Neonatal alloimmune thrombocytopenia

Overview
Neonatal Alloimmune Thrombocytopenia (or -paenia, NAITP or NAIT or NAT for short; or feto-maternal alloimmune thrombocytopenia or -paenia, FMAITP or FMAIT) is a disease that affects fetuses and newborns. Genetic differences between the fetus and mother may result in the expression of certain antigens by fetal platelets, not expressed by the mother. Fetomaternal transfusions result in the recognition of these antigens by the mother's immune system as non-self, with the subsequence generation of allo-reactive antibodies which cross the placenta. NAIT, hence, is caused by transplacental passage of maternal platelet-specific alloantibody and rarely human leukocyte antigen (HLA) allo-antibodies (which are expressed by platelets) to fetuses whose platelets express the corresponding antigens. NAIT occurs in somewhere between 1/800 and 1/5000 live births (more recent studies of NAIT seem to indicate that it occurs in somewhere between 1/800 and 1/1000 live births).

Signs and Symptoms
Frequently, the thrombocytopenia is mild and the affected neonates remain largely asymptomatic. In these cases, therapeutic interventions are not indicated. In case of severe thrombocytopenia, the neonates may exhibit hemorrhagic complication at or a few hours after delivery. The most serious complication is intracranial hemorrhage, leading to death in approximately 10% or neurologic sequelae in 20% of cases.

Diagnosis
Although there are currently no widespread tests for NAIT, platelet antigen genotyping and platelet antibody identification can be performed on the maternal and paternal blood to determine the exact nature of the incompatibility. The Blood Center of Wisconsin Platelet & Neutrophil Immunology Laboratory is currently recognized as a leader on research on NAIT and provides NAIT testing using maternal and paternal blood samples.

Causes
About 80% of cases of NAIT are caused by antibodies against platelet antigen HP-1a, 15% by anti-HP-5a, and 5% by other antibodies. Unlike the hemolytic disease, NAIT occurs during the first pregnancy in to 50% of cases, and the affected fetuses may develop severe thrombocytopenia (<50,000 /μL) very early during pregnancy. Usually, the thrombocytopenia increases as gestation progresses. In utero intracranial hemorrhage occurs in about 10% of affected cases. This complication may also take place before 20 weeks of gestation. The recurrence of NAIT been estimated to be more than 80% in subsequent pregnancies with incompatible fetuses.

During Pregnancy
The use of Intravenous immunoglobulin (IVIG) during pregnancy and immediately after birth has been shown to help reduce or alleviate the affects of NAIT in infants and reduce the severity of thrombocytopenia. The most common treatment is weekly IVIG infusions at a dosage of 1g/kg beginning at 20 weeks of pregnancy and continuing until the birth of the child. In some cases this dosage is increased to 2g/kg and/or combined with a regiment of Progesterone depending on the exact circumstances of the case. Although this treatment has not been shown to be effective in all cases it has been shown to reduce the severity of thrombocytopenia in some.

After Birth
The most rapidly effective treatment in infants with severe hemorrhage and/or severe thrombocytopenia (<30x109/L) is the transfusion of compatible platelets (i.e. platelets from a donor who, like the mother lacks the causative antigen). Additionally if the thrombocytopenia in the infant at birth is not severe enough to warrant a transfusion of platelets (>30x109/L) an infusion of IVIG (1g/kg/day for two days) in the infant has been shown to rapidly increase platelet count and reduce the risk of related injury.

Resources
As of September 23, 2007 there is an active Yahoo group that is dedicated to helping those that are affected by NAIT. The group can be found at http://health.groups.yahoo.com/group/NAIT/