In document Danforth - Obstetrics and Gynecology 9ed (Page 176-180)

Though the first description of erythroblastosis fetalis (hemolytic disease of the newborn) dates back to the 1609, it was not until the early 1900s that the role of

alloimmunization in the pathogenesis of erythroblastosis was established. The early investigators determined that an Rh–negative mother becomes alloimmunized by exposure to Rh–positive fetal erythrocytes during pregnancy or delivery. Maternally produced anti-erythrocyte antibodies pass through the placenta to the fetus where they react with the Rh–positive fetal erythrocytes, causing their destruction. In the past, Rh alloimmunization has also been referred to as sensitization and

immunization. The terms alloimmunization and sensitization will be used interchangeable in this text.

Many other erythrocyte antigens have been described since the discovery of the Rh antigen but only a few are clinically important causes of maternal alloimmunization.

However, with the widespread use of Rh–immunoglobulin prophylaxis and the decline in Rh-D alloimmunization, the overall importance of the “minor antigens” in red cell alloimmunization has increased.

Genetics of the Rh Antigen

The Rh blood group was so named because rabbits immunized with rhesus monkey erythrocytes produced an antibody that agglutinated erythrocytes from 85% of whites. The so-called Rh factor is actually an antibody directed against an erythrocyte surface antigen of the rhesus blood group system. The Rh blood group system has a high degree of polymorphism, with five major antigens and many variant minor antigens. While three systems of nomenclature have been suggested, the Fisher-Race system is probably best suited to understanding the inheritance of the Rh antigen and the clinical management of Rh alloimmunization. It assumes the presence of three genetic loci, each with two major alleles, lettered C, c, D, E, and e. No antiserum specific for a ?d? antigen has been found.

The Rh gene complex is described by the three appropriate letters with eight possible combinations (listed in decreasing order of frequency among whites): CDe, cde, cDE, cDe, Cde, cdE, CDE, and CdE. Genotypes are indicated as pairs of gene complexes, such as CDe/cde. Certain genotypes, and thus certain phenotypes, are more prevalent than others. Although the alleles are always written in the order C(c), D, E (e), the actual order on chromosome one is of the genes coding for the antigens D, C(c), E (e). Anti–C, anti–c, anti–D, anti–E, and anti–e designate specific antibodies directed against the respective antigens. Because the majority of Rh alloimmunization resulting in overt clinical disease results from incompatibility with respect to the D antigen, common convention holds that Rh-positive indicates the presence of the D antigen and Rh-negative indicates the absence of D antigen on erythrocytes.

Unique Rh antibodies have been used to identify more than 30 antigenic variants in the Rh blood group system, the most common of which is the D u antigen, now commonly referred to as weak D. This heterogeneous group of clinically important D-antigen variants is most often found in blacks. At least some weak D–positive patients are capable of producing anti–D, which could presumably result in a weak D–positive mother becoming sensitized to her D–positive fetus, but such an occurrence is exceedingly rare.

The Rh-D antigen appears very early in embryonic life and has been demonstrated on the red blood cells of a 38-day-old fetus. Its precise function is unknown, though it may play a role in maintaining red cell membrane integrity or regulate the asymmetric distribution of different phospholipids through the red cell membrane.

Pathophysiology of Rh Alloimmunization

Rh alloimmunization can only occur in the presence of three conditions:

1. The fetus must have Rh–positive erythrocytes, and the mother must have Rh–negative erythrocytes.

2. The mother must have the immunogenic capacity to produce antibody directed against the D antigen.

3. A sufficient number of fetal erythrocytes must gain access to the maternal circulation.

Incidence of Rh-D Incompatibility and Subsequent Alloimmunization About 15% of whites, 5% to 8% of blacks, and 1% to 2% of Asians and Native Americans are Rh-D–negative. In terms of risk, an Rh–negative woman has about an 85% chance of mating with an Rh–positive man, 60% of whom are heterozygous and 40% of whom are homozygous at the D locus. Assuming that one-half of the conceptions of heterozygous men will be Rh-D–positive, the chance of an Rh–positive man producing an Rh–positive fetus is about 70%. An Rh–negative woman has about a 60% chance of bearing an Rh–positive fetus (0.85 × 0.70). About 10% of pregnancies are Rh-incompatible (0.15 × 0.60). However, fewer than 20% of Rh–incompatible pregnancies result in alloimmunization because fetomaternal

