Rh disease


Rh disease is a type of Hemolytic Disease of the Fetus and Newborn. The term "Rh disease" is commonly used to refer to HDFN as prior to the discovery of anti-Rho immune globulin, it was the most common type of HDFN.
The disease ranges from mild to severe, and occurs in the second or subsequent pregnancies of Rh-D negative women when the biological father is Rh-D positive due to the presence of anti-D antibodies.
Due to several advances in modern medicine HDFN can be prevented by treating the mother during pregnancy and soon after delivery with an injection of anti-Rho immune globulin. With successful mitigation of this disease by prevention through the use of anti-Rho immune globulin, other antibodies are more commonly the cause of HDFN today.

Mechanism

During pregnancy, there is normally a barrier between maternal and fetal blood called the placenta, a temporary organ that connects a mother’s uterus to the umbilical cord to provide nutrients and oxygen to the fetus. However, in certain circumstances, small amounts of fetal blood cells may enter the mother’s circulation. Certain types of events where this occurs are during childbirth, miscarriage or abortion, trauma, and invasive procedures such as amniocentesis. Once the fetal Rh-positive red blood cells enter the bloodstream of a Rh-negative mother, they are recognized as foreign. The mother’s immune system reacts to the Rh-positive red blood cells the same way that it would respond to something like a virus or bacteria, activating B cells—a type of white blood cell that is key to the triggering of an immune response. These activated B cells then differentiate into plasma cells, which produce anti-D antibodies. After the primary exposure, some of these B cells become memory cells that remember the original exposure, and produce IgG antibodies, which are smaller and can cross the placental barrier. Once they cross this barrier into the fetal bloodstream, they bind to fetal Rh-positive cells, triggering opsonization, which marks the red blood cells for destruction. The fetal spleen and liver then begin to break down those red blood cells, thinking that they are a foreign invader when in reality they are just mismatched.

Signs and symptoms

Symptoms of Rh disease include yellowish amniotic fluid and enlarged spleen, liver or heart or buildup of fluid in the abdomen of the fetus.

Pathophysiology

During the first pregnancy, the Rh− mother's initial exposure to fetal Rh+ red blood cells is usually not sufficient to activate her Rh-recognizing B cells. However, during delivery, the placenta separates from the uterine wall, causing umbilical cord blood to enter the maternal circulation, which results in the mother's proliferation of IgM-secreting plasma B cells to eliminate the fetal Rh+ cells from her blood stream. IgM antibodies do not cross the placental barrier, which is why no effects to the fetus are seen in first pregnancies for Rh-D mediated disease. However, in subsequent pregnancies with Rh+ fetuses, the IgG memory B cells mount an immune response when re-exposed, and these IgG anti-Rh antibodies do cross the placenta and enter fetal circulation. These antibodies are directed against the Rhesus factor, a protein found on the surface of the fetal RBCs. The antibody-coated RBCs are destroyed by IgG antibodies binding and activating complement pathways.
The resulting anemia has multiple sequelae:
  1. The immature haematopoietic system of the fetus is taxed as the liver and spleen attempt to put immature RBCs into circulation.
  2. As the liver and spleen enlarge under this unexpected demand for RBCs, a condition called portal hypertension develops, and this taxes the immature heart and circulatory system.
  3. Liver enlargement and the prolonged need for RBC production results in decreased ability to make other proteins, such as albumin, and this decreases the plasma colloid osmotic pressure leading to leakage of fluid into tissues and body cavities, termed hydrops fetalis.
  4. The severe anemia taxes the heart to compensate by increasing output in an effort to deliver oxygen to the tissues and results in a condition called high output cardiac failure.
  5. If left untreated, the result may be fetal death.
The destruction of RBCs leads to elevated bilirubin levels as a byproduct. This is not generally a problem during pregnancy, as the maternal circulation can compensate. However, once the infant is delivered, the immature system is not able to handle this amount of bilirubin alone and jaundice or kernicterus can develop which may lead to brain damage or death. Sensitizing events during pregnancy include c-section, miscarriage, therapeutic abortion, amniocentesis, ectopic pregnancy, abdominal trauma and external cephalic version. However, in many cases there was no apparent sensitizing event. Approximately 50% of Rh-D positive infants with circulating anti-D are either unaffected or only mildly affected requiring no treatment at all and only monitoring. An additional 20% are severely affected and require transfusions while still in the uterus. This pattern is similar to other types of HDFN due to other commonly encountered antibodies.

