The ‘snowman’ sign: placental changes in thrombotic thrombocytopenic purpura leading to fetal death

Maternal intercurrent illness is an uncommon cause of perinatal death. We present a case of thrombotic thrombocytopenic purpura (TTP), with characteristic placental abnormalities, leading to fetal death, and review the literature concerning the onset of the rare condition during pregnancy. A 25-year-old G1P0 woman presented at 20 weeks gestation with a one day history of headaches, dark urine and decreased fetal movements. Initial assessment showed fetal heart activity present. Concurrent investigation revealed thrombocytopenia, and haemo-lytic anaemia consistent with TTP. A fetal morphology scan was performed several days after admission which revealed fetal death. Placental examination revealed large areas of acute infarction with widespread maternal vascular thrombosis. Some maternal vessels showed the characteristic finding of alternating zones of vascular dilatation and constriction (the ‘snowman sign’) consistent with aneurysmal dilatation of maternal spiral arteries with segmental intramural hyaline deposits. The placental findings directly link the TTP to the fetal death. TTP is characterised by absence or inactivation of the plasma protease (ADAMTS13) which breaks down von Willebrand factor multimers. Deficiency leads to platelet aggregation and thrombus formation. In the placenta these changes are seen in conjunction with the effects of trophoblast on the spiral arterioles producing the characteristic histological appearance. TTP rarely occurs during pregnancy, but is associated with a high rate of perinatal death.