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Thrombocytopenia-Absent Radius Syndrome

Last updated Dec. 15, 2019

Approved by: Krish Tangella MD, MBA, FCAP

Thrombocytopenia-Absent Radius Syndrome (TAR Syndrome) is a rare genetic disorder that is characterized by low platelet counts and absence of the long bones, called radius, in both forearms.

What are the other Names for this Condition? (Also known as/Synonyms)

  • Radial Aplasia-Thrombocytopenia Syndrome
  • TAR Syndrome
  • Tetraphocomelia-Thrombocytopenia Syndrome

What is Thrombocytopenia-Absent Radius Syndrome? (Definition/Background Information)

  • Thrombocytopenia-Absent Radius Syndrome (TAR Syndrome) is a rare genetic disorder that is characterized by low platelet counts and absence of the long bones, called radius, in both forearms. It typically manifests within 7-10 days following birth of the child
  • The condition is caused by mutation in the RBM8A gene. This gene codes for a RNA-binding protein. Decreased levels of this protein lead to developmental problems with specific tissues. Having a family history of the condition is a major risk factor for developing TAR Syndrome
  • The chief symptoms of Thrombocytopenia-Absent Radius Syndrome include low platelet count and absence of radius bones in the arms. The low platelets count (termed thrombocytopenia) leads to easy bruising and bleeding, purple skin discoloration, frequent nosebleeds, blood in urine and vomit. Abnormal bone development may also be reflected in facial features, kneecaps, lower legs and spinal cord
  • The diagnosis of TAR Syndrome can be performed via a trans-vaginal ultrasound during pregnancy or by undertaking the genetic testing of fetal blood, if the condition is suspected. After birth, full blood count, imaging tests, and bone marrow sampling may be used in the diagnosis of Thrombocytopenia-Absent Radius Syndrome. If required, a genetic testing can be used to confirm the findings
  • Intracranial hemorrhage is a significant complication that can develop, due to TAR Syndrome, which may be fatal. Other complications include short stature, improper development of bones in the arms and legs, and heart and kidney malformation
  • The treatment of Thrombocytopenia-Absent Radius Syndrome may involve transfusion of platelets during birth, bone marrow transplantation, and surgical measures including physiotherapy for bone-associated abnormalities. The prognosis for TAR Syndrome is reported to be good, if the infant survives beyond 24 months
  • Since Thrombocytopenia-Absent Radius Syndrome is a genetic disorder, currently no preventive measures are available. However, genetic counseling for prospective parents can help them understand the condition better. In the affected individuals, bruising and bleeding episodes may be managed by avoiding injury and trauma

Who gets Thrombocytopenia-Absent Radius Syndrome? (Age and Sex Distribution)

  • Thrombocytopenia-Absent Radius Syndrome is a congenital disorder that manifests in newborns in their first week of life
  • It is a rare disorder that affects less than 1 in 100,000-200,000 newborns worldwide
  • There are no reports of racial or gender bias in the occurrence of TAR Syndrome

What are the Risk Factors for Thrombocytopenia-Absent Radius Syndrome? (Predisposing Factors)

  • Thrombocytopenia-Absent Radius Syndrome is an inherited congenital disorder, and therefore, having a family history of the condition is the greatest risk factor for the disorder
  • In about 25% of the affected individuals, the TAR Syndrome arises due to a spontaneous deletion in the causative gene, RBM8A. In such cases, the risk factors for this genetic abnormality is not precisely known

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Thrombocytopenia-Absent Radius Syndrome? (Etiology)

Thrombocytopenia-Absent Radius Syndrome can be caused by inherited or sporadic mutations. In a majority, TAR Syndrome is caused by a mutation or deletion in the RBM8A gene, which localizes to the long arm of chromosome 1.

  • Under normal circumstances, the RBM8A gene codes for a protein known as the RNA-binding motif protein 8A. The majority of this protein is nuclear, and is important for the synthesis of other proteins involved in crucial cellular functions
  • In individuals with TAR Syndrome, there is reduced or absent RNA-binding motif protein 8A. Since this protein is involved in important cellular processes, the development and function of many cell types in a fetus can be affected. The reduced RNA-binding motif protein 8A can be the result of:
    • A mutation in the RBM8A gene, which is a heritable trait. This type of mutation is inherited in an autosomal recessive manner, in which an individual must possess two copies of the causative gene in each cell of the body for the condition to manifest itself. Typically, both parents are symptomless, but carriers of the condition
    • A 200 kb (200,000 base pairs) deletion of genetic material in the long arm (q) of chromosome 1, which eliminates one copy of the RBM8A gene, leaving only one functional copy. This deletion is known to occur spontaneously and is believed to be non-heritable
    • A combination of a mutation in one copy of the RBM8A gene (inherited) and a deletion of the other copy, leading to absence of the crucial protein. This is known as allelic heterogenicity, where a single abnormality is caused by different modifications in one gene
    • Mutation of more than one gene causing different diseases. This theory is currently under study investigation
  • The mutation or deletion, or a combination of mutation/deletion of the RBM8A gene is believed to result in reduced or missing megakaryocytopoiesis. It is the process of formation and maturation of platelet cells, which is either not found or is reduced, leading to one of the significant symptoms of the disease, known as thrombocytopenia

What are the Signs and Symptoms of Thrombocytopenia-Absent Radius Syndrome?

