Bone marrow - nonneoplastic
Alterations in cellularity
Thrombocytopenia absent radii (TAR) syndrome

Topic Completed: 01 December 2013

Revised: 7 January 2019

Copyright: (c) 2002-2019,, Inc.

PubMed Search: Thrombocytopenia absent radii (TAR) syndrome

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Cite this page: Luca DC Thrombocytopenia absent radii (TAR) syndrome. website. Accessed January 21st, 2019.
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