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Thrombocytopenia-absent radius syndrome

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The thrombocytopenia-absent radius (TAR) syndrome is characterised by bilateral absence of the radius with both thumbs present (absent thumbs suggests other syndromes) and thrombocytopenia which may be transient and tends to remit in later childhood but is usually symptomatic in early infancy (90% within the first 4 months). The main conditions which might be mistaken for TAR syndrome are Holt-Oram syndrome, Roberts syndrome, Fanconi anaemia, thalidomide embryopathy, and Rapadilino syndrome. TAR syndrome was first described by Shaw and Oliver in 1959. A series of 34 patients has been reported from South West England (Greenhalgh KL and colleagues. J Med Genet 2002;39:876–81).

The 34 patients were recruited through Clinical Genetics Services and all had bilateral absence of the radius with thumbs and documented thrombocytopenia (platelets <150 × 109/L). Platelet count at birth (17 cases) ranged from 7 × 109 to 92 × 109 per litre.

The thumbs, though present, were always either hypoplastic or proximally placed. Details of the upper limb abnormalities are provided for 28 cases. Most (20) had relatively mild malformation with hypoplasia of ulna and humerus, normal shoulder girdle and normal strength. Five had more marked limb shortening with underdevelopment of the shoulder girdle and reduced strength and three had severe shortening of ulna and humerus and phocomelia. Thirteen of the 28 had lower limb abnormalities including small patella, knee subluxation, and severe leg bowing associated with abnormalities of hips, knees, and ankles.

Cow’s milk intolerance with vomiting, diarrhoea, and failure to thrive affected almost half (14/30) and was severe, requiring total parenteral nutrition, in two. Other associated abnormalities included renal anomalies (7), cardiac anomalies (5), and dysmorphic facial feature (16). Eight cases had a central facial capillary haemangioma, a feature not previously described in TAR syndrome. Thirteen probands had 20 siblings, four of whom were affected (empirical recurrence risk 20%, “to be interpreted with caution”). Twenty-seven of the 34 subjects were female.

Chromosome analysis, done in 16 cases, showed an abnormality in two (a duplication in chromosome 8 and a translocation involving chromosomes 1 and 7) both of uncertain significance. The pattern of inheritance remains unclear.