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Thrombocytopenia-absent radius syndrome
Thrombocytopenia-absent radius (TAR) syndrome is a very rare congenital malformation syndrome characterized by bilateral radial aplasia and thrombocytopenia.
It occurs in less than 1 per 100,000 live births, equally affecting both sexes.
TAR syndrome presents with bilateral absence of the radius with present thumbs (that is the major distinguishing feature making TAR syndrome different from other disorders featuring radial aplasia), thrombocytopenia, and a number of additional features including skeletal and cardiac anomalies. Ulnar or humeral anomalies, and phocomelia in the most severe cases, may be present. Lower limbs may also be involved (dislocation of the patella and/or of the hips, absent tibiofibular joint, and lower limb phocomelia). Hypomegakaryocytic thrombocytopenia is present in all cases. Most affected individuals have bruising at birth, and typically present with petechiae and severe bleeding (gastrointestinal and, rarely, intracerebral) during the first years of life. Gradually over childhood the number of platelets increases and by adulthood the blood platelet count may be nearly normal or completely normal. Cow's milk intolerance (manifesting by persistent diarrhea and failure to thrive) is frequent. Congenital heart defects (atrial and/or ventricular septal defect, patent arterial duct, tetralogy of Fallot) occur in 15-30% of patients. Dysmorphic features (micrognathia, tall and broad forehead, low and posteriorly rotated ears) and renal malformations have been observed. Intellectual deficit is reported in less than 10% of patients (usually due to intracranial hemorrhage).
The etiology and inheritance pattern remain unclear. In all patients, a deletion on chromosome 1q21.1 has been found. However, at present, it is still unclear how this relates to the cause in TAR syndrome.
Diagnosis is made on the basis of the pattern of limb abnormality and associated features. Skeletal X-ray and examination of blood and bone marrow confirm the diagnosis.
The main differential diagnoses include Holt-Oram syndrome, Roberts syndrome, Fanconi anemia, thalidomide embryopathy and RAPADILINO syndrome (see these terms).
Prenatal diagnosis may be carried out using ultrasonography.
Autosomal recessive and autosomal dominant inheritance with variable penetrance have been suggested.
Management and treatment
There is no specific treatment for TAR syndrome. Prevention of bleeding and hemorrhage in the first years of life is essential to reduce the clinically significant morbidity. Severe thrombocytopenia may require platelet transfusions. Surgical interventions may be required to treat cardiac, urinary or skeletal malformations. Plastic surgery, occupational therapy and physiotherapy should be offered.
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