Homozygous antithrombin deficiency type II causing neonatal thrombosis.

We report four children from different families with homozygous antithrombin (AT) deficiency type II affecting the heparin binding site (p.Leu131Phe mutation). All children had severe spontaneous venous and/or arterial thromboembolic events shortly after birth. This report intends to raise awareness among clinicians about this rare but severe condition. When thrombosis occurs in an otherwise healthy newborn, a severe congenital thrombophilic disorder should be considered. In homozygous AT deficiency type II, AT activity is typically reduced but may also be in the normal range, posing a diagnostic challenge. Rapid diagnosis is important to initiate appropriate therapy. Standard anticoagulation with heparin may prove ineffective in severe AT deficiency, requiring substitution of AT concentrate and early switch to alternative anticoagulants such as vitamin K antagonists.