Background/Purpose: Fetal extralobar pulmonary sequestration (EPS) is sometimes complicated by massive pleural effusion (PE) leading to fetal hydrops. The underlying mechanisms as well as the origin of the fluid are not well understood. This study was performed to find a histological hallmark of an EPS with massive PE. 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 EPS complicated by fetal hydrothorax requiring thoracentesis and eventually thoracoamniotic shunt placement. Total protein content and cell count were measured in the aspirates, which were compared with chylothorax cases (n = 5) requiring fetal shunt placement. After birth, all 3 infants required mass resection for the control of PE. The venous wall thickness was measured on pathology slides stained with Elastica van Gieson staining. Thickness of the media and adventitia was measured in approximately 40 veins per case. They were corrected by external diameter and expressed as percentage of medial thickness and percentage of adventitial thickness. An EPS not associated with PE but with congenital diaphragmatic hernia served as a control. Results: Total protein and the cell count of the EPS related 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%, which were significantly increased compared with the control of 3.1% ± 1.3% (P < .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 control of 2.3% ± 0.7% (P < .01). Conclusions: We conclude that PE associated with EPS is the transudate rather than the lymph. The thickened media and the adventitia found in EPS with PE support the hypothesis that partial obstruction of the venous system leads to an increased transudate production, which ultimately leads to fetal hydrops.
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