Fetal and Neonatal Alloimmune Thrombocytopenia

Abstract

Fetomaternal alloimmune thrombocytopenia (FMAIT) is a relatively uncommon disease, but is the leading cause of severe thrombocytopenia in the newborn. It can cause severe complications and long-term disabilities. The main objective of screening is to reduce both the morbidity and mortality associated with FMAIT, primarily by preventing intracranial hemorrhage. However, controversy surrounds both pre- and antenatal management. This article discusses pathogenesis, screening, diagnosis, and both pre- and neonatal management of FMAIT.

Fetomaternal alloimmune thrombocytopenia (FMAIT) occurs when a woman becomes alloimmunized against fetal platelet antigens inherited from the fetus’s father (which are absent on maternal platelets), leading to fetal thrombocytopenia (< 150,000 platelets/μL). Most cases are mild, with evidence of widespread petechiae and other skin lesions. However, severe cases can cause intracranial hemorrhage (ICH), resulting in death or long-term disability.^1–3 Unlike erythrocyte alloimmunization, FMAIT may appear during first pregnancies, with a high recurrence rate and often with progressively more severe manifestations in subsequent pregnancies.^4–6 FMAIT is the leading cause of severe thrombocytopenia in the newborn,^7,8 and should not be confused with autoimmune thrombocytopenia, in which both mother and fetus are affected due to maternal autoantibodies. The prevalence of FMAIT has been variously reported as between 1 in 350 and 1 in 5000 live births.^5,7,9–11 However, based on genetic probabilities,^7,12 some authors believe that this entity is underdiagnosed and postulate a prevalence nearer to 1 in 1200 live births.^10,13,14 At present, there are no national screening programs for FMAIT and a history of an affected sibling is currently the best indicator of risk to a current pregnancy.^15–17

Etiopathogenesis

FMAIT is produced by the placental transfer of maternal immunoglobulin (IgG) antibodies against fetal platelet antigens inherited from the father. To date, 24 human platelet-specific alloantigens (HPAs) have been described, many of them grouped in biallelic systems.^18–20 Antibodies against these antigens are designated anti-HPA. In white populations, the most common antibody is anti-HPA-1a (75%–80%), which targets the polymorphic Leu/Pro residue of glycoprotein IIIa (GPIIIa) on the platelet membrane.^2,9 The second most common antibody is anti-HPA-5b (10%–15%).^17,21 Many other antibodies have been identified, but are rare.^2,12 Between 1.6% and 4.6% of the general population are negative for the antigen HPA-1a (HPA-phenotype 1b/1b),²² but only approximately 10% will develop anti-HPA-1 antibodies.²³ Interestingly, there appears to be an association between the presence of certain alleles of the HLA II group (HLA DRB3*0101) and the immunogenicity of the HPA-1 antigen, such that alloimmunization is rare in the absence of this allele.^5,7,12,24

The timing and mechanism of immunization is not known with certainty. As a mechanism of immunization, Kumpel and colleagues²⁵ have proposed the expression of HPA-1a not only on fetal platelets, but also on other fetal cells. This is due, in part, to the fact that the number of fetal platelets transferred across the placenta during pregnancy could not be sufficient to produce immunization, and that anti-HPA antibodies are rarely produced after allogenic transfusion, suggesting that a mechanism other than simple platelet exposure is needed to explain alloimmunization. GPIIIa, the platelet glycoprotein carrying the HPA-1a/1b polymorphism, has been identified on the surface of villous syncytiotrophoblast in both first trimester and term placentas,²⁵ and its interaction with the maternal immune system through subcellular villous syncytiotropho-blast microparticles could be one plausible explanation. In susceptible pregnant women, this interaction could result in an immune response to HPA-1, leading to the development of alloantibodies that cross the placenta and produce fetal thrombocytopenia. This reaction to first trimester trophoblasts could explain how FMAIT can present clinically during the first incompatible pregnancy. In prospective screening studies, the frequency of alloimmunization during the first incompatible pregnancy was as high as 24%⁹ and was seen as early as 17 weeks.^5,10

