Third mogul sign is the prominent left atrial appendage visible on the left cardiac border below the pulmonary artery shadow on chest X-ray  (or a similar shadow due to other causes). It can be seen in cases of severe mitral stenosis. Click here to see an X-ray with third mogul sign.
Similar bulges due to post mitral valve replacement left ventricular pseudoaneurysms have also been described . Detection of third mogul sign in such cases is of great significance as left ventricular pseudoaneurysms are more prone for rupture than true aneurysms, with potential catastrophic results.
Moguls are projections of snow seen on a ski slope. First mogul on chest X-ray is the aortic knuckle, second mogul the main pulmonary artery, third mogul the left atrial appendage and fourth mogul the left ventricular apex . Masses in the same location can also cause a third mogul .
Comments by Dr. Jyothish Vijay (edited): Causes of “third Mogul sign” include enlarged left atrial appendage, cardiac aneurysm, pericardial cyst, coronary artery aneurysm, corrected transposition of great arteries – levo posed aorta, Ebstein’s anomaly of tricuspid valve – dilated right ventricular outflow tract,”wooden shoe – Cor en Sabot” of Fallot’s Tetralogy – lifted up and right ventricular type of apex, mediastinal tumor, severe myocardial hypertrophy, pericardial defect (partial absence of left pericardium, through which left atrial appendage bulges out), sinus of Valsalva aneurysm and rarely normal thymus.
Ref: Felson Chest Roentgenology
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