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Last Update: 03/27/2025 10:24 PM

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Commit #309791

SS_OB 1.35 Discuss the pathophysiology and anaesthetic management of the following medical conditions particular to pregnancy:
- ∙ Rhesus iso immunisation

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Commit #309791

Definition:

Development of maternal antibody to fetal Rhesus antigen.

Occurs when the mother is Rhesus (D) negative and the fetus is Rhesus (D) positive.

Pathophysiology:

Isoimmunization does not affect a first pregnancy.
- The mother must have been exposed to Rhesus (D) positive cells in a previous pregnancy and then developed an immune response that remains dormant until the following pregnancy with a Rhesus (D) positive baby.
- In subsequent pregnancies, IgG antibodies cross from maternal to fetal circulation.

Sensitization occurs in:
- Delivery (60%), spontaneous or threatened miscarriage, trauma, amniocentesis, CVS, ECV, or placental abruption.
- During pregnancy, fetal cells may cross the placenta into maternal circulation (fetomaternal hemorrhage, FMH).
  - First 12 weeks: small amount transferred.
  - Weeks 12-24: more is transferred.
  - After 24 weeks: exponential transfer.
- This exposure leads to the development of maternal antibody (isoimmunization).
- IgG antibodies are actively transferred from the mother to the fetus.
- Hemolytic Disease of the Newborn (HDN) occurs when fetal RBC lifespan is reduced by maternal antibody action via placental transfer.
- Affected RBCs are removed by the fetal liver and spleen, resulting in anemia and increased pigment in amniotic fluid (from hemoglobin breakdown).

Severity:

Mild to severe disease:
- Normal delivery at term with mild jaundice requiring phototherapy.
- Preterm delivery of an anemic baby requiring exchange transfusion.
- Delivery at 34 weeks following fortnightly blood transfusions from 26 weeks’ gestation.
- Stillbirth or neonatal death.

Effects on fetus:
- Fetal anemia in utero.
- Extramedullary erythropoiesis (liver, spleen), cardiac decompensation, hydrops fetalis, ascites, pleural and pericardial effusions, polyhydramnios.
- Fetal death if untreated.

Effect of isoimmunization on the fetus worsens with subsequent pregnancies; sensitization cannot be lost, and response will magnify with successive pregnancies.

Management

First Pregnancy in Rhesus Negative Mother:
- Monitor antibody levels at booking, 24 weeks, and 36 weeks.
- 10 IU/ml requires review in fetal medicine center in case invasive assessment and management are needed.
- Delivery is planned if the fetus is at higher gestational age.
- Intrauterine transfusion (IUT) is performed if cordocentesis confirms anemia (intravascularly into the umbilical vein or heart, or intraperitoneally).
- Transfusion is usually stopped by 34 weeks’ gestation, and the fetus is delivered at 36-37 weeks.
- In less severe disease, delivery is planned for 37 weeks.

Previously Sensitized Woman:
- Anti-D antibodies are no longer effective.
- Close surveillance is necessary—monitor antibody levels every 2-4 weeks from booking.
- If antibodies are <10-15 IU/mL, the baby is unlikely to be affected.
- If antibodies are >15 IU/mL and/or features of fetal anemia, fetal medicine opinion must be sought.
- All babies born to Rhesus (D) negative women should have cord blood taken at delivery for FBC, blood group, and indirect Coomb’s test.

Prevention:

Reduces the development of sensitization to <0.2%.
- Anti-D is given only as prophylaxis and is useless once sensitization has occurred.
- Anti-D IG should be given as close to a sensitizing event as possible and within 72 hours.
  - This antibody works by passive immunization and binds to any Rhesus (D) positive fetal cells present in the maternal circulation, so that they can be rapidly cleared by the maternal liver and spleen before an immune response can take place.
- Rhesus (D) negative women (with no preformed antibody) are given an intramuscular injection of anti-D antibody:
  - Routinely at 28 and 34 weeks’ gestation.
  - Routinely at delivery if the baby is Rhesus (D) positive.
  - For post-sensitizing events: Spontaneous or threatened miscarriage, invasive procedures, trauma, placental abruption, termination of pregnancy (TOP), chorionic villus sampling (CVS), amniocentesis, cordocentesis, external cephalic version (ECV), abdominal trauma.
- The Kleihauer-Betke test is used to ensure that the dose is sufficient to remove all fetal cells from maternal circulation.