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Thrombocytopenia-absent radius (TAR) syndrome is a rare genetic condition that is present at birth. Children with TAR syndrome have a decreased number of platelets, blood cells that are required for blood clotting. They also are missing a bone in the arm, the radius. Children with TAR syndrome have thumbs, while children with other similar disorders do not (Fanconi's anemia and Diamond-Blackfan anemia). This helps to distinguish these different disorders, which are treated differently and have a different prognosis.
TAR syndrome is occasionally inherited, or passed down in families, but usually it appears without any other affected people in a family. There do not appear to be any risk factors for having the syndrome other than if a parent is affected. Most people do not have a family history. About 1 child in every 100,000-200,000 babies is affected.
A child with TAR syndrome will have noticeable problems at birth. The most obvious is the lack of a radius in the forearm. Other bones in the arm might also be missing. Hip dislocation at birth can also be present, and occasionally bones in the lower limbs might be missing as well.
The low platelet number starts within the first few weeks of life. These can result in episodes of bleeding that may be serious. The platelet count may change, and generally improves as children get older. Most will have normal platelets by the time they start school. About 20% of children have cow's milk allergy and exposure to cow's milk may worsen the platelet count.
About a fifth of children may have other problems such as simple heart problems (septal defects, or "holes in the heart"). These are usually relatively easy to repair. Some may have kidney problems as well.
TAR syndrome is caused by the loss of a small piece of DNA on chromosome 1. The lost piece of the chromosome is at location 1q21.1 but TAR syndrome is different from two other syndromes (1q21.1 deletion and 1q21.1 duplication syndrome) that also involve losing a small piece of DNA at this location.
The TAR deletion can be detected using a genetic test done from a blood sample. Finding this deletion confirms that a person has TAR syndrome. Occasionally, TAR syndrome may be suspected because of an abnormal ultrasound study during pregnancy. In this case, an amniocentesis could be used to confirm that the baby has TAR syndrome by testing for the deletion. However, it is possible to have the deletion and yet have no symptoms.
Most children with TAR syndrome have normal intelligence and a normal life span. Bleeding episodes are the most serious risk, but can be managed by checking the platelet count frequently and treating as necessary. A serious bleeding episode in the head is the most damaging outcome. Improving the use of the limbs depends on how severe the abnormalities are, but there are a wide variety of prostheses and surgeries that can be helpful.
Genetics Home Reference
Somewhat short, but not excessively technical, summary of TAR syndrome, with some useful links.
TAR Syndrome Support Group
A website with a lengthy review of TAR syndrome that may be accessible with effort to most parents. The support group is based in the UK.
The Wikipedia page is extremely limited, but accurate.