Case report of parapneumonic effusion and mesothelial subdiaphragmatic cyst: relationship or coincidence?

Parapneumonic pleural effusions are common in patients with pneumonia. When colonized by pathogenic bacteria or other microorganisms, these effusions can progress to empyema. Additionally, empyema formation may result in extension of the infection into the infradiaphragmatic region, further complicating the clinical scenario. Many subphrenic collections are found to be mesothelial cysts, which are congenital in origin. However, data regarding the potential association between mesothelial diaphragmatic cysts and parapneumonic effusions are limited. We herein describe a toddler with pneumonia complicated by parapneumonic effusion and a lung abscess with a subphrenic collection. After abscess drainage and a full course of antibiotics, imaging revealed clear lung parenchyma with an interval resolution of the effusion and a persistent unchanged subphrenic collection that was confirmed to be mesothelial diaphragmatic cyst. This case highlights the fact that not every subphrenic collection associated with parapneumonic effusion is a communicated collection formed by seeding. Such a collection can instead be an incidental cyst, which is congenital in origin and known as a mesothelial diaphragmatic cyst. A diaphragmatic mesothelial cyst is an uncommon benign congenital cyst that is unrelated to an adjacent parapneumonic effusion. It is usually incidental and can be monitored without invasive intervention.