Thrombocytopenia absent radius (TAR) affects the radius — the long, thin bone in the forearm — and platelet function. This defect commonly appears at birth or within the first few years of life.

Children with TAR may have prolonged bleeding and skin discoloration. Although experts do not know exactly what causes TAR, they have linked the condition to certain gene mutations.

This article discusses thrombocytopenia absent radius, clinical symptoms, diagnosis, and treatment options.

A doctor holding a baby's feet.Share on Pinterest
Getty Images

Thrombocytopenia absent radius (TAR) is a rare inherited disorder characterized by the absence of the radius on both arms. The radius is one of the two bones in the forearm. TAR also causes a significantly decreased platelet count, called thrombocytopenia.

The severity of skeletal deformity and thrombocytopenia varies for each child. Infants with thrombocytopenia may also have other abnormalities, such as low-set ears, cardiac problems, and a small lower jaw.

Learn about platelet levels with thrombocytopenia.

The primary symptoms of TAR are the absence of the radius bones and other thrombocytopenia features such as prolonged bleeding and skin discoloration.

Other symptoms include:

  • cow milk allergy
  • skeletal abnormalities affecting the limbs, ribs, and vertebrae
  • abnormal development affecting various systems or organs such as kidneys or the uterus.

Learn about primary and secondary thrombocytopenia here.

TAR is an autosomal recessive genetic disorder, which means infants with TAR inherit the defective gene from both parents.

Although the specific cause of the disease is unknown, scientists have linked mutations in particular genes to the condition. For example, a mutation in the RBM8A gene is one of the associations with TAR.

Further research is essential to understand the underlying genetic mechanisms fully and to identify other possible causes of TAR.

Learn about gestational thrombocytopenia.

To diagnose TAR, a doctor starts with a clinical examination of the infant. They typically look for physical features, such as an absent radius, other thrombocytopenia signs, and skeletal or end-organ malformations. The doctor will likely follow up with laboratory tests to confirm their diagnosis.

A doctor may order the following tests:

  • Complete blood count: This assesses the platelet count and confirms thrombocytopenia.
  • X-ray: An X-ray of the forearm can confirm any abnormality of the radius.
  • Ultrasonography: This evaluates organs such as the kidney or uterus.
  • Cardiac tests: These assess heart function.
  • Genetic testing: This confirms a TAR diagnosis.

Learn about the risk factors for thrombocytopenia.

Genetic testing involves examining the DNA for mutations that may cause the condition.

Doctors use genetic testing for screening or diagnostic purposes. If they suspect an infant has TAR, they will conduct genetic testing, which is essential for diagnosis. A test that reveals deletion of the RBM8A gene typically suggests TAR.

However, genetic test findings alone are insufficient to diagnose TAR because the presence of the RBM8A gene does not exclude the disease.

This means that the combination of clinical findings and laboratory and genetic test results is necessary to confirm TAR diagnosis.

Learn about hereditary thrombocytopenia.

The life expectancy and outlook of infants with TAR syndrome typically depend on the severity of the condition.

Infants with severe thrombocytopenia are at risk of developing severe bleeding, which may occur in the brain. Brain hemorrhage is typically life threatening and may also affect cognitive function.

In contrast, the outlook in babies with less severe forms of TAR is typically favorable, especially with early diagnosis and prompt treatment.

Learn about thrombocytopenia in children.

The treatment of TAR is typically a combination of medical and or surgical interventions.

Treatment options include:

  • Medical therapy: Platelet transfusion is crucial in cases of severe thrombocytopenia and avoiding cow milk due to the risk of allergic reactions. Some infants may require specific treatments depending on presenting organ abnormalities.
  • Surgical interventions: Babies with a defective radius that could impact the limb function may require orthopedic surgery.

After treatment, follow-up care is essential in monitoring the infant’s response to therapy. This follow-up care includes physical examination and conducting and interpreting laboratory tests.

Learn more about treating thrombocytopenia.

A caregiver or parent with an infant with TAR may have concerns or questions about the condition. Here are some questions to ask a doctor:

  • What are the potential complications of TAR?
  • What are the symptoms of TAR?
  • What is the future outlook for a child with TAR?
  • What are the specific treatment options for TAR?
  • Is TAR recurrent in future pregnancies?

Thrombocytopenia-absent radius (TAR) is a rare genetic disorder associated with the absence of the radius and thrombocytopenia.

Infants with TAR may also present with different symptoms, such as cow milk allergy, skeletal abnormalities affecting the limbs, ribs, and vertebrae, and abnormal development of various systems or organs.

Scientists have linked mutation in the RBM8A gene as a possible cause of TAR. Treatment options involve medical and surgical intervention.