Background: Neuroendocrine tumors (NETs) can secrete bioactive amines in the bloodstream, resulting in the carcinoid syndrome characterized by diarrhea and flushing. The frequency of occurrence of primary cardiac neuroendocrine neoplasms is lesser than that of metastases, and hence, metastases must be adequately ruled out before diagnosis. Cardiac tumors, both primary and metastatic, mainly result in heart-related symptoms, such as heart failure and acquired valvular dysfunction. Here, we report a unique case of a primary left ventricular neuroendocrine tumor presenting with diarrhea. Case presentation: A 51-year-old female complaining of intermittent diarrhea for 2 years was admitted to our hospital. Enhancement of total abdominal computed tomography scan, echocardiography, and magnetic resonance imaging indicated a mass in the left ventricle. The indexes of myocardial enzymes were normal. Histologically, round cells with well-differentiated neuroendocrine morphology were arranged in typical pseudo-glandular, trabecular, ribbon-like, and solid nest patterns. Immunohistochemically, the tumor cells were positive for cytokeratin, chromogranin, synaptophysin, and CD56. However, they were negative for caudal type homeobox 2, S100, paired box gene 8, thyroid transcription factor 1, and CD20, which ruled out the origin of gastrointestinal, pancreatic, lung, and Merkel cell carcinomas. The symptoms of diarrhea disappeared after the operation. The patient was asymptomatic at the 9-month follow-up. Conclusion: Cardiac neuroendocrine tumors with diarrhea are considerably rare and related clinical research is limited. We presented a case and reviewed related articles to improve the identification, diagnosis, and management of patients with cardiac neuroendocrine tumors. The site of origin of a neuroendocrine tumor is clinically vital, and identification of an occult primary tumor using imaging modalities is necessary. Immunohistochemistry is well-suited to indicate the origin of the tumor. Regular follow-up is necessary for both poorly differentiated and well-differentiated cardiac neuroendocrine tumors. It is suggested to detect some neuroendocrinal markers for patients with unexplained reasons of diarrhea.