Clinical Manifestations and Diagnosis

FMAIT should be suspected if neonatal thrombocytopenia is discovered incidentally, if there are clinical manifestations of hemorrhage, or in women with a previously affected child.^26,27 The clinical presentation depends on the time of onset and severity of the thrombocytopenia. The most common manifestation is bleeding into the skin, which may be the only clinical sign in up to 47% of cases.²⁸ The bleeding may present as mild or widespread petechiae or ecchymoses within a few hours after birth or, in some cases, with hematoma formation at injection sites or bleeding after circumcision.²⁸ In severe cases, FMAIT may present with bleeding into the major organs, such as gastrointestinal, lung, or ICH, resulting in death or long-term disability. ICH affects between 7% and 26% of all cases of FMAIT. Most of these (nearly 80%) occur during intrauterine life and, of these, 42% occur before 30 weeks of gestation.^5,8,22 Mortality rates vary from 1% to 10%, and long-term complications—including neurological sequelae such as mental retardation, cerebral palsy, cortical blindness, and seizures—may occur in as many as 14% to 26% of cases.^5,7,8,22 Thrombocytopenia resulting from anti-HPA-1a immunization is usually more severe than that due to anti-HPA-5b, which usually produces moderate thrombocytopenia and few clinical manifestations. However, ICH can be observed in both instances.^1,8

If thrombocytopenia is suspected in a newborn, a complete blood count should be obtained immediately to document the platelet count and determine whether it is an isolated thrombocytopenia or part of a pancytopenia syndrome, and a maternal blood sample should be sent to exclude low platelets in the mother (suggestive of autoimmune thrombocytopenia). Platelet immunological tests should then be performed to confirm the diagnosis of FMAIT based on the presence of circulating maternal alloantibodies against fetal platelet antigens.^1,2,7,8 Thereafter, both parents and the neonate should be genotyped for HPA antigens. The diagnosis of FMAIT is confirmed when parental antigen incompatibility is found with a corresponding maternal antibody. Detection of specific antiplatelet antibodies can be made using the indirect platelet immunofluorescence test or the monoclonal antibody immobilization platelet antigen (MAIPA) test.^7,8 Such investigations should only be undertaken after consultation with an expert platelet laboratory.

There are two major problems in diagnosing FMAIT. The first is due to the existence in some patients of uncommon antiplatelet antibodies. If the initial tests for HPA antibodies are negative, other monoclonal antibody testing should be used (such as the MAIPA test) and repeated with increasing amounts of maternal serum. It is also recommended that, once common antibodies have been excluded, a cross-match should be performed between the maternal serum and paternal platelets, which can detect low-frequency or private (unique) incompatible antigens.⁷ The second problem has to do with false-negative results. In up to 30% of cases of FMAIT with HPA-1a antigen, maternal anti-HPA-1a antibodies may not be detectable right away, but can become positive a few weeks or even months after childbirth.¹ In order to identify these cases, testing should be repeated in 6 weeks if the clinical index of suspicion is high. Scheffe and associates²⁶ described a noninvasive test based on real-time polymerase chain reaction using cell-free fetal DNA isolated from maternal blood to determine if future pregnancies would be at risk for FMAIT.

Differential Diagnosis

Neonatal thrombocytopenia is a relatively rare condition with a prevalence of 0.9% in unselected populations.² FMAIT is the most common cause of severe thrombocytopenia in the newborn, accounting for 3% of all fetal and neonatal thrombocytopenia and 27% of severe cases (defined as < 50,000 platelets/μL or the presence of ICH).^5,27 Most of the other causes of neonatal thrombocytopenia can be excluded after a careful examination of the neonate and review of the maternal history.^2,7,23,27 The main causes other than FMAIT are infection, autoimmune thrombocytopenia, drug-related destruction of platelets, disseminated intravascular coagulation, necrotizing enterocolitis, hypersplenism, Kasabach-Merritt syndrome, and thrombosis.² Less common causes include genetic abnormalities (eg, congenital amegakaryocytic thrombocytopenia, congenita platelet disorder), bone marrow infiltrative disease (eg, bone marrow metastases, neonatal leukemia), or toxic megakaryocyte injury. Other rare causes are related to conditions such as preeclampsia, hypoxic ischemic injury, and neonatal cold injury.²⁶

