Mesothelial/Monocytic Incidental Cardiac Excrescences (MICE): An Enigmatic Cardiac Pathology.

Mesothelial/monocytic incidental cardiac excrescences (MICE) are rare, reactive tumor-like lesions of the heart. They are typically incidental findings during cardiac surgery. They can mimic thrombi or neoplasms such as myxoma, papillary fibroelastoma, and metastatic carcinoma histopathologically, leading to diagnostic confusion. We report a case of atrial MICE in a 40-year-old woman with rheumatic mitral stenosis and long-standing atrial fibrillation, who underwent mitral valve replacement following a failed percutaneous transvenous mitral commissurotomy. Histopathological examination revealed characteristic features of MICE in the left atrial appendage, with clusters of mesothelial cells and histiocytes embedded in fibrin, confirmed by immunohistochemistry. This case underscores the need for heightened awareness among pathologists to avoid misdiagnosis.