hemorrhage sufficient to trigger a maternal antibody response does not occur in every case. About 16% of untreated Rh–negative women become alloimmunized in their first Rh–incompatible (ABO–compatible) pregnancy. Half produce detectable anti–D antibody within 6 months of delivery while the rest have undetectable

amounts until early in the next incompatible pregnancy. Overall, even before the introduction of Rh–immunoglobulin prophylaxis, only about 1% of pregnant women had anti–D antibody.

Maternal Immunologic Response The probability and severity of Rh-D alloimmunization varies depending on individual patient characteristics. As many as 30% of Rh–negative individuals appear to be immunologic “nonresponders” who will not become sensitized. In addition, ABO incompatibility diminishes the risk of

alloimmunization to about 1.5% to 2.0% after the delivery of an Rh–positive fetus. This is possibly due to rapid clearance of ABO–incompatible fetal cells from the maternal circulation or alteration or damage to the fetal Rh antigen so that it is no longer immunogenic. The effect is most pronounced if the mother is type O and the father is type A, B, or AB.

Fetomaternal Hemorrhage Fetal red cells may gain access to the maternal circulation during pregnancy, delivery, and the immediate postpartum period. Fetomaternal hemorrhage in a volume sufficient to cause alloimmunization is most common at delivery, occurring in 15% to 50% of births. The amount of fetal blood entering the maternal circulation is usually less than 0.1 mL but may be greater than 30 mL in 0.2% to 1% of cases. Risk factors for excessive postpartum fetomaternal hemorrhage include cesarean delivery, multiple gestations, bleeding placenta previa or abruption, manual removal of the placenta, and intrauterine manipulation. However, the majority of cases of excessive fetomaternal hemorrhage occur after uncomplicated vaginal delivery. Antepartum events can also result fetomaternal hemorrhage in sufficient volume to cause in alloimmunization in 1% to 2% of cases, even without obvious disruption of the choriodecidual junction ( Table 18.1). Fortunately, asymptomatic antepartum sensitization rarely occurs before the third trimester.

TABLE 18.1. Antepartum events associated with fetomaternal hemorrhage

Rh-D Immunoglobulin

The prevention of alloimmunization to a specific antigen by the passive administration of antibody is termed antibody-mediated immune suppression. In the case of Rh-D alloimmunization, a high degree of protection was first achieved by administering anti–D immunoglobulin (Rh-D immunoglobulin) to Rh–negative male volunteers who had been infused with Rh–positive red cells. It was later established that the amount of Rh-D immunoglobulin necessary to prevent alloimmunization varies

according to the size of fetomaternal hemorrhage:

300 g of Rh-D immunoglobulin for exposure to 10 mL of fetal blood 20 g of Rh-D immunoglobulin for exposure to 1 mL of fetal erythrocytes 10 g of Rh-D immunoglobulin for 1 mL of whole fetal blood.

Postpartum Alloimmunization Prophylaxis The early Rh-D alloimmunization prevention trials found that administration of Rh-D immunoglobulin within 72 hours of delivery reduced alloimmunization to less than 1.5% in Rh–negative women, a seven- to ten-fold decrease in alloimmunization compared with untreated controls.

