Accessory diaphragm: Uncommon structure in partial anomalous pulmonary venous return (PAPVR) donor

Abstract

This study builds upon an earlier case report, where the authors briefly mentioned the accessory diaphragm. This report provides a more detailed analysis of the embryological origins of the accessory diaphragm malformation, its clinical features, and, importantly, its impact on lung development. Duplication of the diaphragm, a rare congenital anomaly typically found on the right side and often associated with lobar agenesis-aplasia complex, is commonly referred to as an “accessory diaphragm”. The exact cause of diaphragm duplication is not fully understood. However, it is believed that this condition may result from a lack of coordination between the downward migration of the septum transversum and the development of the bronchial system. The accessory diaphragm is characterized as a delicate fibromuscular membrane lined with serosa that attaches to the anterior part of the diaphragm. This membrane extends in a posterosuperior direction to connect with the posterior chest wall, effectively dividing the right hemithorax into two distinct regions. Early formation of the bronchial system may lead to the developing septum transversum being cleaved as it encounters the developing lung. Literature on accessory diaphragms indicates that the associated pulmonary vascular malformations occur during this embryonic period. In all reported cases, aplasia or varying degrees of pulmonary hypoplasia occurred on the affected side. Consequently, the accessory diaphragm divides the right pleural cavity into two sections, trapping part or all of the right middle or lower lobes beneath it.