Thrombocytopenia-absent radius syndrome

Thrombocytopenia-absent radius (TAR) syndrome is a rare congenital disorder characterized by the combination of bilateral absence of the radius bones in the forearms and a reduced number of blood platelets (thrombocytopenia). Importantly, the thumbs are always present, which distinguishes TAR syndrome from other radial ray deficiency conditions such as Fanconi anemia. The condition is present from birth, with skeletal abnormalities being immediately apparent and thrombocytopenia typically most severe during infancy. Low platelet counts can lead to episodes of bleeding, including potentially life-threatening intracranial hemorrhage, particularly in the first year of life. Other skeletal anomalies may include shortening or absence of the ulna, abnormalities of the humerus, and lower limb defects such as hip dislocation, knee joint abnormalities, and phocomelia. Some patients also have cow's milk protein intolerance, which can exacerbate thrombocytopenia, as well as cardiac defects (notably tetralogy of Fallot and atrial septal defects) in a subset of cases. The thrombocytopenia in TAR syndrome tends to improve with age, and many patients experience a significant rise in platelet counts after the first few years of life, though counts may remain somewhat below normal. Management focuses on supportive care, including platelet transfusions during severe bleeding episodes or before surgical procedures, avoidance of cow's milk in intolerant individuals, and orthopedic interventions to optimize limb function. There is no cure for the condition. Genetic counseling is important for affected families. The syndrome is caused by a compound inheritance involving a microdeletion on chromosome 1q21.1 (encompassing the RBM8A gene) inherited from one parent, combined with a low-frequency regulatory single nucleotide polymorphism (SNP) in the RBM8A gene inherited from the other parent. With appropriate management during infancy, the long-term prognosis is generally favorable, and most individuals who survive the early period of severe thrombocytopenia can lead relatively normal lives.

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