Autoimmune thrombocytopenia is often confused with FMAIT, so it is important to emphasize the differences. In FMAIT, only fetal platelets are affected because of the nature of the antibodies. Autoimmune thrombocytopenia is characterized by the presence of autoantibodies in the mother due to maternal disease such as idiopathic thrombocytopenic purpura, systemic lupus erythematosus, or hyperthyroidism, in which antibodies affect both the maternal and fetal platelets. For the fetus and neonate, the severity of the disease is typically greater with FMAIT because the thrombocytopenia is associated with a reduction in the number of platelets as well as platelet dysfunction due to the presence of alloantibodies that are capable of binding both glycoproteins IIb and IIIa.²⁹ On the other hand, the clinical manifestations of autoimmune thrombocytopenia are typically mild and present with petechial purpura several days after delivery.

Predicting Severity

At present, there is no maternal laboratory marker that predicts the severity of FMAIT. The history of a previously affected child, especially if that child developed ICH, is the strongest predictor of recurrence and severity of FMAIT in a future pregnancy.^{4,5,9,28,30,31} The determination of maternal HPA alloantibody status should be considered if there is a history of a previously affected newborn. However, the HPA antibody titer itself does not correlate well with the clinical severity, and severe disease can occur even with low antibody titers. Also, the antibody titer can fluctuate over time, which limits its clinical value.^10,31,32 Fetal HPA group can be obtained by amniocentesis and analysis of cultured amniocytes, and should be considered if there is a previously affected child with FMAIT and a heterozygous partner.¹⁷ However, fetal genotyping using cell-free fetal DNA is becoming a more popular choice in order to avoid the invasive procedure, which (at least in theory) may increase the antibody level and potentially the severity of the disease.²⁷ The recurrence risk in a subsequent pregnancy depends on the zygocity of the father for the relevant antigen.⁶ In general, the clinical severity of the disease and degree of thrombocytopenia in the fetus are usually less than that of the previous newborn.^5,8,28,33

Screening for Anti-HPA-1a Antibodies

The main objective of screening is to reduce both the morbidity and mortality associated with FMAIT, mainly by preventing ICH.^5,9,23,30,34 When considering whether to implement a universal screening policy, it should be noted that (1) the prevalence of HPA-1a (−/−) phenotype in the general population is low (1%–2%); (2) only 10% of these patients will develop anti-HPA-1a antibodies; and (3) of those, only 30% of neonates will develop thrombocytopenia, which will be severe in only 20% of cases.²³ In a study by Knight and colleagues,³⁵ the estimated incidence of clinically detectable FMAIT was 12.4 per 100,000 live births, of which 30% occurred in pregnancies with a history of a previously affected child and, as such, would have been identified using a target screening program.³⁶ Unfortunately, the literature regarding screening for FMAIT is confusing and inconclusive. Universal screening for FMAIT is not currently recommended,³⁶ but has been suggested by a number of key opinion leaders.^5,30,37

At this time, there is no simple, safe, precise, and validated screening test for FMAIT. Using cell-free DNA for noninvasive fetal HPA-1a genotyping, Scheffer and associates²⁶ reported a 100% sensitivity and specificity. However, this study was carried out on only a small number of patients, and other investigators have shown that such a screening strategy would not be cost effective.⁵ An alternative strategy would be to screen all pregnant women for circulating anti-HPA antibodies, but Wu and coworkers³⁸ noted that many cases would be missed as this test is only capable of recognizing certain types of antibodies. An effort to use HLA DRB3*0101 genotyping to reduce the number of pregnant patients needing to be screened for anti-HPA antibodies has also been suggested,³⁸ but has not as yet been tested in large populations. In 2007, Killie and coauthors³⁷ proposed a three-tiered model of screening, including an initial measurement of circulating anti-HPA antibodies and fetal ultrasonography (1), followed by serial antibody titer and fetal testing if either the anti-HPA1 antibody level was > 100 arbitrary units/mL (2), or if the woman had anti-HPA-1bb antibodies and was HLA DRB3*0101 genotype positive, regardless of the anti-HPA1 antibody level (3). The study concluded that this method of screening for FMAIT was cost effective in the population analyzed.³⁷ The same authors suggested that maternal serum levels of anti-HPA1a antibodies at 22 to 34 weeks of gestation could be a good predictor of the degree of thrombocytopenia in the newborn.³⁴ In a systematic review published in 2010, Kamphuis and colleagues⁵ concluded that measuring anti-HPA1a antibodies during the first trimester of pregnancy would allow the identification of pregnant women at risk for FMAIT, but would detect only two cases in a thousand pregnancies, which is not cost effective. Although the early identification of women with circulating anti-HPA antibodies would allow for interventions to prevent adverse outcome, especially ICH, there is as yet no general agreement on how these pregnancies should be managed.³⁵

