HDN

Question 1

What is HDN and how does it arise?

Answer 1

o Foetal RBCs destroyed by the IgG antibodies produced by the mother. Typically, in ABO incompatibility (mild disease), and Rh+ foetus in Rh- mother – can be Rh-C, c, D, E, e (more severe).
o Occurs via either antibody adsorption or foetal-maternal haemorrhage (FMH).
o Antibody adsorption – maternal antibodies enter through the placenta – occurs mainly in Rh+ as these are IgG antibodies. ABO antibodies are IgM and these do not cross the placenta easily.
o FMH – when maternal and foetal blood mixes during labour, ectopic pregnancy, abortion, STI, placental abruption or 3rd trimester. This causes maternal immune system to produce Rh antibodies against the Rh+ antigens from foetal blood. In subsequent pregnancies, these cross the placenta and attack the foetal RBCs.

Question 2

What are the symptoms and their explanations?

Answer 2

o Anaemia – lysis of RBCs
o Jaundice, pale or yellow skin – increase of bilirubin due to breakdown of RBCs
o Urine discolouration – impaired liver function due to increased RBC breakdown, causing decreased bilirubin to be excreted, instead it accumulates in the blood
o Splenomegaly or hepatosplenomegaly – increased destruction of RBCs via macrophages in the spleen and liver (reticuloendothelial system) mean there are many macrophages and lysed RBCs and waste products in these organs causing them to be enlarged
o Hydrops fetalis – oedema and severe anoxia – due to increased breakdown of RBCs via macrophages affecting organs and causing swelling
o Kernicterus – neural damage due to bilirubin – due to increased unconjugated bilirubin accumulating in the bloodstream which then crosses the BBB and accumulates in the brain and glial membranes

Question 3

What are the laboratory findings?

Answer 3

o Anaemia – low RBC and low Hb
o Thrombocytopenia – low platelets
o Hyperbilirubinemia – increased bilirubin
o Increased reticulocyte count
o Prolonged PT and APTT time

Question 4

What further tests can be done?

Answer 4

o Blood film – numerous microcytic and hypochromic nucleated RBCs
o DAT – positive for the antibody of interest
o Kleihauer test and flow cytometry – quantifies FMH
o Biochemistry tests: LDH (increased), haptoglobin (decreased) and albumin (increased)

Question 5

How is HDN treated?

Answer 5

o Treat the anaemia using exchange transfusion: transfuse with fresh CMV negative irradiated Rh- blood that is ABO compatible with both mother and neonate. This is done for 6 weeks to prevent kernicterus.
o Treat the jaundice with phototherapy with blue-green light to convert the unconjugated bilirubin into a water-soluble isomer that can be excreted.
o Treat low BP with IV fluids.
o Treat difficulty breathing with ventilatory support.
o Hydroxycarbamide to increase HbF levels.