Cases of high lying azygos arch and its embryological consideration.

PURPOSE: This study aimed to report cases of high-lying azygos arch and discuss the embryological basis of its development by a thorough evaluation of the anatomical features assessed using computed tomography (CT) images. METHODS: This study was approved by our institutional review board. We retrospectively reviewed chest CT images between November 2011 and November 2018. To determine high-lying azygos arch, we set the upper margin of the T4 vertebral body as the reference level. Regarding the embryological development of high-lying azygos arch, we retrospectively reviewed the CT images of 105 patients with tracheal bronchus to identify the location of the azygos arch. RESULTS: We noted that on three cases CT images, the azygos arch was located higher than the upper margin of the right main bronchus, and drained into the proximal superior vena cava (SVC) at a level higher than the conventional T4 or T5 vertebral level. All 105 patients with right tracheal bronchus showed azygos arch above the tracheal bronchus. CONCLUSION: This variation in the location of the azygos arch can mimic pathological lesion on plain radiographs, and, therefore, it is important to be aware of high-lying azygos arch. Our findings show that the azygos arch may have possibly migrated downward during embryological development.