Also known as: Thrombocytopenia and Absent Radius
Platelets are small, clear bodies in the blood that allow it to clot. In thrombocytopenia and absent radius (TAR) syndrome, there are low platelets and absence of the radius in both arms (bilaterally), but the thumb is still present. The radius is a bone in the forearm that sits on the side of the forearm where the thumb is.
Last reviewed September 2020 by Professor J Hall (retired), Professor Emerita of Pediatrics and Medical Genetics, Department of Pediatrics, British Columbia Children’s Hospital, Vancouver, Canada.
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TAR syndrome is characterised by absence of the radius in both arms and a reduction in the number of platelets. As platelets help blood to clot, a reduction means there may be a higher level of bleeding and bruising. People with TAR syndrome may also be slightly shorter than normal and underdevelopment of other bones in the arms and legs may occur. TAR syndrome may be associated with distinctive facial features including a micrognathia (small lower jaw), a prominent forehead and low-set ears. About 50 per cent of people with the condition experience difficulty digesting cow’s milk (the symptoms of which are diarrhoea, failure to thrive and lowering of platelets), whilst around 30 per cent have a congenital heart defect (see entry Heart Defects). There can also be problems with the kidneys.
Almost everyone diagnosed with TAR syndrome has a deletion from the long (q) arm of chromosome 1 at position 1q21.1. On the other chromosome 1 there is also usually a small change (known as a single nucleotide polymorphism, SNP) in the DNA sequence in the regulatory region of the gene RBMSA.
Diagnosis can be made on the observation of the clinical features of TAR syndrome. X-rays to look at the bones of the skeleton and examination of blood and bone marrow to look for low levels of megakaryocytes (the cells that make platelets) will confirm the diagnosis. Specific testing is available to establish carriers (those that carry the genetic mutation causing the condition, but are not affected).
There is no cure for TAR syndrome. Treatment focuses on relieving the symptoms. Platelet transfusion (receiving platelets directly into one’s circulation) for thrombocytopenia may be required to boost platelet levels and reduce bleeding. If repeated platelet transfusions are needed, a single donor should be identified so as not to develop antibodies.
An orthopaedic specialist will need to advise on interventions to help with limb problems. Cow’s milk should be avoided to reduce the severity of gastroenteritis and to avoid making thrombocytopenia worse. Surgery to mend heart defects may be also be required.
The deletion of genetic material on chromosome 1 associated with this condition can be passed through generations in families. SNPs in the regulatory region of RBMSA can also be passed through many generations. Some deletions occur sporadically (by chance) in people with no history of the disorder in their family. Once TAR has occurred, the recurrence risk for future children of those parents may be as high as 25 per cent. The risk to a person with TAR of having affected children is very small but their partner could be tested for molecular changes that would predispose them to having an affected child. In addition, the siblings of the affected individual could be tested to see if they are carriers.
An ultrasound scan at 12 weeks may show up the absent radii and other limb deformities. Chorionic villus sampling can be offered to detect the genetic change causing TAR syndrome if the mutation in the family is identified.
There is not currently a support group for TAR syndrome in the UK.
Families can use Contact’s freephone helpline for advice, information and, where possible, links to other families. To meet other families with disabled children, join Contact’s closed (private) Facebook group.