Management of fetal and neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia (NAIT) can occur when a mother is immunized against fetal platelet antigens inherited from the father. Early diagnosis and appropriate platelet transfusion therapy are essential to prevent life-threatening intracranial hemorrhage in the thrombocytopenic fetus or neonate. Five major human platelet antigen (HPA) systems are capable of causing this disorder with HPA-1a indicated most frequently. This article reviews the pathophysiology, clinical aspects, and management of NAIT. We also present our experience with treatment of neonates affected with this disorder.

Feto-maternal alloimmune thrombocytopenia: a literature review and statistical analysis

Exploring prognostic factors that determine outcomes in fetomaternal alloimmune thrombocytopenia (FMAIT), a search of Medline was performed covering the years 1966 to April 1998. 376 articles were collected and reviewed; 140 articles contained the case histories of 297 mothers and 433 pregnancies that fulfilled entry criteria. More than 30 data variables were sought from these cases. The data were analysed using SPSS and Arcus Quickstat Biomedical. Nineteen different antigen incompatibilities were documented, the majority being human platelet antigen (HPA)-1a (77.3%), HPA-3a (3.5%) and HPA-5b (3.5%). The relative risk reduction (RRR) in mortality with any intervention was 57% (0.19-0.77) p = 0.009. Treatment of HPA-1a (PlA1) pregnancies with intravenous immunoglobulin (IVIG) increased the likelihood of a neurologically normal outcome, relative risk (RR) 1.68, confidence interval (1.3-2.2) p = 0.0003. Treatment of HPA-1a (PlA1) pregnancies with only antenatal complementary platelet transfusions increased the likelihood of a neurologically normal outcome, RR 1.63 (1.1-2.1) p = 0.01. Despite reviews of more than 400 cases of FMAIT, few prognostic variables are identifiable. Although IVIG appears to reduce the risk of intracranial haemorrhage (ICH), the dosage and timing of IVIG treatment was varied. This study highlights the need for standardised and directed research.

Purpura in infants and children

Hemorrhage into the skin (purpura) may result from abnormalities in any of the three components of hemostasis: platelets, plasma coagulation factors, and blood vessels. The morphology, size, and distribution of the hemorrhagic lesions are helpful diagnostic features. The main causes of purpura in the newborn and the more common hemorrhagic disorders in children are reviewed.

A simple and rapid competitive enzyme-linked immunosorbent assay to identify HPA-1a (PlA1)-negative donor platelet units

BACKGROUND

Alloantibodies to HPA-1a (PlA1) are the major cause of neonatal alloimmune thrombocytopenia and posttransfusion purpura and have been implicated in refractoriness to random-donor platelet transfusions. However, most assays used to phenotype platelets are cumbersome or time-consuming for large numbers of samples.

STUDY DESIGN AND METHODS

A simple, competitive (inhibition) enzyme-linked immunosorbent assay for HPA-1a phenotyping of donor platelets was developed. A segment from the donor platelet unit transfer line was sealed to obtain a small aliquot of platelets. These platelets were washed once and added to a predetermined dilution of serum containing alloantibodies to HPA-1a. Residual anti-HPA-1a binding to the glycoprotein IIb/IIIa purified by lectin and high-performance liquid chromatography and coated on microtiter wells was detected with a conjugated antihuman IgG. A lack of inhibition equivalent to control (no platelets) was used to determine that the platelets were HPA-1b/b.

RESULTS

Of the 557 platelet units tested, 14 (2.5%) were found to be HPA-1a negative, and they were confirmed to be HPA-1b/b by DNA genotyping. Two of the 14 HPA-1b/b units were also HPA-3b/b (approx. 0.35% of the random population). Use of the microtiter format allows 100 to 200 samples to be processed per day.

CONCLUSION

This simple and inexpensive assay is useful for identifying HPA-1b/b units for platelet-compatible transfusions or for platelet antibody investigations.

The prenatal identification of fetal compatibility in neonatal alloimmune thrombocytopenia using amniotic fluid and variable number of tandem repeat (VNTR) analysis

Most severe episodes of neonatal alloimmune thrombocytopenic purpura (NATP) are caused by antiplatelet alloantibodies against the HPA-1a (PlA1) antigen. However, half of subsequent fetuses produced from a HPA-1a/b father (genotypic frequency 28%) will result in a child who is not affected. Some investigators manage NATP by confirming the fetal platelet phenotype using percutaneous umbilical cord sampling, a procedure that carries a low but real risk of fetal morbidity and mortality. More recently, physicians determine the fetal platelet antigen genotype using DNA derived from amniotic fluid or chorionic villus samples. All therapy is withdrawn for a fetus who genotypes as HPA-1b/b. However, since the fetus is the same genotype as the mother, there can be uncertainty about the origin of the genetic material and thus the validity of the fetal genotype. The inappropriate withdrawal of therapy for a erroneously genotyped fetus could be fatal, and consequently many physicians advocate fetal HPA-1 phenotyping with confirmation using percutaneous umbilical blood sampling. In this report we describe the management of two pregnancies with previously affected infants due to anti-HPA-1a alloantibodies. Both husbands were HPA-1a/b. For the current pregnancies, amniotic fluid was collected at 20 or 29 weeks of gestation, and the platelet genotype indicated that the fetuses were HPA-1b/b. The fetal origin of the amniotic fluid derived DNA was confirmed by the forensic technique of DNA profiling using variable number of tandem repeat (VNTR) analysis. All therapy was withdrawn, percutaneous umbilical blood sampling was not performed, and both women vaginally delivered healthy non-thrombocytopenic infants. The application of platelet alloantigen genotyping using DNA from amniotic fluid cells identified the HPA-1b/b fetus, and VNTR analysis confirmed that the tissue was fetal derived, thus avoiding the necessity for percutaneous umbilical blood sampling. The use of this approach in patients at risk will avoid additional investigation and treatment in approximately one-seventh of all NATP pregnancies involving the HPA-1a antigen.