exam 2 - alloimmunization Flashcards
A 39 yo G3 P2002 at 10 weeks EGA presents for prenatal care, and you send off routine labs. Two days later, you note the patient has Rh anti-D antibodies
You contact your perinatologist after arranging for amniocentesis for fetal bilirubin. Over the course of the second trimester, you note that the bilirubin is rising. The perinatologist performs periumbilical blood sampling. The patient is induced at 34 weeks and delivers a liveborn baby boy.
Rh negativity
-in about 10% of the general population
most common antibodies
-A, B, O blood types are the primary blood group but there are others
-Of these, Rh (CDE) is MC
-Consists of 5 antigens: C, c, D, E, and e
-There are also over 30 antigenic variants
-D is either present or absent; those who have the antigen are Rh D-positive and those who do not are Rh D-negative
alloimmunization
-The problem exists when a fetus inherits the Rh antigen (Rh+) from its father and when the parturient lacks the Rh antigen (Rh-)
-There is always some number of fetal RBCs that enter maternal circulation
-If an Rh+ fetus’s RBCs enter into maternal circulation (known as fetomaternal hemorrhage), maternal antibodies may be formed in response to the fetal Rh+ antigen
-However, there must be enough fetal RBCs in maternal circulation to cause alloimmunization, usually <0.1 cc
-These maternal antibodies then enter fetal circulation
-May result in destruction of fetal RBCs by hemolysis as well as other consequences
-Fetomaternal hemorrhage most commonly occurs at delivery
-However, it may occur in first trimester
alloimmunizatoin: events that may cause maternal immune response
-childbirth MC
-threatened abortion
-incomplete abortion
-spontaneous abortion
-voluntary termination of pregnancy
-ectopic gestation
-amniocentesis
-external cephalic version
-direct abdominal trauma
-placental delivery
Pathophysiology and sequelae of alloimmunization
-As noted previously, fetal RBCs are hemolyzed:
-Bilirubin is released due to hemolysis, but cleared by the placenta during pregnancy
-If there is sufficient bilirubin, the neonate may need treatment at birth:
-Ultraviolet light
-Exchange transfusion (much less common today)
-Decreased oncotic pressure due to decreased albumin due to hepatic damage:
-Ascites
-Pleural effusion
-Pericardial effusion
-Kernicterus (bilirubin deposition in basal ganglia of brain)
-Fetal anemia (from RBC destruction)
-Hepatosplenomegaly is caused by hematopoesis
-Sufficiently severe anemia may result in high-output cardiac failure
-When combined with decreased oncotic pressure, resulting in ascites, pleural effusions, etc., may result in worsened CHF, pericardial effusion, or in pulmonary edema
-This is known as hydrops fetalis
-FETAL DEATH MAY ENSUE
Dx of alloimmunization
-Screen for ABO, Rh blood typing at initiation of prenatal care
-May also test for fetal Rh status via NIPT
-If the patient is Rh negative, repeat screen at 30-32 weeks
-If the patient has no antibodies documented at this time, provide passive immunization with Rho(D) immune globulin as soon as possible after obtaining results at 30-32 weeks
-At delivery, if neonate is Rh D positive, and if the patient has threatened, incomplete, spontaneous, or elective abortion, ectopic pregnancy, amniocentesis, etc.
management of maternal antibodies
-obtain antibody titers
-obtain detailed OB hx re possible alloimmunization in past
-obtain paternal ABO/Rh -> may obtain fetal DNA if needed via amniocentesis
-repeat titers Q 4 weeks
-Usually in mid-trimester, begin evaluation of fetal bilirubin via amniocentesis (traditional method of surveillance)
-Consider ultrasound for peak velocity through middle cerebral artery of fetus:
-Velocity is based on blood viscosity
-Increased velocity seen in anemic fetuses
-May also evaluate fetus for hydropic changes via ultrasound
-direct measurement of fetal hematocrit is achieved by percutaneous umbilical blood sampling (PUBS)
-technically difficult study that should be limited to experienced centers and practitioners
-normal fetal hematocrit: 36-44%
-severe fetal anemia: <30%
-may transfuse via PUBS if indicated
-for fetuses with mild hemolysis -> consider induction of labor at 37-38 wks
-in transfusion have been administered -> consider delivery at 32-34wks
-thick scalp
-US evidence of hydrops fetalis
which of the following is the MCC of alloimmunication
-failure to receive Rho(D) immune globulin in pregnancy!!!!
-ectopic
-prior delivery!!!!!
-prior transfusion