Although 300 g of Rh immunoglobulin was used (and continues to be the standard in the United States), it has subsequently been shown that a dose of 100 g to 150 g is probably adequate for routine use. Rh-D immunoglobulin must be given as soon as possible after exposure to Rh-D–positive blood (delivery or other event

associated with fetomaternal hemorrhage) before the primary immune response is established. While 72 hours is the standard recommendation, it is an artifact from the early studies performed using inmates, during which prison officials would allow visits only at 3-day intervals. Prophylaxis beyond 3 days has never been

extensively studied but if for some reason Rh–immune prophylaxis does not occur within 72 hours after exposure, susceptible Rh-D–negative women should be treated up to 14 to 28 days. Further, if the neonatal Rh status is unknown 3 days after delivery, Rh immunoglobulin should be given rather than waiting for the neonatal results.

Antepartum Alloimmunization Prophylaxis One to two percent of susceptible Rh-D–negative women become sensitized during pregnancy in spite of postpartum Rh-D–immune prophylaxis. Most failures can be attributed to antepartum fetomaternal hemorrhage that is often not clinically apparent. Prophylactic administration of Rh-D immunoglobulin at 28 weeks gestation reduces the incidence of alloimmunization from 1.8% to 0.1%. Initial concerns about potential adverse effects of antenatal Rh-D–immune prophylaxis have been refuted by decades of experience with Rh-D immunoglobulin without reports of maternal or fetal complications. Further, routine antepartum prophylaxis is much less expensive than the neonatal intensive care required for severely anemic infants.

Management of the Unsensitized Rh–Negative Pregnant Woman

Prenatal care for Rh-D–negative women, without evidence of alloimmunization is straightforward ( Table 18.2). Every woman should have her ABO blood group, Rh type, and antibody screen checked at the first prenatal visit of all pregnancies. If she is Rh-negative or weak D–negative and has no demonstrable antibody, she is a candidate for 300 g Rh–immunoglobulin prophylaxis at around 28 weeks gestation and again immediately postpartum. The American Association of Blood Banks recommends obtaining another antibody screen before administration of Rh immunoglobulin, including antepartum prophylaxis.

TABLE 18.2. Evaluation and management of an unsensitized Rh-negative, pregnant woman

After delivery, another antibody screen is routinely performed. If negative and the newborn is Rh-D–positive or weak D–positive, alloimmunized women should be given 300 g of Rh-D immunoglobulin. In addition, because up to 1% of deliveries result in a fetomaternal hemorrhage greater than 30 mL (the largest volume of fetal blood adequately covered by a standard 300-g dose of Rh immunoglobulin), a screen for “excessive” fetomaternal hemorrhage should be performed. Most laboratories use the erythrocyte rosette test, a simple and sensitive method for detecting fetomaternal bleeding. If the rosette test is positive, the volume of fetal red cells in the maternal circulation can be calculated using the Kleihauer-Betke test. If the volume of fetomaternal hemorrhage is greater than 30 mL whole blood, an additional 10g of Rh immunoglobulin should be administered for each additional milliliter of fetal blood.

A weak D–positive mother who delivers an Rh–positive infant is not at significant risk of Rh sensitization, probably because the weak D antigen is actually an incompletely expressed D antigen. Therefore, weak D–positive mothers usually do not require Rh immunoglobulin. Occasionally a woman previously typed as

Rh-negative is unexpectedly found to be weak D–positive during pregnancy or after delivery. In this situation, the clinician should be suspicious that the patient's “new”

weak D–positive status is actually due to a large number of Rh–positive fetal cells in the maternal circulation. Appropriate diagnostic studies should be performed, and if fetomaternal hemorrhage is found, the mother should be treated with Rh immunoglobulin.

Several antepartum complications and procedures may also result in fetomaternal hemorrhage (see Table 18.1). First trimester complications, including spontaneous miscarriage, elective abortion, and ectopic abortion, may result in fetomaternal hemorrhage sufficient to result in alloimmunization.

Fetomaternal hemorrhage has also been demonstrated in up to half of women with threatened first trimester miscarriage but is only occasionally associated with alloimmunization. Management is controversial with no clear consensus or evidence-based recommendation on use of Rh immunoglobulin.

