What causes Snowman sign in mediastinum on chest X-ray?
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Answer:
Supracardiac total anomalous pulmonary venous return (TAPVR), a congenital heart defect where all pulmonary veins drain anomalously into the right atrium via a vertical vein, superior vena cava (SVC), and innominate vein, leading to a characteristic mediastinal configuration. It presents in infancy with cyanosis, heart failure, and failure to thrive.
Why is it called so?:
The sign resembles a Snowman sign, with the “head” formed by the dilated SVC on the right, vertical vein on the left, and innominate vein in the midline above the heart base, and the “body” formed by the enlarged heart.
Pathophysiology:
In supracardiac total anomalous pulmonary venous return (TAPVR), pulmonary veins fail to connect to the left atrium and instead converge into a common pulmonary channel that drains cephalad into a left vertical vein, which joins the innominate vein and then the SVC, dilating these vessels. This creates prominent superior mediastinal shadows overlying the heart on chest X-ray, producing the Snowman sign, with an associated interatrial right-to-left shunt via patent foramen ovale.
Alternative names: Figure-of-8 sign, Figure of 8 sign, Snowman appearance
Other associated named signs: None
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