The management of alloimmune neonatal thrombocytopenia

Neonatal alloimmune thrombocytopenia (NAITP), defined as thrombocytopenia (platelet count < 150 x 10(9)/l) due to transplacentally acquired maternal platelet alloantibodies, occurs in approximately I per 1200 live births in a Caucasian population. In such a population, the majority (>75 percent) of cases are due to fetomaternal incompatibility for the platelet specific alloantigen, HPA-la (Pl(Al), Zw(a)). Incompatibility for the HPA-5b (Br-a) alloantigen is the next most frequent cause of NAITP in Caucasians; much less common is NAITP due to incompatibility for HLA, blood group ABO or other platelet-specific antigens. In non-Caucasian populations (e.g. Orientals) HPA-la incompatibility is a rare cause of NAITP and other alloantigens e.g. HPA-4b (Pen(b), Yuk(a)) are implicated. The greatest clinical challange relates to the antenatal management of pregnant women alloimmunized to the HPA-la (Pl(Al), Zw(a)) antigen, and particularly the subset of such women who have a history of a previously affected infant with severe thrombocytopenia and/or intracranial hemorrhage (ICH). The risk of antenatal ICH in the fetus of such women is high enough to merit intervention, either weekly infusion of high-dose intravenous immunoglobulin G (IVIG) with or without corticosteroids given to the mother (the preferred approach in North American centres), or repeated in-utero fetal platelet transfusions (the preferred treatment approach in some European centres). Post-natal management of severely affected infants centres on the rapid provision of compatible antigen-negative platelets harvested from the mother or a phenotyped donor. The value of antenatal screening programs to detect 'at risk' alloimmunized women during pregnancy continues to be debated.