Introduction

Hemolytic disease of the newborn (HDN) is defined as fetal or neonatal shortened red cell (RBC) survival that can be caused by inherited RBC membrane defects, hemoglobinopathies, enzyme deficiencies, acquired intrauterine or perinatal infections, or red cell antibodies.

Maternal alloimmunization occurs when fetal RBC carrying inherited paternal RBC antigens which are not present on maternal RBC pass into the maternal circulation, inducing antibody formation. IgG alloantibodies can then cross the placenta and coat the fetal RBC that in turn are removed by the macrophage system. First time pregnancies are rarely involved. However, for subsequent pregnancies in which the fetus expresses the implicated antigen, severity may increase.

The classic example of maternal alloimmunization is the formation of anti-D in the Rh negative mother of an Rh positive fetus. Up to 17 % of the Rh negative pregnant women exposed to Rh positive fetal red cells will develop anti-D. Only 0.1 mL of red cells are needed to initiate this immune response. However, with the introduction of Rh immune globulin prophylaxis, the risk of anti-D alloimmunization in this patient population has decreased to < 0.1 %. Severity of HDN caused by anti-D is variable ranging from being mild in 50 % of cases to severe in 20 %.

HDN can also be caused by maternal anti-A, anti-B and anti-A,B IgG isoagglutinins mostly present in type O mothers. Although a type O mother of a type A or B infant occurs in 15 % of pregnancies the frequency of HDN due to ABO isoagglutinins is only 1/150 births. Rarely clinically severe, it is manifested mostly in the neonatal period as a rising indirect bilirubin in the first 36 hours of life with minimal anemia. The direct antiglobulin (Coombs) test is weakly positive or negative even in the affected neonates and spherocytes are present in the peripheral blood smear. Most affected neonates will respond to phototherapy alone. Rh alloimmunization is less likely to occur when ABO incompatibility between mother and fetus is present.

Other antibodies against red cell antigens can cause HDN. Implicated antibodies include anti E, anti-c, or anti-K1.

Prenatal Diagnosis and Treatment

The diagnostic evaluation includes maternal prenatal ABO and Rh typing, and an antibody screen. Depending on the results of the antibody screen, maternal antibody titers, and paternal and/or fetal RBC phenotyping are performed. If fetal RBC express the antigen against which maternal alloimmunization has occurred, the pregnancy is then followed by measuring serial maternal antibody titers and abdominal sonograms.

In response to the shortened RBC survival the fetus compensates by markedly increasing erythropoiesis. Extramedullary hematopoiesis in the liver can cause portal hypertension and ascites. If erythropoiesis cannot compensate for the rate of hemolysis, anemia may occur with consequent high output cardiac failure and anasarca. The hemolysis results in increased fetal production of indirect bilirubin that crosses the placenta and undergoes conjugation in the maternal liver, thereby preventing its accumulation in the fetus. Ultrasonography can provide information on polyhydramnios, fetal organomegaly and edema. Abnormal ultrasound findings signs are an indication to measure fetal hemoglobin by percutaneous umbilical blood sampling and/or amniocentesis to measure bile pigments in the amniotic fluid. The fetus is monitored with serial sonograms, serial hemoglobin measurements and serial bile pigment measurements. Depending on the gestational age and fetal lung maturity, intrauterine RBC transfusions are given or early delivery is performed. Blood used for intrauterine transfusion should be type O negative, CMV negative, irradiated, Sickledex negative, < 7 days old, crossmatched compatible with mother’s serum, and red cell antigen negative for the offending antibody.

Neonatal Intervention

After delivery a direct antihuman globulin test (Coombs), hemoglobin, and bilirubin level is performed on a cord blood sample. After birth, the maternal liver is not available for conjugation of the accumulating bilirubin and the neonatal liver is too immature to handle the bilirubin load, therefore the neonatal unconjugated bilirubin levels starts to rise. Accumulation of the unconjugated bilirubin puts the neonate at risk of developing kernicterus with the accompanying neurological sequelae. The rate at which the indirect bilirubin levels increase dictates what kind of intervention to follow, with the alternatives including observation, phototherapy, or red cell exchange transfusion. Exchange transfusion serves several purposes including: reducing the levels of indirect bilirubin, removing circulating maternal antibodies and antibody coated neonatal RBC and correcting the anemia.

Prevention

Gestational alloimmunization with anti-D in Rh negative mothers can be prevented by the use of RhIg. The risk of alloimmunization has decreased from 17 % to 0.1% with RhIg prophylaxis. The immunosuppresive effect is believed to result from an interference with antigen recognition during the induction phase of the primary immunization. RhIg is a formulation prepared from the plasma of humans sensitized with anti-D. Its preparation and processing makes it a viral free blood derivative, similar to albumin. Currently it is available as an intramuscular or intravenous preparation. One vial contains 300 ug of anti-D which will protect against 30 mL of Rh positive whole blood or 15 mL of Rh positive RBC. It is recommended that RhIg be given at 28 weeks gestation and within 72 hours of delivery of an Rh positive neonate. It is known that the majority of the alloimmunization occurs after 28 weeks of gestation although the expression of the Rh antigen in the fetus can be expressed as early as the 4th week of gestation. The amount of fetomaternal hemorrhage (FMH, circulating fetal blood in the mother) is estimated after delivery in order to provide an adequate RhIg dose. An initial screening test, the rosette test, will detect a FMH > than 10 mL. If positive, a quantitative test, the Kleihauer-Betke acid elution test is performed. The amount of RhIg to be given is calculated based on the quantity of fetal RBC in the maternal blood. It is estimated that 0.3% of FMH are greater than 10 mL. RhIg is also recommended for any obstetrical intervention or maternal abdominal trauma in the mother which could potentially cause FMH (i.e. abortion, amniocentesis, chorionic villi sampling, cordocentesis).