Background/Purpose:Fetal extralobar pulmonary sequestration(EPS)is sometime s complicated by massive pleural effusion(PE)leading to fetal hydrops.The und erlying mechanisms as well as the origin of the fluid are not well understood.T his study was performed to find a histological hallmark of an EPS with massive P E.We hypothesized that venous obstruction has a role to play in the mechanisms of fluid production by EPS.Methods:We recently experienced 3 cases of fetal EP S complicated by fetal hydrothoraxrequiring thoracentesis and eventually thoraco amniotic shunt placement.Total protein content and cell count were measured in the aspirates,which were compared with chylothorax cases(n = 5)requiring feta l shunt placement.After birth,all 3 infants required mass resection for the co ntrol of PE.The venous wall thickness was measured on pathology slides stained with Elastica van Gieson staining.Thickness of the media and adventitia was mea sured in approximately 40 veins per case.They were corrected by external diamet er and expressed as percentage of medial thickness and percentage of adventitial thickness.An EPS not associated with PE but with congenital diaphragmatic hern ia served as a control.Results:Total protein and the cell count of the EPS rel ated PE were 0.6 ±0.3 mg/dL and 28 ±14/μL(mean ±SD),which were significantly lower than those of PE i n chylothorax(2.2 ±0.2 mg/dL and 1900 ±1100/μL).Percentages of adventitial thickness of EPS with PE were 9.8%±3.8%,10.4%±3.6%,and 8.3%±3.7%,whi ch were significantly increased compared with the control of 3.1%±1.3%(P < 0.01).Percentages of medial thickness of EPS with PE were 7.0%±1.9%,7.3%±1.4%,and 6.6%±2.3%,which were significantly increased compared with the cont rol of 2.3%±0.7%(P< 0.01).Conclusions:We conclude that PE associated with EPS is the transudate rather than the lymph.The thickened media and the adventi tia found in EPS with PE support the hypothesis that partial obstruction of the venous system leads to an increased transudate production,which ultimately lead s to fetal hydrops. Background / Purpose: Fetal extralobar pulmonary sequestration (EPS) is sometime s complicated by massive pleural effusion (PE) leading to fetal hydrops. The und erlying mechanisms as well as the origin of the fluid are not well understood. His study was performed to find a histological hallmark of an EPS with massive P E.We hypothesized that venous obstruction has a role to play in the mechanisms of fluid production by EPS. Methods: We recently experienced 3 cases of fetal EP S complicated by fetal hydrothorax requiring thoracentesis and eventually thoraco amniotic shunt placement. Total protein content and cell count were measured in the aspirates, which were compared with chylothorax cases (n = 5) requiring feta l shunt placement. After birth, all 3 infants required mass resection for the co ntrol of PE. venous wall thickness was measured on pathology slides stained with Elastica van Gieson staining. hickness of the media and adventitia was mea sured in approximately 40 veins per case. they were corrected by external diamet er and expressed as percentage of medial thickness and percentage of adventitial thickness. Ann EPS not associated with PE but with congenital diaphragmatic hern ia served as a control. Results: Total protein and the cell count of the EPS rel ated PE were 0.6 ± 0.3 mg / dL and 28 ± 14 / μL mean ± SD, which were significantly lower than those of PE in chylothorax (2.2 ± 0.2 mg / dL and 1900 ± 1100 / μL). Percentages of adventitial thickness of EPS with PE were 9.8% ± 3.8%, 10.4% ± 3.6%, and 8.3% ± 3.7%, whi ch were significantly increased compared with the control of 3.1% ± 1.3% (P <0.01) .Percentages of medial thickness of EPS with PE were significantly increased compared to the cont rol of 2.3% ± 0.7% (P <0.01) .Conclusions: We conclude that PE associated with EPS is the transudate rather than the lymph. The thickened media and the adventi tia found in EPS with PE support the hypothesis that partial obstruction of the venous syste m leads to an iincreased transudate production, which ultimately leads to fetal hydrops.