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Thrombocytopenia-absent radius syndrome
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Thrombocytopenia-absent radius syndrome

Thrombocytopenia-absent radius (TAR) syndrome is a rare genetic condition defined by the absence of forearm bones and a deficiency in blood-clotting cells.

Overview

Thrombocytopenia-absent radius (TAR) syndrome is characterized by the absence of the radius bone in the forearms and a shortage of platelets (thrombocytopenia). The platelet deficiency is most severe during infancy and can lead to easy bruising, nosebleeds, and life-threatening hemorrhages.

Symptoms

  • Skeletal issues: Absence of the radius in both arms; the ulna may also be underdeveloped or absent. Unlike other conditions with absent radii, individuals with TAR syndrome typically have thumbs.
  • Blood issues: Thrombocytopenia, which may improve over time.
  • Other features: Potential heart or kidney malformations, and facial features such as a small lower jaw, prominent forehead, and low-set ears.
  • Allergies: Approximately half of affected individuals have an allergy to cow's milk, which can exacerbate the platelet deficiency.

Causes

TAR syndrome is caused by mutations in the RBM8A gene. Most affected individuals have one mutation in the RBM8A gene and a deletion of genetic material on chromosome 1 that includes the other copy of the gene. This reduction in the RNA-binding motif protein 8A is believed to disrupt tissue development, though the exact mechanism remains under study.

Inheritance

TAR syndrome is inherited in an autosomal recessive pattern. Affected individuals inherit an RBM8A mutation from one parent and, in about 75 percent of cases, a specific 200-kb deletion on chromosome 1 from the other parent. In other cases, the deletion occurs spontaneously during early development. Parents who carry these genetic changes typically do not exhibit symptoms of the syndrome.

Frequently Asked Questions

Platelet counts in TAR usually start low at birth and may gradually rise over the first few years of life. Many children see a significant increase by age 2–3, though some remain thrombocytopenic into adolescence. Regular monitoring by a hematologist is essential to adjust treatment plans and to determine when platelet transfusions or other interventions can be safely reduced.
void non‑steroidal anti‑inflammatory drugs (NSAIDs), aspirin, and other medications that impair platelet function unless prescribed by a specialist. Limit contact sports that increase the risk of injury, and use protective gear during play. Prior to any dental work, surgery, or invasive procedures, coordinate with a hematologist to plan platelet transfusions or antifibrinolytic agents as needed.
Seek emergency care if the child has uncontrolled bleeding from a cut, nosebleeds lasting more than 5–10 minutes, bleeding into the gums or eyes, bruising that spreads rapidly, or any signs of internal bleeding such as abdominal pain, vomiting blood, or blood in the stool. Persistent or severe bleeding that does not stop with basic first‑aid measures warrants urgent evaluation.
Many children with TAR have a cow’s milk allergy that can exacerbate platelet counts. An elimination diet that removes cow’s milk and dairy products, confirmed by IgE or skin‑prick testing, can improve platelet levels in some cases. Work with an allergist and a dietitian to ensure adequate calcium, vitamin D, and protein intake from alternative sources while avoiding dairy. Regular monitoring of platelet counts after dietary changes helps assess the impact.===
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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