Hemolytic disease of the fetus and newborn (HDFN) is an immune-mediated red blood cell (RBC) disorder in which maternal antibodies attack fetal or newborn RBCs. HDFN can cause significant morbidity and mortality, especially in healthcare limited resource settings. Effects of HDFN range from mild anemia to hydrops fetalis in the fetus and hyperbilirubinemia and kernicterus in the newborn. Through early detection, management, and prevention of this disease, the incidence and prevalence of HDFN has exponentially decreased in the past 50 years.
There are two main mechanisms in which maternal antibodies target fetal or newborn RBC antigens:
ABO incompatibility is a congenital, inherent mismatch between maternal and fetal blood types. Conversely, alloimmunization due to fetomaternal hemorrhage (FMH) is an acquired immune-mediated mechanism that typically affects subsequent pregnancies rather than the pregnancy in which the FMH happens.
Hemolytic disease of the fetus and newborn was first described by Dr. Louis K. Diamond in 1932 when he wrote about erythroblastosis fetalis in the newborn based on peripheral smears. Rhesus D-negative (RhD) immunoprophylaxis was first introduced in 1968, which dropped the incidence of HDFN from 1% of all newborns worldwide (with 50% mortality) to 0.5%. The incidence of HDFN decreased even further to 0.1% with the administration of antepartum RhD immunoprophylaxis. However, despite adequate RhD immunoprophylaxis, an estimated 1 to 3 in 1000 Rh-negative women still develop alloimmunization today, thus it is important to stay vigilant for the development of HDFN.
Rh incompatibility varies by race, ethnicity, and risk factors. The Rh-negative blood type is most predominant in white races (15%) compared to African Americans (5 to 8%) or Asians and Native Americans (1 to 2%). Among white women, an Rh-negative woman has an 85% chance of mating with an Rh-positive man. This frequently occurring Rh-incompatibility increases the risk of HDFN with any FMH event.
As little as 0.1 mL of fetal blood entering maternal circulation is sufficient to cause alloimmunization. In fact, 15 to 50% of gestations have sufficient fetomaternal hemorrhage to cause alloimmunization, and only 1 to 2% of all Rh alloimmunization is caused by antepartum FMH. As such, it is important to consider HDFN at all stages of the pregnancy where FMH may occur.
As mentioned, there are two mechanisms causing hemolytic disease of the fetus and newborn. First, the fetomaternal pair can have inherent ABO incompatibility, which occurs in 15 to 25% of pregnancies. Only about 1% of those pairs, those with high IgG titers, will develop HDFN due to ABO incompatibility. In ABO incompatibility, naturally occurring antigens against A or B blood types are present in mothers with O blood type. If the mother's fetus has an A or B (or AB) blood type, these maternal anti-A and/or anti-B antibodies, respectively, will attack the foreign blood type of the fetus. The anti-A and anti-B antibodies are IgG, which can cross the placenta and affect the developing fetus. Compared with FMH, ABO incompatibility generally causes a less severe form of HDFN. Postulated theories for this include fetal RBCs express less ABO blood group antigens than adult levels and that ABO blood group antigens are expressed by many tissues, which reduce the chance that antibodies target specifically the antigens on fetal RBCs.
The second mechanism most commonly causing HDFN is through fetomaternal hemorrhage (FMH), where maternal antibodies develop after exposure to fetal blood. When fetal RBCs enter the maternal blood circulation, maternal antibodies can develop to an antigen presented on the fetal RBC surface. The most common antigen involved in this mechanism is the Rhesus D antigen. It is estimated that 1.5 to 2.5% of obstetric patients will develop antibodies to other "minor" antigens. While most of these cases of alloimmunization do not cause significant hemolytic disease of the newborn, some can cause severe anemia at low titer thresholds. Antibodies against the antigens of the Kell blood group, for example, are associated with an increased risk of severe anemia and/or death of the fetus. These patients should be monitored very closely throughout the pregnancy.
Antigens in fetal blood that are foreign to maternal blood are inherited from paternal genes. For example, an Rh-negative woman can have an Rh-positive fetus due to her partner being Rh-positive. Antibodies that develop due to FMH put subsequent pregnancies at risk for HDFN as the first antibodies to develop are of IgM type, which cannot cross the placenta. In subsequent encounters with the Rh-D antigen, maternal antibodies rapidly develop IgG antibodies, which do cross the placenta.
As infant RBCs are attacked and broken down, infants develop hemolytic anemia. The breakdown of heme leads to bilirubin, which is removed by the placenta in utero. At birth, the liver begins processing the bilirubin. Indirect bilirubin, or unconjugated bilirubin, is conjugated to direct bilirubin using the enzyme uridine diphospho-glucuronosyltransferase (UDP-glucuronosyltransferase). This conjugated bilirubin is excreted in bile, where it ultimately will be excreted in feces and urine.
