Comprehensive Study Guide on Rhesus Alloimmunization and Hemolytic Disease of the Fetus

Definition and Pathophysiology of Rhesus (Rh) Alloimmunization

Rhesus (Rh) alloimmunization is a clinical condition involving the immune response of a mother to Rhesus antigens present on the surface of fetal red blood cells (RBCs). The Rhesus factor is a protein inherited on the surface of red blood cells. If the protein is present, the individual is designated as Rh positive $(+)$; if it is absent, they are Rh negative $(-)$. In the latter case, no Rh antigens exist on the RBC surface. Alloimmunization occurs when an Rh negative mother’s immune system recognizes the Rh positive cells of her fetus as foreign bodies, triggering the production of antibodies to attack and destroy them. The term "Allo" refers to an immune response against antigens from another individual of the same species (human).

There are approximately $1200$ known Rhesus antigens, but the D antigen is responsible for about $90\%$ of all clinical cases of sensitization. Other antigenic forms include $C$, $c$, $E$, $e$, and the Kell antigen. Sensitization occurs in two distinct stages: the primary response and the secondary response. The primary response occurs during the first exposure to the Rh antigen. The body slowly produces a limited amount of Immunoglobulin M $(IgM)$. Because $IgM$ has a high molecular weight, it does not cross the placenta, is considered weak, and typically does not affect the fetus during the first pregnancy.

The secondary response occurs upon subsequent exposure to the Rh antigen. This is a rapid "memory" response characterized by the production of large quantities of Immunoglobulin G $(IgG)$. Unlike $IgM$, $IgG$ is small enough to cross the placental barrier, where it attacks fetal RBCs, leading to Hemolytic Disease of the Fetus and Newborn (HDFN). This can result in varying degrees of fetal anemia. If the destruction is mild, the fetus compensates by increasing RBC production in the liver or bone marrow, leading to the presence of reticulocytes or nucleated red blood cells.

Clinical Consequences of Hemolytic Disease and Hydrops Fetalis

Severe hemolysis leads to profound fetal anemia, resulting in high-output congestive heart failure. This state progresses to "Hydrops Fetalis," a condition characterized by abnormal fluid accumulation in two or more fetal compartments. Specific manifestations include pericardial effusion (fluid around the heart), pleural effusion (fluid around the lungs), and ascites (fluid in the abdominal cavity). Clinical signs detectable via ultrasound include skin edema, which appears as a double-contour halo around the fetus, and hepatosplenomegaly (enlargement of the liver and spleen) caused by extra-medullary hematopoiesis.

The placenta also becomes thickened and enlarged (placentomegaly) due to the hydrops. As the heart fails to pump effectively against the severe anemia, circulatory collapse occurs, eventually leading to fetal death in the final stages. The presence of fluid in the pericardial, pleural, or peritoneal spaces is a pathological marker of severe distress, often referred to as the "ascites sign" or "black zone" on ultrasound imaging.

Fetomaternal Hemorrhage and the Placental Barrier

In a healthy pregnancy, maternal and fetal blood do not mix directly because they are separated by the placental blood-barrier. This barrier consists of multiple layers: the syncytiotrophoblast, the cytotrophoblast, the mesodermal connective tissue, and the vascular endothelium of the fetal capillaries. However, as the pregnancy nears its end, the cytotrophoblast layer thins and disappears, leaving only a thin layer of syncytiotrophoblast and the endothelium. By the end of the term, the barrier is almost exclusively composed of the vascular endothelium, increasing the risk of leakage.

Fetomaternal hemorrhage (FMH) is the crossing of fetal red blood cells into the maternal circulation. This can occur due to abdominal trauma, placental complications such as abruption (placental abruption), placental tumors, or invasive obstetric procedures like amniocentesis. In many cases, the cause remains unknown. Even a small volume of fetal blood, often referred to as a "hidden leak," can trigger maternal sensitization.

One specific theory, known as "The Grandmother Theory," suggests that a female fetus who is Rh negative may be sensitized if her own mother was Rh positive and fetal-maternal bleeding occurred during her own birth. In this scenario, the girl is born already possessing antibodies (sensitized) and may experience HDFN during her very first pregnancy if she carries an Rh positive child.

Screening, Diagnosis, and the Kleihauer-Betke Test

Clinical management begins with determining the Rhesus status of the mother during her first prenatal visit. If the mother is Rh negative, her husband’s status and the fetal status must be assessed. The fetal Rh status can be determined non-invasively using Cell-Free Fetal DNA (cfDNA) screening from maternal blood as early as the $9^{th}$ week of pregnancy. This is a safe but expensive procedure. In genetics, Rh positivity is dominant; thus, if the mother is Rh negative $(dd)$ and the father is homozygous Rh positive $(DD)$, $100\%$ of the children will be Rh positive $(Dd)$. If the father is heterozygous $(Dd)$, there is a $50\%$ chance the child will be Rh positive and a $50\%$ chance the child will be Rh negative.

To detect maternal sensitization, the Indirect Coombs Test is performed on maternal serum to check for free Rh antibodies. The Direct Coombs Test is performed on fetal RBCs to see if they are already coated with maternal antibodies. If the mother is not sensitized, she is given Anti-D immunoglobulin.

