As a result of the remarkable success achieved in prevention of Rh hemolysis, fetomaternal ABO incompatibility currently is the most common cause of immune hemolytic disease of the newborn. Although ABO incompatibility occurs in approximately 20% to 25% of pregnancies, hemolysis develops in only a small fraction of infants born subsequently, and in general the disease is much milder than Rh incompatibility. In part the less severe course of disease can be attributed to the expression of A and B antigens on many cells other than red cells which act like sponge for the transferred antibody. ABO hemolytic disease occurs almost exclusively in infants of blood group A or B who are born to mothers of blood group O. The normal anti-A and anti-B isohemagglutinins in group O mothers usually are of the IgM type and therefore do not cross the placenta. However, for reasons not well understood, certain group O women possess IgG antibodies directed against group A or B antigens (or both) even without previous sensitization. Therefore, the firstborn may be affected. There is no effective method of preventing hemolytic disease resulting from ABO incompatibility.
The major causes of nonimmune hydrops include those disorders associated with cardiovascular defects, chromosomal anomalies, and fetal anemia.
Structural cardiovascular defects and functional abnormalities (i.e., arrhythmias) may result in intrauterine cardiac failure and hydrops. Among the chromosomal anomalies, 45,X karyotype (Turner syndrome) and trisomies 21 and 18 are associated with fetal hydrops; the basis for this disorder usually is the presence of underlying structural cardiac anomalies, although in Turner syndrome there may be an abnormality of lymphatic drainage from the neck leading to postnuchal fluid accumulation (resulting in cystic hygromas).
Fetal anemias resulting from causes other than Rh or ABO incompatibility also may result in hydrops. In fact, in some parts of the world (e.g., Southeast Asia), severe fetal anemia caused by homozygous α-thalassemia probably is the most common cause of fetal hydrops.
Transplacental infection by parvovirus B19 is increasingly recognized as an important cause of fetal hydrops. The virus gains entry into erythroid precursors (normoblasts), where it replicates. The ensuing cellular injury leads to the death of the normoblasts and aplastic anemia. Parvoviral intranuclear inclusions can be seen within circulating and marrow erythroid precursors (
Fig. 7.28
Fig. 7.28 BonemarrowfromaninfantinfectedwithparvovirusB19.Thearrows pointtotwoerythroidprecursorswithlargehomogeneousintra-nuclearinclusionsandasurroundingperipheralrimofresidualchromatin.
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The basis for hydrops in fetal anemia of immune and nonimmune causes is tissue ischemia with secondary myocardial dysfunction and circulatory failure. Additionally, secondary liver failure may occur, with loss of synthetic function contributing to hypoalbuminemia, reduced plasma osmotic pressure, and edema.
MORPHOLOGYTheanatomicfindingsinfetuseswithintrauterinefluidaccumulationvarywithboththeseverityofthediseaseandtheunderlyingetiology.Aspreviouslynoted,hydrops fetalis representsthemostsevereandgeneralizedmanifestation(