In infants, especially preterm infants, liver processing is less efficient, which often leads to natural physiologic jaundice. With excess breakdown products from HDFN, these immature processing mechanisms are overwhelmed, and the resulting hyperbilirubinemia can be prominent. The accumulation of unconjugated bilirubin can lead to neurologic dysfunction as the unbound bilirubin crosses the blood-brain barrier and deposits in the brain of the developing newborn. Prompt recognition and treatment of hyperbilirubinemia and HDFN are paramount to avoiding long-term neurologic dysfunction in these infants.
An in-depth history can be essential in raising suspicion for hemolytic disease of the fetus and newborn. Emphasis should be placed on identifying any events that may have led to fetomaternal hemorrhage. These include previous pregnancies with HDFN or hydrops fetalis, miscarriages, ectopic pregnancies, early pregnancy terminations, blood transfusions in the mother, chorionic villous sampling, amniocentesis, or documentation of bleeding in pregnancy.
Early in the first trimester, blood typing should be done on all pregnant women. Those with blood type O naturally express antibodies to A and B blood types, thus they should be monitored for the development of HDFN, especially at delivery and directly postpartum. The antibodies expressed in mothers with O blood type are typically immunoglobulin G (IgG) and can cross the placenta. Conversely, mothers with blood type A have antibodies against blood type B, which are predominantly immunoglobulin M (IgM) and do not cross the placenta. It is generally standard practice to check the blood types of infants born to mothers with blood type O at birth, whereas blood types of infants whose mothers have blood types A, B, or AB may not be checked (if the Rhesus factor is positive).
The main signs of hemolytic disease in the newborn (HDN) are anemia and hyperbilirubinemia, which may present as lethargy, jaundice, conjunctival icterus, pallor, hepatosplenomegaly, tachycardia or bradycardia, increased oxygen requirement, and/or apnea.
Hemolytic disease of the fetus and newborn should be considered in the differential diagnosis of newborns with jaundice/hyperbilirubinemia and certainly in the case of neonatal anemia. Diagnosis of HDFN can be made by identifying the presence of maternal RBC antibodies (agglutination in an indirect antibody test) and/or a positive direct antibody test (DAT) in the infant's serum. If a pregnant woman is identified to have alloimmunization, the first step in further evaluation is to determine the paternal RBC antigen status. If positive, the next step is to identify the fetal blood type, typically done through amniocentesis.
According to the American Academy of Pediatrics (AAP), "if a mother has not had prenatal blood grouping or is Rh-negative, a direct antibody test (or Coombs' test), blood type, and an Rh (D) type on the infant's (cord) blood are strongly recommended."
If hemolytic disease of the fetus and newborn is identified or suspected in utero, a consult to maternal-fetal medicine should be placed as early as possible in the pregnancy. Affected pregnancies can be managed by monitoring antibody titers, and fetal middle cerebral artery velocities, intrauterine transfusions, and possibly early delivery as infants with severe anemia may not tolerate term labor well.
Hemolytic disease of the newborn is managed by treating hyperbilirubinemia with phototherapy and exchange transfusions if needed. Routine universal screening with transcutaneous bilirubin (TcB) often occurs at 24 hours of life, but screening should be conducted as soon as hyperbilirubinemia is suspected. An elevated TcB should always be verified with a serum total bilirubin (TB). The hour-specific Bhutani nomogram is then used to risk stratify the amount of bilirubin in the infant's blood. This nomogram provides a recommended threshold for starting phototherapy versus early transfusions depending on the infant's risk level.
Phototherapy was introduced in the 1970s and has become the mainstay of hyperbilirubinemia management in newborns. Photo isomerization causes the transformation of bilirubin into a water-soluble isomer that can then be excreted by the kidneys and stool without the need for processing in the liver. The main determinants of phototherapy efficacy are the wavelength of light used, the intensity of that light, the total light dose (time exposed and surface area exposed), and the threshold at which phototherapy is initiated. The AAP recommends the use of intensive phototherapy in HDFN. Optimal light used for phototherapy has a wavelength of 460-490nm. The light should be at a close distance (about 20cm above the infant), and double phototherapy has proven to be more efficacious than single. There is limited data on the efficacy of continuous versus intermittent phototherapy for infants >2000g. During the use of phototherapy, mothers should be encouraged to breastfeed their infants at timely intervals despite needing to remove them from phototherapy to do so.
