Box-Shaped Heart in Ebstein Anomaly
The box-shaped (or "globular") cardiac contour seen on chest X-ray in Ebstein anomaly results from massive right atrial enlargement combined with a small or inconspicuous aortic root, creating a characteristic squared-off cardiac silhouette. 1
Anatomical Basis for the Box-Shaped Appearance
The distinctive radiographic appearance stems from several key anatomical features:
Right Atrial Enlargement
- Massive right atrial dilation is the primary contributor to the box-shaped contour, occurring because the large and compliant right atrium accepts all regurgitant flow from severe tricuspid regurgitation with minimal pressure rise. 1
- The right atrium enlarges progressively due to chronic volume overload from tricuspid regurgitation and atrialization of the right ventricular inflow portion. 1, 2
Small Great Arteries
- The great arteries are typically small, and the aortic root appears inconspicuous or even absent on chest X-ray, contributing to the squared-off superior cardiac border rather than the normal rounded contour. 1
- This creates a flattened or straight upper cardiac border that contrasts with the prominent lateral bulging from right atrial enlargement. 1
Clear Lung Fields
- Despite severe cardiomegaly, the lung fields remain clear because the pathology is primarily right-sided with volume overload rather than pulmonary congestion. 1
- This combination of massive cardiac enlargement with clear lungs is highly characteristic of Ebstein anomaly. 1
Severity Correlation
The degree of cardiac enlargement on chest X-ray correlates with disease severity:
- Mild cases may show a nearly normal chest X-ray without the box-shaped appearance. 1
- Severe cases demonstrate marked cardiomegaly with the prominent globular/box-shaped cardiac contour. 1
- A cardiothoracic ratio greater than 65% is a poor prognostic factor in Ebstein anomaly. 3
Clinical Pitfall
The jugular venous pressure may appear normal on physical examination despite severe tricuspid regurgitation, precisely because the massively enlarged and compliant right atrium accommodates the regurgitant volume without significant pressure elevation. 1 This can lead to underestimation of disease severity if relying solely on clinical examination rather than imaging.