Chronic, neutrophilic pericarditis with free‐floating intrapericardial mesothelial‐lined fibrous nodules in a cat

A 15‐year‐2‐month‐old, male, neutered domestic shorthair cat was presented for a 1‐week history of increased respiratory effort and declining appetite. Point‐of‐care ultrasound diagnosed tri‐cavitary effusion, and echocardiography revealed the presence of multiple free‐floating hyperechoic nodules within the pericardium. Pericardial fluid analysis indicated a high‐protein transudate. After no definitive cause was identified on computed tomography, surgical pericardiectomy was performed. Histopathology confirmed chronic neutrophilic pericarditis with fibrosis. The nodules consisted of mesothelial‐lined fibrous tissue, supporting a chronic, reactive inflammatory process. Despite surgical and medical management, including antibiotics and anti‐inflammatory therapy, the cat developed recurrent pleural effusion and was euthanased 6 months after pericardiectomy. This case describes an unusual presentation of multiple free‐floating intrapericardial nodules diagnosed as mesothelial‐lined, fibrous nodules caused by a non‐neoplastic chronic inflammatory process. This condition should be considered as a differential diagnosis for intrapericardial masses in cats.