An exchange transfusion may be needed for severely anemic newborns, which involves replacing infant RBCs with antigen-negative RBCs, thereby preventing further hemolysis. 5mL/kg aliquots are removed and replaced over several minutes for a total of 25-50mL/kg exchange of RBCs. Exchange transfusions are recommended by the AAP if total bilirubin levels remain above the transfusion threshold despite intensive phototherapy or if signs of bilirubin encephalopathy are present. If an exchange transfusion is being considered, an albumin level should be measured. Albumin of 3.0 g/dL or less is considered an independent risk factor for hyperbilirubinemia and lowers the phototherapy threshold. Without sufficient albumin to bind bilirubin, the amount of free, unconjugated bilirubin increases, thereby increasing the risk for kernicterus.
Anemic infants may require blood transfusions with ABO-matched packed RBCs. If immediate transfusion is thought to be needed, O-type, Rh-negative blood that has been leukodepleted and irradiated should be available at delivery.
Other treatment modalities have been considered, but are still controversial. Intravenous immunoglobulin (IVIG) in the infant may block Fc receptors on macrophages, thereby decreasing the breakdown of antibody-coated RBCs. IVIG is recommended by the AAP if total serum bilirubin continues to rise despite intensive phototherapy or is within 2-3 mg/dL of the exchange transfusion level. Administration of IVIG to mothers prior to delivery has not been shown to be efficacious and is not currently recommended. Other agents such as albumin, phenobarbital, metalloporphyrins, zinc, clofibrate, and prebiotics have been studied as possible treatment options for hyperbilirubinemia, but none are currently recommended. In a recent randomized control trial of 70 infants with Rh-alloimmunization, delayed cord clamping was shown to improve anemia without increasing the incidence of adverse events. Delayed cord clamping had no significant impact however, on the need for exchange transfusion or duration of phototherapy.
Hemolytic disease of the fetus and newborn should be included in the differential diagnosis of infants with early, severe, or prolonged jaundice and anemia. Other etiologies of jaundice and hyperbilirubinemia in the newborn include physiologic jaundice, prematurity, breast milk, and breastfeeding jaundice, G6PD deficiency, thalassemia, sepsis, birth trauma, Gilbert's syndrome, and hypothyroidism. The history and physical as well as simple laboratory evaluation, as described above, can help differentiate these causes.
The American Association of Blood Banks recommends repeat antibody screening prior to administration of Rh-D immunoprophylaxis (Rh-D IgG) at 28 weeks of gestation, postpartum, and in FMH events. In a network meta-analysis conducted in China, the most effective protocol for preventing maternal alloimmunization was the administration of Rh-D immunoprophylaxis at 28 and 34 gestational weeks in Rh-negative women.
The standard dose of anti-RhD administered in the second and third trimester, as well as postpartum, if needed, is 300mcg. If needed in the first trimester, the recommended dose is 150mcg. A one-time dose of 300mcg anti-RhD should prevent isoimmunization when 15mL or less of fetal RBCs (or 30mL of whole blood) enters maternal circulation. A rosette test is a qualitative test to assess potential FMH. If positive, the rosette test should be followed by the Kleihauer-Betke test to quantify the amount of fetomaternal blood mixing to then determine if additional doses of Rh-D immunoprophylaxis are needed.
The overall prognosis of HDFN is good if identified and treated promptly. While permanent neurologic dysfunction may result from delays in care, this is now a rare occurrence with the advancements in monitoring as well as prophylaxis against HDFN.
Acute bilirubin encephalopathy from the buildup of bilirubin in an infant's brain may manifest as hypotonia or poor suck reflex, which then progresses to irritability and hypertonia with retrocollis and opisthotonos. Long term consequences of chronic bilirubin encephalopathy may lead to cerebral palsy, auditory dysfunction, paralysis of upward gaze, and permanent intellectual dysfunction. Thus, early recognition and treatment are imperative to prevent the detrimental progression of HDFN.
Patient education regarding routine lab testing and Rh-D immunoprophylaxis in Rh-negative women are necessary to ensure possible FMH events are reported and appropriately treated in pregnant women. Additionally, parents of newborns can be educated on signs and symptoms to be aware of in hyperbilirubinemia to assist the interprofessional team in early identification of possible HDFN cases.
Enhancing interprofessional team outcomes for patients with hemolytic disease of the fetus and newborn requires close collaboration between both OB/GYN and pediatric providers, nurses, pharmacists, and blood bank personnel. With HDFN, there are two patients to consider at all times - the mother and the fetus/newborn. When HDFN is identified in utero, the delivery team should be well versed and prepared ahead of time to identify signs and symptoms of HDFN as these infants may need timely transfusions at birth. Throughout the pregnancy, pharmacists and providers must identify if and when Rh-D immunoprophylaxis is indicated to prevent future HDFN cases. Through the development of Rh-D immunoprophylaxis and newborn work-up protocols, the incidence of HDFN has dramatically dropped in the past 50 years, but it will take continued interprofessional collaboration to ensure the incidence of HDFN remains low.
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