The signs and symptoms associated with Thrombocytopenia Absent Radius Syndrome include the following:

  • Related to thrombocytopenia:
    • Purpura: Red or purple discoloration appears on the skin
    • Petechiae: Red or purple spots in the body caused by the breakage of blood vessels
    • Epistaxis or nose bleeding
    • Melena: Black feces that are caused by gastrointestinal hemorrhage
    • Hemoptysis: Coughing up blood from the lungs
    • Hematuria or blood in urine
    • Hematemesis or vomiting blood
  • Related to the head and neck region:
    • Hemorrhage or bleeding in the skull
    • Unusually small or missing cerebellum
    • Disproportionately large or wide head
    • Micrognathia or disproportionately small jaw
    • Drooping eyelids, small upturned nose
    • A vascular birthmark on forehead caused by overgrowth of capillaries
  • Related to upper limb:
    • Absence of radius bone in both forearms
    • Underdevelopment of tissues
    • Absence of the ulnar bone in the forearm
    • Abnormal humerus, which is the long bone extending from the shoulder to the elbow
    • Carpal bone hypoplasia or fusion: The carpal bones that make up the wrist and allow them to rotate and move vertically, are either underdeveloped or merged together
    • Thumbs appear normal, but there is difficulty in bending them
  • Related to the abdomen:
    • Pancreatic cyst: An unusual growth in the pancreas
    • Meckler’s diverticulum: A small bulge in the small intestine present at birth
    • Intolerance to cow’s milk
  • Related to spine and pelvis:
    • Spina bifida: Underdevelopment of the spinal cord and its coverings
    • Delayed motor developments
    • Dislocation of the hip
    • Coxa volga: An abnormality that occurs in the hip joint, wherein the angle between the neck and the shaft of the femur is abnormally wide.
  • Related to the lower limbs:
    • Knee subluxation: The knee cap is partially or totally dislocated
    • Patellar dislocation: Injury caused to the knee by twisting of the leg
    • Femoral or tibial torsion: Upward twisting of the thigh bone and inward twisting of the shin bone
    • Absence of fibula: The outer of the two bones in the lower leg is totally missing
    • Hypoplastic phalanges: Improper bone development in the toes
  • Developmental issues:
    • Short stature
    • Learning difficulties

How is Thrombocytopenia-Absent Radius Syndrome Diagnosed?

Thrombocytopenia-Absent Radius Syndrome may be diagnosed through the following methods:

  • A complete physical examination and assessment of signs and symptoms
  • A thorough evaluation of family medical history
  • Blood tests to check for counts of different blood cell types (complete blood count)
  • Imaging tests to determine bone abnormalities
  • Fetal blood sampling from the umbilical cord, to check for fetal genetic abnormalities
  • Bone marrow sampling, to check for the formation and maturation of different blood cell types
  • Genetic testing for confirming mutation in or deletion of RBM8A gene, as well as to distinguish TAR Syndrome from other condition with overlapping symptoms

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Thrombocytopenia-Absent Radius Syndrome?

Some potential complications of Thrombocytopenia Absent Radius Syndrome include:

  • Bruising and bleeding from small injuries can lead to significant blood loss
  • Chronic blood loss can lead to anemia
  • Bleeding in the skull, which can be fatal
  • Effect on quality of life, owing to bone deformities and other developmental issues

How is Thrombocytopenia-Absent Radius Syndrome Treated?

The treatment options for Thrombocytopenia Absent Radius Syndrome may include:

  • Transfusion of platelets, to compensate for reduced blood platelets
  • Avoiding physical injury and trauma to minimize bleeding events
  • Avoiding blood-thinning medication
  • Bone marrow transplants may be considered, when platelet transfusion is ineffective
  • If warranted, surgery may be undertaken to correct bone deformities
  • Surgical removal of spleen (splenectomy) for treatment of thrombocytopenia may be helpful, in some cases. A splenectomy may help in improving low platelet counts
  • Physical and adaptive aids to help with independent movement
  • Physiotherapy and occupational therapy

How can Thrombocytopenia-Absent Radius Syndrome be Prevented?

Thrombocytopenia-Absent Radius Syndrome is a genetic condition, and therefore, there are no guidelines or specific methods of preventing the condition from developing.

  • Genetic testing of the expecting parents (and related family members) and prenatal diagnosis (molecular testing of the fetus during pregnancy) may help in understanding the risks better during pregnancy
  • If there is a family history of the condition, then genetic counseling will help assess risks, before planning for a child
  • Active research is currently being performed to explore the possibilities for treatment and prevention of inherited and acquired genetic disorders such as TAR Syndrome
  • Regular medical screening at periodic intervals with tests and physical examinations are strongly recommended

Although the TAR Syndrome cannot be prevented, it may be possible to minimize bleeding events by:

  • Avoiding physical injuries or trauma
  • Keeping the lower limbs elevated in case of injury to the legs
  • Avoiding anti-inflammatory, anti-platelets, steroidal and blood-thinning medications (such as aspirin), when possible

What is the Prognosis of Thrombocytopenia-Absent Radius Syndrome? (Outcomes/Resolutions)

The prognosis of Thrombocytopenia-Absent Radius Syndrome is dependent upon the extent and severity of the symptoms:

  • If an affected infant survives the first 2-years successfully, the outcome is generally good
  • The prognosis for upper extremity function is good, if the affected individual only has radial aplasia

Additional and Relevant Useful Information for Thrombocytopenia-Absent Radius Syndrome:

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Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: April 3, 2017
Last updated: Dec. 15, 2019