To quantify the volume of fetomaternal hemorrhage, the Kleihauer-Betke (KB) test is utilized. This test relies on the principle that fetal hemoglobin $(HbF)$ is more resistant to acid elution than adult hemoglobin $(HbA)$. A maternal blood smear is exposed to an acid solution; the $HbA$ dissolves, leaving behind empty "ghost cells," while $HbF$ remains intact in the fetal cells. The lab technician counts the fetal cells against the maternal cells. To calculate the volume of fetal blood leaked into the mother, the following ratio is used: $\text{Ratio} = \frac{\text{Fetal Cells}}{\text{Maternal Cells}}$. This ratio is then multiplied by the mother's total blood volume (estimated as $10\%$ of her body weight).

Management with Anti-D Immunoglobulin

Anti-D immunoglobulin (RhoGAM) is administered to Rh negative, non-sensitized mothers to destroy any fetal Rh positive RBCs in the maternal circulation before the mother's immune system can recognize them and produce its own antibodies. A standard dose of $300\,\mu g$ (one vial) of Anti-D is sufficient to neutralize approximately $30\,ml$ of fetal whole blood. If the KB test indicates a hemorrhage greater than $30\,ml$, additional doses are required.

Routine administration of Anti-D occurs at $28\,weeks$ of gestation and within $72\,hours$ after delivery if the newborn is confirmed Rh positive. It is also administered after events that increase the risk of FMH, such as spontaneous or induced abortion, ectopic pregnancy, or invasive procedures like amniocentesis. If the mother and fetus are ABO-incompatible (e.g., Mother Type O, Fetus Type A), the risk of Rh sensitization is actually decreased because the mother’s natural anti-A or anti-B antibodies destroy the fetal cells before the Rh-specific immune response can be initiated. If they are compatible, the risk of Rh sensitization increases.

Monitoring Fetal Anemia via Spectrophotometry and Doppler

When a mother is sensitized (titer > 1/16), the degree of fetal hemolysis must be monitored closely. Historically, this was done via amniocentesis to measure bilirubin levels in the amniotic fluid. Bilirubin is a byproduct of RBC breakdown. In the fetus, bilirubin is usually cleared by the placenta; however, in cases of severe hemolysis, it enters the amniotic fluid through tracheal secretions or the umbilical cord. Bilirubin is unstable and turns transparent when exposed to light, so samples must be kept in opaque tubes.

Spectrophotometry measures the optical density of the amniotic fluid at a wavelength of $450\,nm$ (the peak absorbance of bilirubin). The results are plotted on a Liley Graph or a Queenan Graph. These graphs are divided into zones: Zone 1 (mild or no disease), Zone 2 (moderate disease requiring frequent monitoring), and Zone 3 (severe disease requiring immediate intervention such as delivery or intrauterine transfusion).

Currently, a non-invasive alternative is the Middle Cerebral Artery (MCA) Doppler. There is a direct correlation between fetal anemia and increased peak systolic velocity in the MCA. If the velocity exceeds $1.5$ times the median $(MoM)$, it is highly suggestive of severe fetal anemia. This screening is typically performed between $17$ and $35\,weeks$ of gestation.

Intrauterine Transfusion and Fetal Pharmacotherapy

In cases of severe fetal anemia identified before the fetus is viable for delivery (usually before $34\,weeks$), an intrauterine transfusion may be performed. Concentrated, fresh RBCs (Hematocrit $\approx 70-80\%$) that are CMV-negative and compatible with the mother's serum are infused. The transfusion can be performed into the umbilical vein (Percutaneous Umbilical Blood Sampling - PUBS) or the fetal peritoneal cavity. The amount for a peritoneal transfusion is calculated as: $\text{Volume (ml)} = (\text{Gestation Weeks} - 20) \times 10$. If the transfusion is intravenous, the amount is calculated based on fetal weight, usually representing $62\%$ of the fetal weight for a targeted hematocrit.

To facilitate the procedure, a paralytic agent like Curare is injected into the fetal thigh using a $22\text{-gauge}$ spinal needle to prevent movement. Transfusions are often repeated every $3\,weeks$ because fetal hematocrit typically drops by approximately $1\%$ per day. Additionally, mothers can be given Phenobarbital two weeks before delivery to stimulate the fetal liver’s glucuronyl transferase enzyme, which helps conjugate bilirubin and reduce the risk of neonatal jaundice (kernicterus). A side effect of Phenobarbital is Vitamin K deficiency, so prophylactic Vitamin K must be given to the newborn.

Post-Procedure Testing and the Rosette Test

The Rosette test is a qualitative screening tool used after delivery to detect the presence of Rh positive fetal cells in a maternal blood sample. If the mother has already been treated with Anti-D, the injected antibodies will surround the Rh positive fetal cells, forming a "rosette" appearance under the microscope. A positive Rosette test confirms that fetal-maternal bleeding occurred and helps verify that the dose of Anti-D was sufficient if free antibodies are still detected. If the Rosette test is negative after treatment, it may indicate that the fetal cells have been successfully cleared.