An Rh–negative, unsensitized patient who has antepartum bleeding or suffers an unexplained second or third trimester fetal death should receive Rh–immunoglobulin prophylaxis and be evaluated for the possibility of massive fetomaternal hemorrhage. Several procedures may also result in fetomaternal hemorrhage sufficient to cause alloimmunization including chorionic villus sampling, amniocentesis, and external cephalic version.

For first trimester pregnancy complications and procedures, 50 g of Rh immunoglobulin is protective. Beyond 12 weeks, a full 300-g dose is indicated even if in the absence of detectable hemorrhage. In addition, because excessive fetomaternal hemorrhage may occur with any complication or procedures performed in the second and third trimester, an assessment of the volume of fetal whole blood should be performed and the appropriate amount of Rh-D immunoglobulin should be given.

Failure to administer Rh immunoglobulin when indicated is responsible for one-fourth of new cases of alloimmunization. This inexcusable oversight may be due to:

failure to type the patient's blood at the first prenatal visit or to order Rh-D immunoglobulin when indicated error in transmitting the proper blood type to the mother's chart and to the physician

error in typing the mother's, father's, or baby's blood

failure to administer Rh-D immunoglobulin when ordered unrecognized fetomaternal hemorrhage during pregnancy

inadequate Rh-D-immunoglobulin for the volume of fetomaternal hemorrhage patient refusal.

Management of the Rh-D–Alloimmunized Pregnancy

Obstetric History A well-documented obstetric history is essential to guide management of alloimmunized pregnancy. Fetal hemolytic disease tends to become more severe in subsequent pregnancies. If hydrops occurred in a previous pregnancy, the next Rh–incompatible fetus has an 80% to 90% chance of becoming hydropic as well. With this in mind, patients are grouped into one of three categories:

mildly affected fetuses, which can be allowed to remain in utero until they have achieved pulmonary maturation

moderately affected fetuses, which may need to be delivered before pulmonary maturity but who do not need fetal treatment

severely affected fetuses, which require active intervention to reach a gestational age at which the risks of delivery and neonatal intensive care are less than the risks of in utero therapy.

In general, hemolysis and hydrops develop at about the same time or somewhat earlier in subsequent pregnancies; this can be used as a rough guide for timing initial fetal studies and transfusions. Fetal hydrops seldom develops in a first sensitized pregnancy.

Maternal Antibody Titers Severe erythroblastosis or perinatal death does not occur if antibody levels remain below 1:16. Some centers use an anti–D titer of 1:8 because of variations in reliability and methods of titration. In general, women with anti–D titers of 1:8 or less, and no history of a previously affected infant, can be safely followed with anti–D titers every 2 to 4 weeks and serial fetal ultrasound assessment. Those with anti–D titers of 1:16 or greater should be referred for amniocentesis. Once the critical anti–D antibody titer has been reached in a sensitized pregnancy, more antibody titers are not useful in the current pregnancy or subsequent pregnancies. Titers may remain stable in up to 80% of severely affected pregnancies. Variability between maternal antibody levels and severity of fetal disease is explained by the fact that antibody concentration is only one factor influencing the degree of anemia. Other factors include antibody subclass and degree of glycosylation, placental transfer of antibody, antigen expression on fetal erythrocytes, functional maturity of the fetal reticuloendothelial system, and the presence of HLA-related antibodies that inhibit fetal erythrocyte destruction.

Determination of the Fetal Antigen Status The possibility that the fetus is Rh-negative (not at risk) should always be considered. A reasonable first step in this process is to determine paternal Rh–antigen status and zygosity:

If the father is Rh-negative, the fetus must also be Rh-negative and no further testing is necessary.

If the father is Rh-positive, but has previously fathered Rh–negative children, he is heterozygous and the probability that this fetus is Rh-negative is 50%.

If the father is Rh-positive without other Rh–negative children, zygosity can be established using either DNA analysis or Rh antisera. If the father is homozygous, this fetus is Rh-positive and no other testing is necessary.

