TARsyndrom

TAR syndrome, short for thrombocytopenia-absent radius syndrome, is a rare congenital disorder defined by two hallmark features: low platelet counts present in the newborn period (thrombocytopenia) and bilateral radial ray abnormalities, typically the absence or severe hypoplasia of the radius bones in the forearms. Thumbs are variably affected, and some individuals may have additional congenital anomalies such as heart or kidney defects. Bleeding due to thrombocytopenia often leads to easy bruising and petechiae in infancy, though platelet counts commonly improve with age.

Genetic basis and inheritance: Most TAR cases arise from a two-hit genetic mechanism. A microdeletion at chromosome 1q21.1 is inherited from one parent, while a hypomorphic RBM8A allele is inherited from the other parent, resulting in reduced RBM8A gene expression. This combination underlies the TAR phenotype. The condition is typically described as autosomal recessive-like because two pathogenic events are usually required, though parents may be unaffected carriers.

Diagnosis and management: Diagnosis combines clinical examination, complete blood counts showing thrombocytopenia, radiographic confirmation of radial ray defects, and genetic testing for 1q21.1 deletions and RBM8A variants. Management is multidisciplinary and supportive: careful handling to minimize bleeding, platelet transfusions for significant hemorrhage, orthopedic assessment for limb reconstruction or function-preserving procedures, and physical therapy to improve function. Bleeding risk often lessens with age, though limb and growth-related issues may require ongoing care.

Epidemiology and prognosis: TAR is very rare, with the exact prevalence unknown. The condition is lifelong, but many individuals experience improvements in thrombocytopenia during childhood; outcomes depend largely on the severity of limb anomalies and associated health issues.