TAR Syndrome

Synonyms: Thrombocytopaenia with absent Radius (hence TAR) syndrome, Tetraphocomelia-thrombocytopenia syndrome

• This is a rare autosomal recessive condition.
• One study in Spain found a frequency of 0.42 cases per 100,000 live births.¹

• Thrombocytopaenia - symptoms usually present in first week of life, with purpura, petechiae, epistaxis, melaena, haemoptysis, haematuria, haematemesis.
• Head and neck - Intracranial bleed, mental retardation, cerebellar hypoplasia, brachycephaly, micrognathia, squint, ptosis, small upturned nose, naevus flammeus on forehead.
• Upper limb abnormalities - bilateral absence of radii, hypoplasia or absence of ulna (may be bilateral), abnormal humerus, carpal bone hypoplasia or fusion, hypoplastic phalanges (but thumbs always present). The thumbs are usually of normal appearance, but held in a position of metacarpophalangeal flexion.
• Abdomen - pancreatic cysts, Meckel's diverticulum.
• Spine and pelvis - spina bifida, delayed motor development, hip dislocation, coxa valga.
• Lower limb anomalies (47%) - knee subluxation, patella dislocation, femoral or tibial torsion, fibula may be absent.

• Fanconi syndrome
• Holt-Oram syndrome - abnormalities of the upper limbs and heart
• RAPADILINO syndrome - radial hypoplasia/aplasia, patellar hypoplasia/aplasia, cleft or highly arched palate, diarrhoea, dislocated joints, small stature and other abnormalities
• Roberts syndrome - pre- and postnatal growth retardation, craniofacial anomalies, limb deficiencies and genital hyperplasia
• Trisomy 18 (Edward syndrome)
• VACTERL association - a syndrome of congenital anomalies including vertebral dysgenesis and renal and limb anomalies

Laboratory tests¹

• Full blood count may show:
  • Low platelet count - 15-30 X 10⁹/L
  • Eosinophilia (in 50% of patients)
  • Leukocytosis - WBC greater than 35 X 109/L with left shift, leukemoid reaction
  • Anaemia secondary to bleeding
• Genetic tests - although thought to be an autosomal recessive condition, genetic testing is usually normal, helping differentiation from other congenital conditions (see Differential diagnosis, above)

Radio-imaging

Ultrasound - transvaginal ultrasound can pick up skeletal involvement as early as 13 weeks gestation. The presence of radial aplasia signals the need to scan for abnormalities of the face, extremities and kidneys to differentiate this condition from other syndromes.³

Procedures¹

• Cordocentesis for fetal sampling may detect thrombocytopaenia prenatally.³ The risks are:
  • 1-2% fetal loss
  • Prolonged bleeding from the puncture site
• Bone marrow sampling may reveal:
  • Normal or hypercellular bone marrow
  • Decreased, absent, or immature megakaryocytes
  • Small, basophilic, vacuolated megakaryocytes
  • Erythroid hyperplasia

• Cow's milk intolerance (47%)
• Renal abnormalities (23%)
• Cardiac abnormalities (e.g. ASD, VSD, Fallot's Tetralogy) (15%)

Medical

• Prenatal transfusion of platelets has successfully been carried out on a fetus identified as having severe thrombocytopaenia on cordocentesis.⁵
• General measures to reduce the risk of bleeding should be taken if the platelet count drops below 80x10⁹/L. They include:
  • Avoidance of trauma (e.g. children should wear a soft helmet)
  • Avoidance of antiplatelet medication (e.g.aspirin and NSAIDs)
  • Prolonged pressure on injection sites
• Acute bleeding from a limb injury should be treated with pressure and limb elevation. The patient should be kept warm and conveyed to hospital as soon as possible.
• Platelet transfusion should be considered if the risk of transfusion (infection, anaphylaxis, haemolytic reaction) is superseded by the risk of significant bleeding. The rule of thumb is a platelet count below 40x10⁹).
• Leucocyte-reduced platelet concentrations may be indicated in patients at risk of severe bleeding.
• Patients who do not respond to transfusion may be helped by HLA-matched platelets from family members, although this does not guarantee success, as non-HLA antigen reactions may occur.
• Bone marrow transplantation should be considered for patients resistent to platelet transfusions.

Non-medical

• Physiotherapy and occupational therapy may help to improve function.
• Surgical correction of deformities may be necessary. This is sometimes preceded by splinting of the affected part to improve function.
• If surgery is not possible the goal should be to improve independence and functioning as much as possible. Adaptive devices and aids may be needed to help with washing, dressing and feeding.
• Prostheses are generally not helpful due to muscular weakness.
• Splenectomy is usually an effective cure for thrombocytopaenia persisting into adulthood.

Complications are usually related to haemorrhage, especially intracranial haemorrhage.

• If patients survive the first two years of life, the life expectancy is normal.
• The main cause of mortality is haemorrhage.
• Bleeding, especially intracranial haemorrhage, may result in significant morbidity.
• The prognosis regarding hand and upper extremity function is usually good, providing radial aplasia is the only skeletal abnormality.

Genetic counselling and early prenatal diagnosis raises the possibility of termination should the fetus be shown to be affected.⁶ A technique called array comparative genomic hybridisation - a method of assessing the errors in DNA reproduction associated with the condition - has considerably assisted the genetic counselling process.⁷