Prenatal Management

The prenatal management of pregnancies at risk for FMAIT includes general measures such as a planned delivery and avoiding the consumption of nonsteroidal anti-inflammatory drugs and aspirin. The antenatal management of FMAIT remains controversial, and currently involves three treatment options: maternal intravenous immunoglobulin (IVIG), maternal steroid administration, or serial intrauterine platelet transfusions (IUPT). Studies have shown that both maternal IVIG and IUPT can prevent severe thrombocytopenia in the fetus and its related complications,^39,40 but IUPT is associated with additional complications, such as fetal hemorrhage or intrauterine death.^40,41 As such, the tendency among many consensus bodies has been to favor noninvasive treatment and close monitoring.^1,17,19,34 As first-line treatment, many authors propose administering IVIG (1 g/kg body weight) to the mother at weekly intervals starting at 20 weeks of gestation, depending on the previous history. Some maternal-fetal medicine specialists have suggested using a lower dose of IVIG (0.5 g/kg) and starting treatment between 12 and 20 weeks of gestation. These same specialists often recommend fetal blood sampling (FBS) to measure the fetal platelet count at 28 weeks, which is typically 8 weeks after starting IVIG treatment.^17,34 Bussel and associates³⁹ demonstrated that treatment with IVIG produced a significant increase in the platelet count of fetuses with FMAIT, and additional observational studies have suggested an improvement in clinical outcome and reduction in the risk for ICH when IVIG was administered to the mother throughout pregnancy.¹⁷ Indeed, maternal therapy with IVIG results in a fetal platelet count > 50,000/mL in 67% of pregnancies with a history of a prior sibling affected by FMAIT,³³ thereby reducing the need for FBS and IUPT and their attendant complications. In cases that fail to respond to maternal IVIG, it is recommended to double the dose of IVIG and/or consider starting corticosteroids, and to repeat the FBS in 2 to 4 weeks. With regard to antenatal corticosteroids, prednisolone (0.5 mg/kg) is generally preferred over dexamethasone, because the latter has been associated with fetal oligohydramnios.²³ Adverse effects to the mother of continuing corticosteroids therapy throughout pregnancy include an increased risk for hypertension and osteoporosis.¹⁷

The other strategy of antenatal management is to measure fetal platelet counts by serial FBS and perform IUPT when needed. Such procedures carry a significant risk for fetal morbidity and mortality, including bleeding, fetal cardiac arrhythmia, and amniotic fluid contamination which may result in fetal loss. For this reason, many authors recommend this therapy only in cases refractory to IVIG and steroids.^17,30 Although the optimal management of FMAIT remains unclear and further clinical trials are required to determine the ideal treatment algorithm for this condition, the existing literature suggests that the noninvasive approach is a more sensible initial strategy.^1,8,17,20

Neonatal Management

Neonatal management depends on the clinical presentation. Initial evaluation should include a cranial ultrasound examination (to exclude ICH), in addition to blood tests to evaluate the severity of the thrombocytopenia. Platelet transfusion is the treatment of choice. Transfusion is indicated in term neonates with signs of bleeding or if the platelet count is < 30,000/μL during the first 24 hours of life. In preterm infants or those with evidence of ICH, transfusion should be considered at a higher platelet count.^2,31 The aim of treatment is to maintain acceptable platelets levels, especially within the first 72 to 96 hours of life. This is generally defined as > 30,000/μL without active bleeding, and > 100,000/μL if there is evidence of bleeding. Thereafter, platelet transfusions should be performed only if there is persistent hemorrhage. Serial cranial ultrasound examinations may be required in neonates with platelet counts persistently < 50,000/μL.^1,31,32,42

