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At a glance

TAR is an acronym that stands for Thrombocytopenia, Absent Radius. Rare condition most severe in the neonatal period and early infancy. TAR is associated with thrombocytopenia and bilateral radial aplasia and presents with abnormalities in skeletal, gastrointestinal, hematologic, and cardiac systems. Pancytopenia similar to Fanconi. The major cause of mortality is hemorrhage. The incidence of hemorrhage is limited to the first 14 months of life.

Synonym

Tetraphocomelia-thrombocytopenia Syndrome.

History

It was first described in 1951. It has been proposed that the three medical conditions: ☞SC Phocomelia Syndrome, TAR Syndrome, and ☞Roberts Syndrome are not separate.

Incidence

Rare. The frequency is 2 to 10 cases per 1,000,000 live births in Spain. Male-to-female ratio is equal. This medical condition is very rare in the United States.

Genetic inheritance

Autosomal recessive.

Pathophysiology

Microdeletion on Chromosome 1 including the RBM8A gene and a single nucleotide polymorphism (SNP) either at the 5’ untranslated region (5’UTR) or within the first intron of RBM8A. RBM8A is located within the microdeleted region on chromosome 1q21 and encodes a protein designated Y14, a core subunit of the exon junction complex. The underlying mechanisms for the isolated thrombocytopenia and for the bilateral radius aplasia remain elusive.

Diagnosis

Finding of bilateral absence of the radius and thrombocytopenia.

Clinical aspects

Skeletal anomalies in addition to absent radii include ulnar hypoplasia, humeral hypoplasia, shoulder girdle hypoplasia, various hand anomalies, and lower limb dysplasias. Mandibular and maxillary hypoplasia are occasional findings. Congenital cardiac disease occurs in 30% of patients, most commonly tetralogy of Fallot and atrial septal defect. Additional findings may include hepatosplenomegaly and renal abnormalities. Symptomatic thrombocytopenia occurs in 50% of patients by the age of 1 week, although it may not occur until early adult life. Purpura, petechiae, epistaxis, gastrointestinal hemorrhage, and hematuria are common modes of presentation. Patients presenting outside the neonatal period may give a history of easy bruising. Thrombocytopenia may be episodic and is commonly precipitated by infection. In those patients surviving to adult life, the thrombocytopenia tends to be less severe and may be asymptomatic. Laboratory investigations may reveal a normal or low platelet count and eosinophilia. During episodes of thrombocytopenia, a leukocytosis with left shift is common. Anemia is usually secondary to blood loss rather than marrow aplasia. Megakaryocytes are not seen on examination of the bone marrow. The thrombocytopenia has been treated by steroids and splenectomy.

Precautions before anesthesia

Careful assessment of airway for maxillary/mandibular hypoplasia. Cardiovascular assessment: examine for evidence of cardiac disease; consider ECG and echocardiography. Cardiac catheterization as indicated by proposed procedure and cardiac status. Laboratory investigations: full blood cell count, electrolytes, urea, and ...

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