TAR | Thrombocytopenia with Absent Radius

Definition

  • Rare congenital disorder
  • Thrombocytopenia with bilateral absent radius
  • Thumbs present

Aetiology

  • Autosomal recessive
  • 1q21.1 microdeletion including RBM8A on one allele
  • Hypomorphic noncoding RBM8A variant on the other allele

Clinical Features

  • Bilateral complete absence of the radius in all cases (Bayne and Klug type IV)
  • Thumbs present, possibly hypoplastic with weak opposition
  • Radially deviated wrist and hand with a short, bowed ulna
  • Humeral hypoplasia, phocomelia in severe cases
  • Lower limb anomalies common, including knee stiffness, patellar dislocation or absence, hip dysplasia and clubfoot
  • Cardiac anomalies (tetralogy of Fallot, ASD), renal anomalies and facial capillary haemangiomas
  • Cows’ milk intolerance

Pathology

  • Hypomegakaryocytic thrombocytopenia, most severe in the first year
  • Intracranial haemorrhage in infancy can be fatal
  • Bleeding triggered by cows’ milk intolerance and infection
  • Platelet numbers improve after infancy
  • Marrow shows reduced megakaryocytes, other lines normal apart from leukaemoid reactions

Differential Diagnosis

  • Thumb present with absent radius distinguishes TAR
  • Fanconi anaemia, autosomal recessive, thumb hypoplasia, later pancytopenia and positive chromosome breakage
  • Holt-Oram syndrome, autosomal dominant, cardiac defects, absent or triphalangeal thumbs
  • VACTERL association
  • Nonsyndromic radial longitudinal deficiency

Investigations

TAR | Thrombocytopenia with Absent Radius, radiograph of the forearm showing an absent radius and radially deviated hand in thrombocytopenia absent radius syndrome
Radiograph of the forearm showing an absent radius and radially deviated hand in thrombocytopenia absent radius syndrome. Image by Riaz Ahmad Department of Orthopaedics, Weston General Hospital, Weston-Super-Mare, UK, Wikimedia Commons, CC BY 2.0.
  • Full blood examination and film
  • Chromosome breakage testing to exclude Fanconi anaemia
  • Genetic testing for 1q21.1 deletion and RBM8A variant
  • Echocardiography and renal ultrasound
  • Upper and lower limb radiographs

Management

  • Multidisciplinary, with platelet transfusion for bleeding and before surgery
  • Surgery deferred until platelets improve after the first year
  • Stretching and splinting first
  • Centralisation or radialisation corrects radial deviation, sometimes after soft tissue distraction
  • Many achieve good hand function without surgery, as the thumb is present
  • Correction risks recurrence, stiffness and ulnar physeal injury, so surgery is selective
  • Knee deformity may need release, osteotomy or guided growth, with variable results

Prognosis

  • Mortality highest in infancy from haemorrhage
  • Normal intelligence unless intracranial haemorrhage occurred
  • Haematological problems improve with age

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.