Bone marrow nonneoplastic
Alterations in cellularity
Thrombocytopenia absent radii (TAR) syndrome
Author: Dragos C. Luca, M.D.
Last author update: 1 December 2013
Last staff update: 13 November 2023
Copyright: 2002-2025, PathologyOutlines.com, Inc.
PubMed Search: Thrombocytopenia absent radii (TAR) syndrome
Table of Contents
Definition / general | Clinical presentation and diagnosis | Laboratory | Radiology images | Case reports | Treatment and prognosis | Microscopic (histologic) description | Molecular / cytogenetics description | Differential diagnosis | Additional referencesCite this page: Luca DC. Thrombocytopenia absent radii (TAR) syndrome. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/bonemarrowtarsyndrome.html. Accessed April 2nd, 2025.
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)
Laboratory
- 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
Images hosted on other servers:
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
- Congenital amegakaryocytic thrombocytopenia: biallelic mutations in C-MPL [TPO receptor] at 1p34
- Fanconi anemia: absent thumbs, permanent thrombocytopenia, DEB testing (Am J Med Genet A 2004;124A:35) and mitomycin C testing (C R Biol 2013;336:29) helpful
- Trisomy 18: absent thumbs, permanent thrombocytopenia, karyotyping
Additional references
- Foucar: Diagnostic Pathology: Blood and Bone Marrow, 1 Har/Psc edition, 2011, Foucar: Non-Neoplastic Disorders of Bone Marrow (Atlas of Nontumor Pathology), 1st edition, 2009, Penachansky: Pediatric Bone Marrow, 2004, OMIM: Thrombocytopenia-Absent Radius Syndrome; TAR, eMedicine: Thrombocytopenia-Absent Radius Syndrome
