Peripheral villous stromal hyperplasia: a distinctive placental lesion in hemoglobin bart hydrops fetalis.

In hydrops fetalis (HF) the placenta can be markedly enlarged and the villi show stromal edema, increased Hofbauer cells, and reduced numbers of capillaries. Hemoglobin (Hb) Bart is the most severe form of thalassemia, causing HF due to profound anemia. We report a previously undescribed histologic finding based on a review of Hb Bart HF cases, termed "peripheral villous stromal hypercellularity." This change was noted in 15 of 18 (83%) placentas with Hb Bart HF but not in placentas of 21 cases of HF due to other causes, including 11 cases involving anemia. The hyperplastic stromal cells were determined to be myofibroblastic by immunohistochemistry and electron microscopy, associated with a more complex capillary network in villi than is seen with other causes of HF. The authors hypothesize that this angiogenesis in villi is a response to fetal anemia from Hb Bart. In turn, there is increased villous blood flow, resulting in edematous villous stroma, leading to narrowing of the intervillous space in the placenta. Hyperplasia of myofibroblasts might then be a compensatory change, in that contraction by these cells would reduce the vascular lumina and the size of placental villi, thereby widening the intervillous space to improve capacity for maternal blood circulation. Curiously, this histologic change was restricted to cases of HF caused by Hb Bart. We speculate that in Hb Bart disease, the hypoxia and hydrops develop earlier in gestation, compared to other causes of HF, allowing the time for these adaptive changes to occur in the placenta.