Diagnosis

Maternal blood

In the United States, it is a standard of care to test all expecting mothers for the presence or absence of the RhD protein on their RBCs. However, when medical care is unavailable or prenatal care not given for any other reason, the window to prevent the disease may be missed. In addition, there is more widespread use of molecular techniques to avoid missing women who appear to be Rh-D positive but are actually missing portions of the protein or have hybrid genes creating altered expression of the protein and still at risk of HDFN due to Anti-D.
  • At the first prenatal visit, the mother is typed for ABO blood type and the presence or absence of RhD using a method sensitive enough to detect weaker versions of this antigen and a screen for antibodies is performed.
  • * If she is negative for RhD protein expression and has not formed anti-D already, she is a candidate for RhoGam prophylaxis to prevent alloimmunization.
  • * If she is positive for anti-D antibodies, the pregnancy will be followed with monthly titers of the antibody to determine if any further intervention is needed.
  • A screening test to detect for the presence or absence of fetal cells can help determine if a quantitative test is needed. This is done when exposure is suspected due to a potential sensitizing event.
  • If the screening test is positive or the appropriate dose of RhoGam needs to be determined, a quantitative test is performed to determine a more precise amount of fetal blood to which the mother has been exposed.
  • * The Kleihauer–Betke test or Flow Cytometry on a maternal blood sample are the most common ways to determine this, and the appropriate dose of RhoGam is calculated based on this information.
  • There are also emerging tests using Cell-free DNA. Blood is taken from the mother, and using PCR, can detect fetal DNA. This blood test is non-invasive to the fetus and can help determine the risk of HDFN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol.

    Paternal blood

Blood is generally drawn from the biological father to help determine fetal antigen status. If he is homozygous for the antigen, there is a 100% chance of all offspring in the pairing to be positive for the antigen and at risk for HDFN. If he is heterozygous, there is a 50% chance of offspring to be positive for the antigen.

Prevention

The protection that is offered today against Rh incompatibility involved preventive measures that primarily utilize Rh immunoglobulin, also known as RhoGAM. The aim of these treatments are to prevent the mother's immune system from becoming sensitized to the Rh antigen, which reduces the risk of hemolytic disease in future pregnancies. RhoGAM, Rh immunoglobulin administration, is a product that contains antibodies to the Rh antigen; it is used to prevent the mother from developing an immune response to fetal red blood cells. RhIg 'coats' any Rh-positive fetal red blood cells that enter the mother’s bloodstream, effectively 'hiding' them from the mother's immune system. RhoGAM is typically administered at around 28 weeks of pregnancy, then again within 72 hours after childbirth. It is also given during other events that happen during pregnancy like miscarriages, ectopic pregnancies, amniocentesis, and abdominal trauma.
In Arar, Saudi Arabia, results of a study showed that women had a low level of knowledge regarding maternal–fetal blood incompatibility. Regarding their knowledge about anti-D, researchers found that 68.5% of the mothers had knowledge about it, while only 51% of the mothers had knowledge about the administration of prophylactic anti D after delivery.

Management

As medical management advances in this field, it is important that these patients be followed by high risk obstetricians/maternal-fetal medicine, and skilled neonatologists postpartum to ensure the most up to date and appropriate standard of care.

Antenatal

  • Routine prenatal labs drawn at the beginning of every pregnancy include a blood type and an antibody screen. Mothers who are Rh negative and have anti-D antibodies need to determine the fetus's Rh antigen. If the fetus is also Rh negative then the pregnancy can be managed like any other pregnancy. The anti-D antibodies are only dangerous to Rh positive fetuses.
  • *The fetal Rh can be screened using non-invasive prenatal testing. This test can screen for the fetus's Rh antigen at the 10th week of gestation using a blood sample drawn from the mother. The Unity test uses NGS technology to look for Rh alleles in the cell free fetal DNA in the maternal bloodstream. In healthy pregnancies, at least 5% of the cell free DNA in the maternal bloodstream comes from the fetus. This small fraction of cell free DNA from the fetus is enough to determine the fetus's Rh antigen.
  • Once a woman has been found to have made anti-D, she is followed as a high risk pregnancy with serial blood draws to determine the next steps.
  • Once the titer of anti-D reaches a certain threshold, serial Doppler ultrasound examinations are performed to detect signs of fetal anemia.
  • * Detection of increased blood flow velocities in the fetus are a surrogate marker for fetal anemia that may require more invasive intervention.
  • If the flow velocity is found to be elevated a determination of the severity of anemia needs to ensue to determine if an intrauterine transfusion is necessary.
  • * This is normally done with a procedure called percutaneous umbilical cord blood sampling.
  • Intrauterine blood transfusion
  • * Intraperitoneal transfusion—blood transfused into fetal abdomen
  • * Intravascular transfusion—blood transfused into fetal umbilical vein—This is the method of choice since the late 1980s, and more effective than intraperitoneal transfusion. A sample of fetal blood can be taken from the umbilical vein prior to the transfusion.
  • * Often, this is all done at the same PUBS procedure to avoid the needs for multiple invasive procedures with each transfusion.