DESCRIPTION

Neonatal alloimmune thrombocytopenia (NAIT) is one of the major causes of severe thrombocytopenia in the newborn.

• Analogous to ABO/Rh incompatibility but involves platelets instead of RBCs
• Presents with bleeding complications including petechiae, bruising, mucosal bleeding, and/or intracranial hemorrhage (ICH) that can occur in utero

EPIDEMIOLOGY

1:1,000 live births

GENERAL-PREVENTION

In pregnancies known to be at risk of NAIT, maternal treatment with IVIG in the antenatal period can be considered.

PATHOPHYSIOLOGY

Antibody-mediated platelet destruction

ETIOLOGY

Maternal IgG antibodies, directed against paternally inherited platelet-specific antigens in the fetus, cross the placenta, enter the fetal circulation, and attack fetal platelets.

• HPA-1a (formerly PLA-1) incompatibility is by far the most common cause of NAIT in those of Caucasian ancestry, accounting for ~75% of cases. The disease happens when the mother is HPA-1a negative (HPA 1b/1b) and father is HPA-1a positive (HPA 1a/1a or 1a/1b). If the fetus inherits HPA-1a from father, maternal exposure to HPA-1a–positive fetal platelets during pregnancy causes mother to generate anti–HPA-1a IgG antibodies. Anti–HPA-1a antibodies crosses the placenta and causes platelet destruction.
• Other common antigens implicated in NAIT include HPA-2, HPA-3, HPA-5, HPA-9, and HPA-15.
• HPA-4 incompatibility accounts for the majority of cases in Asian populations.
• At least 23 other low-frequency antigens have been reported in a small fraction of cases.
• HPA-1a–negative mothers who are HLA-DRB3*0101 positive are far more likely to develop antibodies than those who are DRB3 negative.
• The role of HLA platelet antigens in NAIT is controversial.