Bone marrow nonneoplastic

Alterations in cellularity

Thrombocytopenia absent radii (TAR) syndrome

Last author update: 1 December 2013
Last staff update: 13 November 2023

Copyright: 2002-2024,, Inc.

PubMed Search: Thrombocytopenia absent radii (TAR) syndrome

Dragos C. Luca, M.D.
Page views in 2024 to date: 119
Cite this page: Luca DC. Thrombocytopenia absent radii (TAR) syndrome. website. Accessed July 13th, 2024.
Definition / general
  • Also called selective aplasia, hypoplasia of megakaryocytes with missing radii
  • Congenital megakaryocytic disorder with thrombocytopenia and decreased / absent megakaryocytes
  • Impaired responsiveness to thrombopoietin (THPO) and high plasma THPO levels
Clinical presentation and diagnosis
  • Rare autosomal recessive disorder of unknown etiology with bilateral absence of radii (thumbs are present) and isolated thrombocytopenia (single line aplasia)
  • Lower limbs, GI, cardiovascular and other systems may be involved
  • Prenatal diagnosis is possible by detecting bone abnormalities (ultrasound) and thrombocytopenia (cordocentesis)
  • Usually severe thrombocytopenia ( < 50,000/μL)
  • May have anemia related to extensive bleeding
  • Normal or increased WBC
  • 50% have leukemoid reaction with WBC > 40,000/μL and left shift
Radiology images

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Absent radius

Case reports
  • Child with thrombocytopenia absent radius syndrome showing a larger 1q21.1 deletion than his healthy mother has (Eur J Med Genet 2012;55:120)
Treatment and prognosis
  • Platelet transfusions as needed; rarely stem cell transplantation may be required
  • May have increased risk of acute leukemia (Eur J Haematol 2003;70:246)
  • Platelet counts spontaneously reach normal levels by age 12 months
  • Peripheral blood hematological improvement is associated with normal megakaryocytes in bone marrow
Microscopic (histologic) description
  • Megakaryocytes and megakaryocyte precursors are absent or extremely scarce
  • Stains for antiplatelet membrane glycoproteins do not detect small immature megakaryocytes
  • Bone marrow may show erythroid hyperplasia if significant bleeding has occurred
Molecular / cytogenetics description
  • Deletions of 1q21.1 (RBM8A)
Differential diagnosis
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