In the past, determination of fetal blood type required direct analysis of fetal blood obtained by umbilical cord blood sampling with its attendant risks of fetal loss and fetomaternal hemorrhage. The development of DNA tests that use polymerase chain reaction (PCR) has made it possible to determine fetal Rh status from uncultured amniocytes obtained from as little as 2 mL of amniotic fluid or 5 mg of chorionic villi. Though highly accurate, DNA testing for fetal Rh status is equivocal in about 1% of cases, probably because of the presence of gene rearrangements near the Rh-D locus that can be missed by standard DNA primers used for PCR. Most laboratories recommend simultaneous testing of paternal blood and amniotic fluid. Fetal antigen testing from amniocytes has become routine at most centers in the United States.

Most alloimmunized women have fetal antigen typing at the time of the first amniocentesis for amniotic fluid bilirubin analysis. Alloimmunized women having chorionic villus sampling (CVS) or second trimester amniocentesis for other unrelated conditions can have fetal antigen typing earlier. However, except for patients with severe Rh sensitization who would consider termination of an Rh–positive pregnancy, CVS and amniocentesis are not usually offered for detection of Rh-D fetal antigen status alone. If the DNA test indicates an Rh–negative fetus, the small likelihood of misdiagnosis should be discussed and the patients offered standard antenatal

surveillance. In the future, fetal Rh–antigen status will likely be performed on fetal cells obtained from the maternal circulation or by preimplantation genetics.

Amniotic Fluid Optical Density Analysis Assessment of amniotic fluid in Rh alloimmunization is based on the original observations that spectrophotometric determinations of amniotic fluid bilirubin correlated with the severity of fetal hemolysis. A by-product of fetal hemolysis, bilirubin is excreted into the amniotic fluid through fetal pulmonary and tracheal secretions and by diffusion across the fetal membranes and the umbilical cord. Using a semilogarithmic plot, the curve of optical density of normal amniotic fluid is approximately linear between wavelengths of 525 and 375 nm. Bilirubin causes a shift in the spectrophotometric density with a peak at a wavelength of 450 nm. The amount of shift in optical density from linearity at 450 nm (the ?OD450) is used to estimate the degree of fetal red cell hemolysis ( Fig.


FIG. 18.5. Spectrophotometric scan of amniotic fluid containing bilirubin. An arbitrary line ( thick line) has been drawn to show where the scan would have been traced if there had been no increase in bilirubin. The peak absorption of bilirubin occurs at 450 µm. The difference between the peak and the arbitrary line equals 0.204.

Liley was the first to correlate amniotic fluid ?OD450 values with newborn outcome by dividing a semilogarithmic graph of gestational age versus ?OD450 into three zones ( Fig. 18.6). Unaffected fetuses and those with mild anemia had ?OD450 values in zone I (the lowest zone), while severely affected fetuses had ?OD450 values in zone III (the highest zone). Fetuses with zone II values (the middle zone) had disease ranging from mild to severe, indicated primarily by the trend of the

determinations of amniotic fluid bilirubin. Because there is a tendency for amniotic fluid bilirubin to decrease as pregnancy advances, the boundaries of the zones slope downward as gestational age increases. Implementation of Liley's method reduced perinatal mortality from 22% to 9% over a 5-year period.

FIG. 18.6. This form is used to record serial data on each Rh-immunized patient. The modified Liley graph is divided into three zones to predict the outcome of the pregnancy in terms of umbilical cord blood hemoglobin, intrauterine deaths, or unaffected fetuses.

FIG. 18.6. This form is used to record serial data on each Rh-immunized patient. The modified Liley graph is divided into three zones to predict the outcome of the pregnancy in terms of umbilical cord blood hemoglobin, intrauterine deaths, or unaffected fetuses.

In document Danforth - Obstetrics and Gynecology 9ed (Page 176-180)