Thrombocytopenia-absent radius syndrome (TAR), initially described almost 50 years ago, is a rare congenital disorder of incompletely disclosed etiology that presents with two clinical hallmarks - bilateral agenesis of the radius and thrombocytopenia [1] [2] [3] [4] [5]. Several studies have attempted to delineate the exact mutation responsible for this condition [1] [3] [5], and so far the RBM8A gene located on chromosome 1 seems to be the main location of mutations [2], but solid conclusions are still lacking. The complete absence of radial bones evidently manifests as a markedly shorter length of the forearms, but the bones of the hand are frequently preserved [1] [2] [6]. Still, a variable degree of functional impairment (limited finger extension and marked radial deviation of the hands) is seen in the majority of cases [2] [3]. Conversely, thrombocytopenia, arising from a defective maturation process of megakaryocytes in the bone marrow [3] [4] [5], develops in virtually all patients during the first several months of life, resulting in petechiae and predisposition to gastrointestinal or intracerebral bleeding that may be severe or even life-threatening [1] [2] [3]. In addition to bilateral radial agenesis and thrombocytopenia, other notable features of TAR syndrome include skeletal abnormalities (dysgenesis of axial skeleton, lower limbs and hip dislocation), cow's milk allergy (some authors associate this phenomenon with thrombocytopenia) that may be accompanied by diarrhea, improper development of the genitourinary organs (the female reproductive tract in particular), and congenital heart disease (eg. Tetralogy of Fallot and atrial septal defect) [1] [2] [3].

• Atrial Septal Defect

  Tetralogy of Fallot and atrial septal defect). [symptoma.com]

  Associations cleft lip /- palate congenital cardiac anomalies in 30% tetralogy of Fallot atrial septal defect (ASD) dextrocardia jaw under development : micrognathia partial absence of lower limbs fibula aplasia : 50% 8 Antenatal ultrasound Sonographic [radiopaedia.org]

  Cardiac abnormalities eg Atrial septal defect (ASD), ventricular septal defect (VSD), Fallot's tetralogy. [patient.info]

  The heart problems include holes in the atrial chamber of the heart (atrial septal defect) and tetralogy of Fallot. The name tetralogy of Fallot means there are four different defects of the heart. [encyclopedia.com]

  septal defects being the most commonly reported lesions. 1, 4 The genetic basis of TAR syndrome is uncertain. [jmg.bmj.com]

• Turkish

  Consanguinity in a Turkish family with thrombocytopenia with absent radius (TAR) syndrome. Hum Hered 1983 ; 63 : 94 –6. Shalev E, Weiner E, Feldman E, Cohen H, Zurkerman H. Micrognathia - prenatal ultrasonographic diagnosis. [jmg.bmj.com]

• Cardiomegaly

  Two patients not included in this figure had cardiac murmurs, in one associated with cardiomegaly. 1 ASD and tetralogy of Fallot were each seen in three patients making these the most frequent defect seen in the study of Hall et al. 1 ASD and VSD defects [jmg.bmj.com]

• Primary Amenorrhea

  A 22-year-old woman with TAR syndrome referred to our department with primary amenorrhea. Patient history, physical and laboratory examination, diagnostic laparoscopy, and karyotyping. Physical features in the combination of syndromes presented. [ncbi.nlm.nih.gov]

Because of the rather specific clinical and laboratory findings encountered in thrombocytopenia-absent radius syndrome, a proper physical examination may be sufficient to gain enough evidence about the presence of this ailment. Physicians should completely inspect the patients and observe prominent bilateral forearm shortening with a somewhat preserved anatomy of the hand, and it is not uncommon for the diagnosis to be made prenatally. In fact, fetal ultrasonography can be highly useful for screening the condition prenatally [6]. If thrombocytopenia is identified in the complete blood count (CBC, often being < 50 /109 mm3), or by obtaining a sample through cordocentesis (the preferred method to look at the fetal blood count) solid clinical suspicion could be raised, but it must be pointed out that the platelet count normalize over time [3] [4] [6]. Finally, genetic testing to confirm deletions on chromosome 1 and subsequent RBM8A gene mutations may be indicated as a definite measure to solidify the diagnosis [3]. Some authors classify thrombocytopenia-absent radius syndrome as an autosomal recessive genetic disorder [2] [3], indicating that a family history might provide useful information as well, and thus should be considered as a key component of the diagnostic workup.

1. Greenhalgh K, Howell R, Bottani A, et al. Thrombocytopenia-absent radius syndrome: a clinical genetic study. J Med Genet. 2002;39(12):876-881.
2. Toriello HV. Thrombocytopenia Absent Radius Syndrome. 2009 Dec 8 [Updated 2016 Dec 8]. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2017.
3. Klopocki E, Schulze H, Strauß G, et al. Complex Inheritance Pattern Resembling Autosomal Recessive Inheritance Involving a Microdeletion in Thrombocytopenia–Absent Radius Syndrome. Am J Hum Genet. 2007;80(2):232-240.
4. Ballmaier M, Schulze H, Strauss G, et al. Thrombopoietin in patients with congenital thrombocytopenia and absent radii: elevated serum levels, normal receptor expression, but defective reactivity to thrombopoietin. Blood. 1997;90(2):612-619.
5. Letestu R, Vitrat N, Massé A, et al. Existence of a differentiation blockage at the stage of a megakaryocyte precursor in the thrombocytopenia and absent radii (TAR) syndrome. Blood. 2000;95(5):1633-1641.
6. Bertoni NC, Pereira DC, Araujo Júnior E, Bussamra LC de S, Aldrighi JM. Thrombocytopenia-absent radius syndrome: prenatal diagnosis of a rare syndrome. Radiol Bras. 2016;49(2):128-129.