Thrombocytopenia

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Chapter 44 THROMBOCYTOPENIA

Theodore X. O’Connell

General Discussion

Normal platelet counts in children and term infants are 150,000 to 450,000/μL of blood. Normal platelet counts in preterm infants are in this same range but tend to average on the lower end of normal. Thrombocytopenia is defined as a platelet count of less than 150,000/μL of blood.

Idiopathic (immune) thrombocytopenic purpura (ITP) is the most common cause of thrombocytopenia in childhood and usually occurs in children between the ages of 2 and 4 years. In about 90% of children with ITP, the disease is an acute, self-limited process that usually resolves within 6 months. Chronic ITP is more common in children older than 10 years and in those younger than 1 year of age. In ITP, the complete blood count (CBC) will show an isolated thrombocytopenia with normal white (WBC) and red blood cell (RBC) counts. Examination of the peripheral smear is important to evaluate platelet size, WBC differential and morphology, RBC morphology, and to rule out the presence of microangiopathic changes such as schistocytes, which might suggest another diagnosis. If the history, physical examination, CBC, and peripheral smear are normal except for thrombocytopenia, a bone marrow biopsy may be deferred.

Neonatal thrombocytopenia is relatively common, occurring in 2% to 3% of healthy term infants and 20% to 30% of all sick neonates. In general, the thrombocytopenia seen in a sick infant is likely related to the primary disease process and resolves as the primary process improves.

Neonatal alloimmune thrombocytopenia occurs in 1 in 1000 births. The typical presentation is that of a full-term infant who is otherwise healthy, born after an uncomplicated pregnancy and delivery, and found to have petechiae or bruising on examination and confirmed to have a significant degree of thrombocytopenia. The thrombocytopenia is caused by transplacental passage of maternal alloantibodies directed against fetal platelet antigens inherited from the father but absent on maternal platelets. This process is the platelet equivalent of Rh-hemolytic disease of the newborn. In contrast to Rh-hemolytic disease of the newborn, neonatal alloimmune thrombocytopenia often develops in the first pregnancy of an at-risk couple.

The thrombocytopenia seen in autoimmune thrombocytopenia is less severe than in alloimmune thrombocytopenia, with only 10% to 15% of patients having a neonatal platelet count less than 50,000/μL. As such, autoimmune thrombocytopenia is associated with a lower bleeding rate. The history will reveal a low maternal platelet count, maternal ITP, or possibly maternal systemic lupus erythematosus (SLE) or hypothyroidism. All neonates of mothers with autoimmune disease should be considered for cord blood platelet count at birth and again at 24 hours to evaluate for thrombocytopenia. In affected children, spontaneous recovery of the platelet count usually occurs within 3 weeks after birth.

If the history and physical examination are not suggestive of thrombocytopenia, pseudothrombocytopenia or “artifactual” thrombocytopenia should be considered. The platelet count can be repeated using a citrated tube to assess the count in the absence of platelet clumping.