Snowman sign refers to the configuration of the heart and the superior mediastinal borders resembling a snowman. This is seen in total anomalous pulmonary venous return (TAPVR) type I (supracardiac type).
It is an abnormality of the fetal circulation wherein the entire pulmonary venous flow is directed to the right atrium via the superior vena cava (SVC). All pulmonary veins join to form a common pulmonary vein which drains into a vertical vein. The vertical vein drains into the SVC via the innominate (brachiocephalic) vein.
The paratracheal shadow on the right is the prominent SVC and on the left is the vertical vein. The innominate vessel lies in the midline above base of heart. These three prominent vessels together form the head of the 'snowman'. The body is formed by the rest of the heart.
It is also referred to as the figure of 8 sign.