Background: Small congenital diaphragmatic defects are usually occult, but can present as a problem in patients with increased abdominal fluid as unilateral massive hydrothorax. We present a unique case presentation of a small congenital diaphragmatic defect.
Case report: A 79 female immediately postoperatively developed a hydrothorax following low anterior resection for rectal cancer. The hydrothorax turned out to be due to an intrabdominal bleed in a patient with a previously undiagnosed congenital diaphragmatic hernia. The respiratory ball value mechanism worked as a pump to move blood from the abdomen to the chest and caused a tension hemothorax.
Results: Surgeons should be aware that a post operative hydrothorax could be associated with an intrabdominal complication of recent surgery.
Conclusion: A small occult congenital diaphragmatic defect and associated ball value mechanism may provide an unexpected but clinically significant communication between the abdominal and thoracic cavities.

Keywords: congenital diaphragmatic defect, surgical complication, rectal cancer