What causes Shmoo sign in the heart on chest radiograph?
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Answer:
Shmoo sign represents left ventricular enlargement, typically due to chronic pressure or volume overload from conditions such as aortic stenosis, aortic regurgitation, hypertension, or ischemic cardiomyopathy, resulting in a smoothly contoured, enlarged left ventricular silhouette with associated aortic dilatation on anteroposterior chest radiographs.
Why is it called so?
The sign is named after the fictional cartoon creature “Shmoo” from Al Capp’s Li’l Abner comic strip (1948), due to the resemblance of the prominent rounded left heart border (Shmoo body) and dilated ascending aorta (Shmoo head) to the creature’s shape. It is a descriptive, didactic term popularized in teaching radiology texts rather than a term originating in the primary peer-reviewed journal literature.
Pathophysiology
Chronic left ventricular pressure overload (e.g., aortic stenosis, hypertension) or volume overload (e.g., aortic regurgitation) leads to eccentric or concentric hypertrophy and dilatation of the left ventricle, displacing the cardiac apex inferiorly and laterally while smoothing the left heart border; concomitant aortic root dilatation enhances the characteristic contour visible on AP chest radiographs.
Alternative names: None.
Other associated named signs: Boot-shaped heart (tetralogy of Fallot), box-shaped heart (pericardial effusion) — other cardiac silhouette descriptors on chest radiograph.
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