Cardiac embryonal rhabdomyosarcoma in a domestic shorthair cat

•Primary cardiac neoplasms are uncommon, and cardiac rhabdomyosarcoma (RBM) is extremely rare in domestic cats.•This is the first documented case of a cardiac embryonal rhabdomyosarcoma (ECRBM) in a domestic shorthair cat.•The cat exhibited signs of cardiac dysfunction, ultimately succumbing to ECRBM.•Comprehensive pathological and immunohistochemical analyses revealed the distinctive features of this exceptionally rare cardiac tumor in the cat.•ECRBM, even rare, should be included in the differential diagnosis of thoracic neoplasms. Rhabdomyosarcomas (RBMs) are rare neoplasms arising from striated muscle tissues in domestic animals and are exceptionally uncommon, affecting the heart of domestic cats. A case of cardiac embryonal RBM was detected in a two-year-old female domestic shorthair cat. The cat exhibited dyspnea, anorexia, pericardial and pleural effusions, ultimately succumbing to severe respiratory arrest. Gross post-mortem examination revealed a firm yellow-tan mass measuring 2.5 × 1.5 cm protruding from the right atrium, along with metastatic nodules of various sizes dispersed throughout the liver, spleen, and lungs. On histology, the right atrium exhibited a highly cellular infiltrative and vascularized mass predominantly composed of spindle and small round cells arranged in dense cellular bundles supported by a scant fibrous stroma interspersed with scarce regions of loosely myxoid stroma. Comparable histological features were noted in the metastatic lesions. Immunohistochemical analysis revealed positive immunolabeling for vimentin, desmin, MyoD1, and fast myosin, while no immunostaining was observed for smooth muscle actin, chromogranin, and S100. Histological and immunohistochemical features supported the diagnosis of metastatic cardiac embryonal RBM spindle cell variant in the cat. Cardiac embryonal RBMs should be included in the differential diagnosis of cardiac and thoracic neoplasms in domestic cats presenting with clinical signs indicative of cardiac arrest or sudden death.