Definition: Congenital disorder described by Gross in 1956 characterized by bilateral radial aplasia with normal thumbs and thrombocytopenia at a level < 100.000/mm3.
Synonyms: TAR syndrome, Megakaryocytopenia-absent radius, radial aplasia-thrombocytopenia syndrome.
Etiology: Autosomal recessive disorder.
Recurrence risk: The recurrence risk is 25%.
Diagnosis: The major finding is that of bilateral club hand due to the missing radii. In some cases the abnormal position of the hands has been detected in the first timester. A fetal blood sampling is then performed to assess the level of platelet and the number of megakaryocytes,. When cardiac and renal anomalies are present, oligohydramnios may develop during the second trimester, making the evaluation of the limbs more difficult. In such a case, both amnioinfusion and fetoscopy have been proposed.
Genetic anomalies: Not known. Radial ray aplasia may result from chromosomal, teratogenic, genetic, and multifactorial causes.
Differential diagnosis: The most common differential diagnoses include Trisomy 18 and Holt-Oram syndrome. Other differential diagnoses include other limb reduction abnormalities, in particular those that may present associated thrombocytopenia (Fanconi pancytopenia: usually have aplasia of the thumbs and chromosome fragility; Roberts-SC syndrome: usually present cleft lip and palate and premature separation of centromeric heterochromatin; Aase syndrome: usually present just mild forms of skeletal deformities).
Associated anomalies: The following anomalies may associate with thrombocytopenia absent radius in variable incidence: anemia, eosinophilia, and skeletal defects of upper and lower extremities such as ulnar, humeral, and femoral hypoplasia, congenital hip dislocation, toe syndactyly, talipes equinovarus and genu varum. Cardiac (30% of the cases) and renal defects may be found. An increase susceptibility to infections is also observed.
Prognosis: 40% of the affected die during early infancy. Hemorrhage and heart disease are the main causes of death. 7% have mental impairment. Motor developmental retardation is expected due to the skeletal defects. Hematological profile improves with age6. The prognosis improves after the first year of age.
Management: Termination of pregnancy can be offered before viability. The infusion of platelet has been advocated before delivery to prevent hemorrhages during delivery. Postnatally transplantation of allogeneic bone marrow from histocompatible sibling may correct persistently low platelet count.
 Gross. H, Groh C, Weippl G. Congenitale hypoplastische thrombopenie mit radialaplasie. Neue Osterr Z Kinderheilkd 1956, 1:574.
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