NEONATAL ALLOIMMUNE THROMBOCYTOPENIA (DUE TO HPA-3B)
Neonatal alloimmune thrombocytopenia (NAIT) is caused by maternal alloimmunization to a fetal (paternal) platelet antigen not present on the mother's platelets, the platelet equivalent of hemolytic disease of the newborn (HDN). Unlike HDN, up to 60% of the cases of NAIT occur during the first pregnancy. The incidence is estimated to be 1/1000 live births, accounting for approximately 20% of neonatal thrombocytopenia. Maternal alloantibodies (IgG) cross the placenta and bind to platelets, resulting in their removal by the reticulo-endothelial system.
The human platelet antigen system consists of different platelet antigens of which fourteen have been associated with NAIT of varying severity. Expression of these antigens is genetically determined and is associated with platelet memBrane glycoproteins (GP). Polymorphism of platelet antigens usually involves single amino acid substitution caused by a single nucleotide substitution in the coding gene. For example, expression of human platelet antigen (HPA) 1a or 1b is determined by whether Leucine or Proline is in position 33 of GP IIIa. Expression of platelet specific antigens varies greatly among different ethnic groups and is associated with the specificity of NAIT. In Caucasians, the most commonly described and the most clinically important antigen is HPA-1a (PLA1). Incompatibility of HPA-1a (PLA1) is found in 80-90% of all NAIT cases. The second most commonly associated antibody specificity with NAIT in Caucasians is anti HPA-5b (Bra), accounting for 5-15% of the cases. Other less frequent antibodies associated with NAIT in Caucasians include anti HPA-3a (Baka) and anti HPA-1b (PLA2). In Asians, NAIT associated with anti HPA-1a (PLA1) virtually never occurs, whereas anti HPA-4b is the most important cause of NAIT. HLA antibodies and ABO antibodies have been implicated in a few cases.
The existence of incompatibility between the mother and fetal/paternal platelets appears to be necessary but not sufficient for maternal alloimmunization to occur. For example, only 1 in 20 HPA-1a (PLA1) negative women with an HPA-1a (PLA1) positive fetus will develop anti HPA-1a (PLA1) antibodies. There appears to be a genetic restriction on the immune response. Several studies have described association between specific maternal expression of HLA class II types and the development of alloimmunization against platelet specific antigens: anti HPA-1a (PLA1) sensitization is associated with maternal expression of the DR52a and HPA-5b (Bra) is associated with the expression of the DRw6.
Regardless of the antibody specificity, NAIT tends to progress in severity with subsequent pregnancies. Other factors that may influence the degree of severity include the antigen density and location and the ability of the antibody to fix complement. In anti HPA-1a (PLA1) related NAIT, thrombocytopenia can be severe and intracranial hemorrhage (ICH) occurs in 10-30% of the cases, of which approximately 50% occurs intrauterine. HPA-5b (Bra) associated NAIT is clinically less severe.
Most cases of neonatal alloimmune thrombocytopenia are discovered after birth. Mildly affected infants may be asymptomatic. In those with severe thrombocytopenia, the most common presentation is the presence of petechiae or a cephalohematoma at birth in an otherwise healthy newborn. There is evidence of central nervous system (intracraneal) hemorrhage in 10-20% of affected neonates which is associated with neonatal death in 10% and neurological sequelae in 20% of the affected infants, respectively. If untreated, the neonatal thrombocytopenia normally lasts 2-3 weeks, resolving spontaneously. Other entities that need to be part of the differential diagnosis include infection induced thrombocytopenia, maternal idiopathic thrombocytopenic purpura, maternal drug exposure, and other rare causes of neonatal thrombocytopenia.
Complete blood counts on the affected neonate uniformly show thrombocytopenia; however, the maternal platelet count is normal. Neonate hemoglobin levels may be decreased secondary to bleeding.
