Usual position of the vagus nerve.The vagus nerve (arrow) is located posterior or lateral to the reference axis, which comprised the carotid artery and the internal jugular vein (red dot line). A variation in position was defined as the vagus nerve being located anterior or medial to the carotid-internal jugular vein axis, and variations were classified into four types-anterolateral (AL), anteromiddle (A), anteromedial (AM), and medial (M)-based on the relative location of the vagus nerve to the carotid artery.