Fetomaternal or Neonatal Alloimmune Thrombocytopenia

A rare but potentially serious condition that causes bleeding in the newborn is neonatal alloimmune thrombocytopenia (NAIT).

NAIT is caused by the mother's IgG alloantibodies against the baby's platelet specific alloantigen. The fetus and newborn are at risk of thrombocytopenia and intracranial haemorrhage.(1)

Feto-maternal or neonatal alloimmune thrombocytopenia (FMAIT or NAIT) is the platelet equivalent of haemolytic disease of the newborn.

This is a rare, potentially serious condition and specialist advice is required. It occurs in 1:2000 to 1:3000 live births among Caucasians.

NAIT is caused by maternal IgG alloantibodies against a fetal platelet specific alloantigen in the Human Platelet Antigen (HPA) system.

Approximately 75% of cases in a Caucasian population are caused by anti-HPA-1a and 20% by anti-HPA-5b. In oriental populations, anti-HPA-4b is more common than anti-HPA-1a.

There may or may not be a history of thrombocytopenia in a previous infant. NAIT often (>60%) occurs during the first pregnancy.

Affected infants may be severely thrombocytopenic and at high risk, especially for intracranial bleeding.
 

Management of the neonate

The condition is self-limiting, usually resolving within 2 weeks. It occasionally persists for up to 6 weeks.

Rapid treatment is required when there is bleeding or a platelet count is <30 x 109/L. Several transfusions of compatible platelets may be needed.

The treatment of choice is to give platelets lacking the relevant HPA antigen. In the absence of suitable donor platelets, the mother's platelets may be used. They must be plasma-reduced (to remove maternal antibody) and irradiated.

Surprisingly, however, provision of untyped (presumably HPA-1a-positive) platelets may be clinically effective in many cases of haemorrhage due to NAIT associated with anti-HPA-1a alloantibodies, and are preferable to delaying any transfusion during attempts to identify HPA-1a-negative donors.(2)

High dose intravenous immunoglobulin (IVIg) at a dose of 1–2 g/kg body weight) given to the neonate is effective in about 75% of cases.

Additional doses may be required 2–4 weeks after the initial response due to recurrence of thrombocytopenia.
 

Management of subsequent pregnancies

Subsequent pregnancies should be carefully monitored. Intracranial bleeding in utero can occur, and most obstetricians will monitor the fetal platelet count and administer intravenous immunoglobulin IVIG to the mother, and intrauterine platelet transfusions should the platelet count be very low.

When possible, compatible platelets should be maintained on hand at the expected delivery time.

Maternal serum is the test sample usually required for the reference laboratory investigation of possible NAIT. Usually the mother’s serum is tested against both panel cells from known HPA-typed donors and against platelets from the father.

Reactivity with only the father’s platelets may indicate the presence of antibody directed to a low-incidence antigen.

Antibodies to HLA antigens have also been reported to cause NAIT although the extent of their involvement is controversial.

 

Reference
  1. JA Peterson, JG McFarland, BR Curtis, RH Aster. Neonatal alloimmune thrombocytopenia: pathogenesis, diagnosis and management. British Journal of Haematology. 2013,6:3-14.
  2. Kiefel V, Bassler D, Kroll H, Paes B, Giers G, Ditomasso J, et al. Antigen-positive platelet transfusion in neonatal alloimmune thrombocytopenia (NAIT). Blood 2006;107(9):3761–3763.