There are various options available for platelet transfusion. Transfused platelets should be compatible with the maternal antibody specificity so they are not destroyed in the circulation of the newborn, which is why the mother is considered the best donor. Platelets must first be washed and irradiated in order to remove maternal antiplatelet antibodies and to prevent graft-versus-host disease.^1,2,8 The disadvantage of this approach is that it commonly takes 12 to 24 hours to prepare the platelets for transfusion, and so it is not frequently used in clinical practice.⁸ In emergent cases, for example if the newborn has severe thrombocytopenia and is hemorrhaging, a transfusion of typed and matched random donor platelets with or without IVIG can be started while maternal platelets are being processed.^1,7,8 A further option includes the administration of platelet concentrates that are HPA-1a-negative/HPA-5b-negative, but these are not generally available and are expensive.^28,31,32

As additional therapy, some authors have suggested adding high-dose IVIG (400 mg/kg/d for 3–4 days or 1 g/kg/d for 1–3 days) to reduce the time taken for the platelet count to recover.^1,2,8 Although this is a reasonable adjunct, IVIG should not be relied upon as the only treatment because it takes 18 to 24 hours to work.^1,2 Administration of intravenous methylprednisolone (1 mg/kg every 8 hours for 1–3 days) has also been proposed, but steroids are not generally recommended as routine therapy for the neonatal management of FMAIT.³² It is important to follow the platelet count serially in the newborn, especially if the initial platelet count is < 30,000/μL, as the FMAIT platelet count tends to get worse during first few days of life. A cranial ultrasound examination should always be performed to exclude ICH. When ICH is absent, the prognosis is usually favorable and the platelet count typically recovers to normal values within 8 to 10 days of life.^1,2,42

Conclusions

FMAIT is a relatively uncommon disease, but is the leading cause of severe thrombocytopenia in the newborn. It can cause severe complications and long-term disabilities, especially if ICH develops. The diagnosis should be suspected if a neonate presents with unexplained hemorrhage or if there is a family history of FMAIT. The cornerstone of management in the neonatal period is platelet transfusion to maintain platelet counts above a threshold level depending on the clinical manifestations. Antenatal management should focus on maternal IVIG/corticosteroid administration or IUPT. Both treatments can prevent severe thrombocytopenia and its related complications, and the current tendency is to try to avoid the attendant complications of invasive procedures. A history of a previously affected child, especially if that child developed ICH, is the strongest predictor of recurrence and severity of FMAIT in a future pregnancy. At this time, there is no simple, safe, precise, and validated screening test for FMAIT, and universal screening is not recommended.

Main Points.

• Fetomaternal alloimmune thrombocytopenia (FMAIT) occurs when a woman becomes alloimmunized against fetal platelet antigens inherited from the fetus’s father. Most cases are mild; however, severe cases can cause intracranial hemorrhage (ICH), resulting in death or long-term disability.

• Diagnosis is difficult, but FMAIT should be suspected if neonatal thrombocytopenia is discovered incidentally, if there are clinical manifestations of hemorrhage, or in women with a previously affected child.

• Universal screening for FMAIT is not currently recommended, but has been suggested by a number of key opinion leaders. The main objective of screening is to reduce both the morbidity and mortality associated with FMAIT, mainly by preventing ICH. Unfortunately, the literature regarding screening for FMAIT is confusing and inconclusive.

• Prenatal management of pregnancies at risk for FMAIT includes general measures such as a planned delivery and avoiding the consumption of nonsteroidal anti-inflammatory drugs and aspirin. Antenatal management of FMAIT remains controversial, and currently involves three treatment options: maternal intravenous immunoglobulin, maternal steroid administration, or serial intrauterine platelet transfusions.