Cornelia de Lange syndrome

  • Duane ray syndrome/Okihiro syndrome

  • Fanconi anemia

  • Nager syndrome

  • AFD, Rodriguez type

  • Omphalocele–radial ray complex

  • Rothmund-Thomson syndrome

  • TAR (thrombocytopenia–absent radius) syndrome

  • Trisomy 13

  • Trisomy 18

  • Trisomy 10 mosaicism

  • Trisomy 16 mosaicism

  • 22q11 deletion

  • VATER/VACTERL association

  • Roberts SC phocomelia

  • Goldenhar syndrome

  • Treacher Collins syndrome

  • Baller-Gerold syndrome

  • Steinfeld syndrome

  • Gollop-Wolfgang complex

  • Fetal varicella syndrome

  • Valproic acid/thalidomide/aminopterin exposure

VATER/VACTERL, V ertebral, a nal, c ardiac, t racheal, e sophageal, r enal, and l imb anomalies.

Detailed history for similarly affected children will provide diagnostic clues to recessively inherited disorders such as Fanconi anemia. Parents should be evaluated for subtle hand abnormalities, particularly their thumbs, because autosomal dominantly inherited Holt-Oram syndrome can show variable expressivity, with some affected individuals only manifesting slight thumb abnormalities and no cardiac abnormalities. Thrombocytopenia–absent radius (TAR) syndrome has a complex inheritance pattern, but is inherited as a recessive disorder. For CDLS, almost 60% of cases result from heterozygosity for mutations in the NIPBL gene that encodes for homolog of delangin. A smaller percentage of CDLS also results from heterozygosity for mutations in SMC1A, SMC3 , and RAD21 , all encoding components of the cohesin complex. There are two X-linked forms of the disease due to mutations in SMC1A and HDAC8 .

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