The thrombocytopenia-absent radius syndrome, a rare medical entity, is distinguished by bilateral aplasia of the radius on both arms and thrombocytopenia. Additional anomalies include congenital heart disease, agenesis of the genitourinary system, skeletal abnormalities of the lower limbs and the axial skeleton, as well as cow's milk intolerance. The diagnosis rests on clinical findings, ultrasonography (either prenatal or postnatal), laboratory studies, and genetic testing, although the exact mechanism of the disorder remains to be elucidated.
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].
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.