What causes the egg-on-a-string sign on the chest radiograph?
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Answer:
The egg-on-a-string sign, also called the egg-on-its-side appearance, is the classic radiographic appearance of d-transposition of the great arteries. The cardiac silhouette is oval or globular, lying on its side like an egg, because of biatrial and right ventricular enlargement together with increased pulmonary blood flow. The superior mediastinum is abnormally narrow, forming the string, for three reasons: the transposed great arteries lie in an anteroposterior rather than side-by-side relationship so they overlap on the frontal projection, the lungs are hyperinflated, and stress-induced thymic involution removes the normal thymic shadow. The result is a large egg-shaped heart suspended from a narrow vascular pedicle. The sign is usually seen only after the first few days of life, once pulmonary vascularity increases and the thymus involutes, and its absence does not exclude the diagnosis; echocardiography remains the diagnostic study.
Why is it called so?
The oval cardiac silhouette resembles an egg lying on its side, and the narrow vascular pedicle above it looks like the string from which the egg hangs.
Pathophysiology
In d-transposition the aorta arises from the right ventricle and the pulmonary trunk from the left, placing the two vessels in a direct anteroposterior relationship instead of the normal crossing arrangement. Superimposed on the frontal radiograph, they occupy a much narrower transverse width than normal, narrowing the vascular pedicle. Meanwhile the parallel systemic and pulmonary circulations cause volume overload of both atria and the right ventricle and increased pulmonary blood flow, enlarging the heart into an oval, transversely oriented silhouette. Physiological stress accelerates thymic involution, removing the shadow that would otherwise widen the upper mediastinum.
Alternative names: Egg on its side, oval heart
Other associated named signs: Boot-shaped heart (tetralogy of Fallot), snowman sign (supracardiac TAPVR), and the box-shaped heart of Ebstein anomaly
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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