Special platelet studies are necessary to confirm the diagnosis of NAIT. Maternal and paternal platelet antigen typing is performed by serological and/or DNA-based assays. The maternal serum is screened by serological assays for platelet antibodies against the paternal platelets and/or selected donor platelets in which the specific antigen is present or absent. The different methods available include platelet immunofluorescence test, solid- phase red cell adherence assay, monoclonal antibody immobilization of platelet antigens. Up to 85% of the maternal sera tested will show antibodies specific for the implicated antigen. However, the absence of detectable maternal antibodies does not exclude the diagnosis of NAIT.
Postnatal treatment focuses on increasing the neonate's platelet count. Therapeutic intervention involves IVIG and platelet transfusions. Platelet transfusions are recommended for any infant with severe hemorrhage or a platelet count <30,000/mL. The platelets to be transfused should be compatible with maternal serum. Generally, the most readily available source of such platelets is the mother. Maternal platelets can be collected from a unit of whole blood or by plateletpheresis. In order to avoid any prolongation of the neonatal thrombocytopenia, the plasma portion of the maternal platelets should be reduced to minimize the passive transfer of the maternal antibodies. Maternal platelets should be irradiated to reduce the risk of transfusion-associated graft-versus-host disease. Alternative platelet donors include the mother's siblings or volunteer donors that lack the target platelet antigen. In the presence of severe thrombocytopenia and or clinically significant bleeding, if an antigen negative platelet product is not readily available random donor platelets should not be withheld, as adequate response to a unit of random platelets, although temporarily can be expected in 40% of cases. High dose intravenous IgG has been successful in increasing platelet counts within 48-72 hours. The recommended dose for intravenous immunoglobulin (IVIG) therapy is 400 mg/kg per day over 5 days. The use of IVIG may eliminate the need for platelet transfusion in the less severe thrombocytopenic neonates. It can also be used as an adjunctive therapy for those with severe thrombocytopenia.
Management of Subsequent Pregnancies
The primary purpose of antenatal diagnosis and treatment is to prevent intrauterine ICH which may occur in 2-7% of NAIT patients. Genetic counseling as well as obstetrical high risk support for future pregnancies is strongly recommended once the diagnosis has been established. A couple with an affected child should be referred to a high risk obstetrics specialist; recurrence of NAIT in subsequent pregnancies is frequent (90%), with equal or increasing severity. The first issue, with regards to genetic counseling is to determine the father's zygosity. For example, approximately one in four HPA-1a (PLA1) fathers are heterozygotes. If the father is PLA1/PLA1 (homozygous), the next infant will be an obligate heterozygote, and will be as affected as the previous one. If the father is PLA1/PLA2 (heterozygous), there would be a 50:50 chance that the next infant would be affected. Fetal platelet alloantigen genotype can be determined by polymerase chain reaction (PCR) by 18 weeks gestation by cordocentesis. If the fetus is found to be PLA1 negative, no further diagnostic testing is indicated. If the fetus is found to be PLA1 positive, cordocentesis should be considered to determine the fetal platelet count. A fetus is considered affected if the platelet count is <100,000/mL.
Maternal therapy of fetal thrombocytopenia is based on IVIG dosed as 1 gm/kg/week which has been shown to improve the fetal thrombocytopenia in 75% of cases. If IVIG treatment fails to give a response, high-dose steroids (60 mg prednisone/day) can be added. In the presence of treatment failure (persistent fetal platelet count < 100,000/mL), frequent fetal blood sampling will be needed to assess severity of the thrombocytopenia and to provide intrauterine transfusion of platelets for fetal platelet counts <50,000/mL Early delivery after 34 weeks gestation in the presence of fetal lung maturity might be necessary. Antigen-negative irradiated platelets, preferentially maternal platelets, are used for the fetal platelet transfusion. Maternal platelets can be collected from a unit of whole blood or by plateletpheresis. The plasma portion of the maternal platelets should be reduced to minimize the passive transfer of the maternal antibodies.
Contributed by Lirong Qu, MD, PhD, Theresa Nester, MD, and Ileana Lopez-Plaza, MD