Thrombocytopenia with absent radius (TAR) syndrome is primarily characterized by the following two features:
absent fetal radii (bilaterally) with the presence of both thumbs
On this page:
Epidemiology
The condition is extremely rare, with an estimated incidence of 0.4 per 100,000 births ref.
Associations
-
congenital cardiac anomalies in ~30% ref
partial absence of lower limbs: fibula aplasia in ~50% 8
Pathology
The principal defect is considered absent or arrested development of the megakaryocyte progenitor cell or a progenitor cell with a maturational defect as there is minimally deleted region on chromosome 1q21.1 (deletion/duplication syndrome) ref.
Genetics
It is thought to carry an autosomal recessive inheritance with variable penetrance 3,8.
Radiographic features
Ultrasound
Sonographic features include:
bilateral absence of radii with normal thumbs 3 (crucial to the diagnosis)
radial deviation of the hands ref
the ulna is abnormal (usually shortened and malformed but may be absent bilaterally in ~20% of cases and unilaterally in ~10% of cases) ref
the humerus is abnormal in almost 50% of cases and absent bilaterally (humeral aplasia) in 5-10% of cases (a five-fingered hand arises from the shoulder ref
asymmetrical limb shortening, abnormalities of the shoulder joint and hypoplasia of the glenoid fossa, scapulae and clavicles and soft tissues of the arm and shoulder have been reported ref
Treatment and prognosis
Although TAR syndrome is not uniformly fatal, early prenatal diagnosis is important as the morbidity and mortality associated with this condition may be high. The prognosis is thought to be good if the patient survives to at least one year of age 6 when there is an improvement of the thrombocytopenia.
Management options include intrauterine platelet transfusion to prevent fetal hemorrhage in labor; a planned atraumatic delivery (cesarean section) or termination of pregnancy in the second trimester may be contemplated.
Complications
intracranial hemorrhage: especially during the first year of life can impair mental development
History and etymology
The TAR syndrome was initially described by H Gross et al. in 1956 4.
Differential diagnosis
The following diseases show radial hypoplasia as a component manifestation: