Pathology of thrombotic thrombocytopenic purpura in the placenta, with emphasis on the snowman sign.

Thrombotic thrombocytopenic purpura (TTP) commonly affects women of childbearing age. Although cases of TTP in pregnancy have been reported, the pathology of TTP in the placenta has not been described. In other organs, TTP has a distinctive pathology of arteriolar aneurysms with segmental intramural hyaline deposits. Published many decades ago, these classic papers antedate modern computerized literature searches, so this information has not been included in many modern reviews on TTP. We report 2 cases of TTP in pregnancy, both leading to fetal loss in the 2nd trimester. We noticed a distinctive pathology of TTP in the spiral arteries on the maternal surface of the placenta, particularly the "snowman sign," which differs somewhat from TTP in other organs due to the fact that TTP is superimposed on trophoblast-induced changes in these spiral arteries. In one of our cases, where the clinical diagnosis of TTP was difficult, the distinctive placental findings helped reinforce the clinical decision to perform potentially life-saving plasmapheresis.