History and clinical findings: A 61 year old man presented with newly developed tachyarrhythmia. Echocardiographically a tumour of unclear origin in the left atrium was diagnosed.
Investigations: The heart rate in the ECG was arrhythmic at 146/min; intermittently sinus rhythm was established. Transoesophageal echocardiography revealed a 90 x 50 mm tumour with sharp borders displacing both atria. The chest x-ray showed a mediastinal rounded mass in the dorsal part overlapping the right mediastinal border. Computer tomography was unable to discriminate the atrial wall from the tumour masses; hence the difference between an (intra-)cardiac tumour (i.e. myxoma) and a tumour of extracardiac (mediastinal) origin could not be established. Additional magnetic resonance tomography led to the most probable diagnosis of a bronchogenic cyst, which spread ventrally up to the ascending aorta and dorsally to the paravertebral region, thus displacing and shrinking the left pulmonary artery and the left atrium.
Therapy: For certain exclusion of malignancy and because of displacement of other thoracic structures the patient was operated on this mediastinal mass by anterolateral thoracotomy. Histologically a dysontogenetic, benign cyst with an inner layer of mesothel and nerval as well as vascular structures in the capsule was diagnosed. Since the operation the patient has persistent sinus rhythm without symptoms of relevant rhythm disorders. An overview of different kinds of intrathoracical cysts is given.