Today we will be discussing fetal and neonatal alloimmune thrombocytopenia, also referred to as NAIT, which affects about 1 in 1,000 births. NAIT is caused by maternal alloantibodies to specific platelet antigens, usually human platelet antigen 1a also referred to as HPA-1a. These antigens may be inherited from the father but are absent in the mother. Only 2% of women are HPA-1a negative and at risk to produce these alloantibodies which can lead to fetal thrombocytopenia.
Dr. Bussel, who wrote an Editorial on the subject, discusses the morbidity and mortality associated with NAIT.
“If the platelets are low enough, there is a serious risk of bleeding. When the fetal platelet get low enough, then the fetus may bleed in the brain and most of the bleeding in the brain that happens in fetuses and neonates affected by NAIT, occurs in utero, not after birth.”
Currently, there are no screening guidelines to identify mothers and fetuses that are at risk for NAIT. Therefore, it is usually diagnosed when the fetus or newborn has a low platelet count. In order to identify risk factors associated with NAIT and intracranial hemorrhage, a recent study published in TRANSFUSION examined gynecologic and immunogenetic variables in almost 100 patients with NAIT.
The study found that 59% of the fetuses with intracranial hemorrhage died. A greater number of pregnancies was associated with higher concentrations of maternal HPA-1a alloantibodies. Furthermore, the presence of certain class II histocompatibility alleles, such as DRB3*0101, contributes to whether HPA-1a alloantibodies are made. These alleles, for example those of DRB4, may also help predict whether antenatal intravenous immunoglobulin therapy will be effective.
Dr. Bussel summarizes the primary implications of the study:
“I think the big take home message is related to the fact that we cannot predict who will be seriously thrombocytopenic and which thrombocytopenic fetus will have an intracranial hemorrhage. The Kaplan study looks at the immunogenetics of the mother to see which patients who would otherwise look identical are the ones most at-risk to be most affected. This very extensive and original study lays the groundwork for future studies to further define the risk of fetal and neonatal ICH.”
- Delbos F, Bertrand G, Croisille L, Ansart-Pirenne H, Bierling P, Kaplan C. Fetal and neonatal alloimmune thrombocytopenia: predictive factors of intracranial hemorrhage. Transfusion 2016;56: 59–66.
- Bussel J. What do we know about intracranial hemorrhage in fetal and neonatal alloimmune thrombocytopenia? Transfusion 2016;56:17-18.