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Thrombocytopenia Absent Radii (TAR)

TAR patients are missing one of the two bones from each lower arm. The missing bone is called the radius, and it runs from the elbow to the wrist on the side of the thumb. The thumbs are not missing, as they are in some of the other bone marrow failure disorders (see Fanconi anemia, Diamond-Blackfan anemia). Bruising is due to decreased production of platelets (the cells which help the blood to clot) by the bone marrow and is usually apparent at birth. The gene for TAR has not been identified. Males and females are affected equally.

  1. What are the major findings on physical examination?
    1. The radius bone is missing from both lower arms.
    2. The thumbs are present.
    3. Patients may have small shoulders.
    4. Patients may have abnormal knees such as bow legs or knock knees.

  2. What is the age at diagnosis?
    1. Patients are almost always diagnosed at birth.

  3. What is the pattern of bone marrow failure?
    1. The illness begins with a low platelet count.
    2. Patients with TAR do not develop aplastic anemia (a condition seen in some of the other bone marrow failure disorders, when all 3 types of cells (red cells, white cells and platelets) are abnormally low because the bone marrow is not producing them).

  4. What specific kinds of cancer develop?
    1. Leukemia (cancer of the blood and bone marrow)
    2. It is not yet clear whether patients with TAR are truly at increased risk of developing cancer.

  5. How is TAR specifically diagnosed?
    1. The diagnosis is made by physical examination, in which the radius bones in the arms are found to be missing.
    2. A low platelet count
    3. The specific gene or genes for TAR remain to be discovered. There is an association with a deletion (absence) of part of chromosome 1 inherited from one parent (who is normal), but the mutated gene from the other parent has not been identified.
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