[A case of an idiopathic enlargement of the right atrium, lacking in prominent right atrium contour and mimicking left pericardial defect on the chest X ray].

A 47-year-old man was admitted for evaluation of heart murmur in 1982. On admission, two-dimensional echocardiogram showed a giant right atrium with mild tricuspid regurgitation, but a chest X ray showed no prominent right atrium contour. Echocardiographic finding at various postures and chest X ray with artificial pneumothorax indicated little likelihood of left pericardial defect. An angiocardiogram and computed tomography showed the dilated right atrium with clockwise rotation. There was no indication of right atrial overload. Therefore, we made the diagnosis of an idiopathic enlargement of the right atrium. For about 10 years after discharge, we have followed up this patient. He has received no treatment and has been asymptomatic. On the other hand, echocardiography shows that the right atrium has gradually enlarged. Electrocardiogram has clearly revealed only a tall and slender peaked P wave in chest leads for the last 5 years. This may indicate not only an increasing load on the right atrium but also that the enlarged atrium is getting closer to the walls of the chest. We report this case of an idiopathic enlargement of the right atrium lacking in prominent right atrium contour, and mimicking left pericardial defect on chest X ray.