Piece them together - Corner Case -1​ : August 2021

Contributed by
Dr Megha Sheth
Consultant Radiologist
U N Mehta institute of cardiology and research centre, Ahmedabad, Gujarat

An 8 months old female patient, presented to the outpatient department with respiratory distress. On 2D echocardiography of the heart, findings were partial atrio-ventricular canal defect, atrial septal defect and bilateral superior venae cavae. The patient was referred to the radiology department at U. N. Mehta institute of cardiology and research centre, Ahmedabad, Gujarat for MSCT angiography study for detailed evaluation.

See the images below.

Findings:

Figure 1. On CT evaluation, there is fusion of posterobasal segments of right and left lungs behind pericardial reflections, anterior to aorta. 

Figure 2a and 2b: The resulting isthmus of pulmonary parenchyma receives the blood supply from the right pulmonary artery, and the respective bronchi, from the right bronchial tree. So, the vessels and bronchus of the isthmus cross the midline.

Figure 2c : 3D VRT image showing branches of RPA supplying horse shoe lung.

Figure 3 : showing atrial septal defect.

  • The pulmonary vein from horseshoe lung crosses midline to drain into left atrium via right inferior pulmonary vein. Associated anomalies in our case were partial atrio-ventricular canal defect, atrial septal defect, hypoplastic right lung and bilateral superior venae cavae.
  • Lower attenuation of horseshoe lung parenchyma compared to surrounding normal lung is observed and considered to represent air trapping due to compression of isthmic bronchus.
  • Learning points

    MSCT demonstrates the following characteristic findings:

    • Anomalous course of pulmonary artery crossing mid line. Identification of this vessel is clue for diagnosis.
    • Clear demonstration of pulmonary venous drainage of isthmus. This anatomical information is important if patient undergoes surgical intervention.
    • Associated cardiac as well as extra-cardiac anomalies.
  • Horseshoe lung is a rare congenital malformation in which an isthmus of pulmonary parenchyma extends from the right lung base across the midline behind the pericardium and joins the posterobasal segments of the lungs.

    In most cases, horseshoe lung is associated with scimitar syndrome.

    • Other associated findings include a hypoplastic right pulmonary artery, an abnormal systemic arterial supply from the aorta to the right lung, and bronchial sequestration.
    • Although invasive pulmonary angiography was once considered the diagnostic modality of choice, now-a-days it is entirely replaced by non invasive MSCT pulmonary angiography. Non-invasive MSCT pulmonary angiography with 3D reconstructed and Maximum Intensity Projection (MIP) images provide excellent 3-D anatomical mapping with demonstration of characteristic diagnostic findings of horseshoe lung.

Summary

  • The present case is unusual in not demonstrating findings of scimitar syndrome. Almost 80 % of horseshoe lung cases are associated with scimitar syndrome.

 

References:

  1. Freedom RM, Burrows PE, Moes CAF. “Horseshoe” lung: report of five new cases. AJR Am J Roentgenol 1986;146:211-5.
  2. Spencer H. Pathology of the lung, 3rd ed. Oxford: Pergamon Press, 1977:83.
  3. Singh N, Agarwal R, Gupta D. Pseudohorseshoe lung. Can Med Assoc J 2008;4:178.
  4. Dische MR, Teixeira ML, Winchester PH, Engle MA. Horseshoe lung associated with a variant of the “scimitar” syndrome. Br Heart J 1974;36:617-20.