This post is part of our “Understanding Upper Limb Differences” series from Dr. Paul Dell. Click here to see more from this collection. Do you have a question or topic you’d like to see Dr. Dell cover? Click here to make a submission.
Radial Longitudinal Deficiency is the most common upper extremity congenital longitudinal deficiency and presents with a wide spectrum of upper extremity differences from mild thumb hypoplasia to absent radius. The embryological development of the upper extremity occurs along 3 axes of the limb bud, one of which is the radial-ulnar axes. As the upper limb bud is developing in utero the growth advances from the ulnar (little finger side) to the radial side (thumb side). Normal formation of the hand and upper extremity is complete by the 8th week of pregnancy and if interrupted R.L.D. occurs in an estimated 1 in 5,000 live births.
There is a specialized area of the limb bud called the apical ectodermal ridge which responds to various “signaling factors“ to induce and stimulate cellular proliferation. The radial-ulnar axis development differentiates under the influence of what is called SONIC HEDGEHOG EXPRESSION IN THE ZONE OF POLARIZING ACTIVITY. Simply put, this control mechanism within the developing embryo activates a particular zone in the limb bud. It is thought that this development occurs from an ulnar to radial direction. Consequently, if development is interrupted by factors which we don’t understand, the ulnar side of the upper extremity forms normally while the radial aspect may not.
Radial Longitudinal Deficiency which is a musculoskeletal anomaly commonly occurs in conjunction with other congenital anomalies. Holt-Oram Syndromeis where radial aplasia occurs with congenital heart anomalies. There are serious blood disorders that may be associated with radial dysplasia. TAR SYNDROME is absent radius associated with deficient number of platelets (THROMBOCYTOPENIA), present at birth but later in infancy corrects. This association can be determined immediately at birth with a simple blood count. Fanconi’s Anemia develops later in childhood can also be diagnosed by blood work looking for a particular gene. Finally, VACTERL Syndromeincludes Vertebral Abnormalities, Anal atresia, Cardiac abnormalities, Tracheoesophageal fistula,Renal agenesis, and Limb defects. These associations grow stronger the more severe cases of RLD
Hereditary: Most of these diagnoses are recessive genes, i.e. both parents would need to carry the gene for their child to present with RLD. Remember, not every case may be explained by genetics and may be a mistake as the complex development of the upper limb develops.