The superior piercing variant, the most common brachial plexus variant, identified using ultrasonography in the present study: (A) unlabeled; and (B) labeled. Note the lack of fascial plane separating the anterior scalene muscle (AS) and the superior trunk (ST) of the brachial plexus in this condition. The ST nerve fibers travel through the AS muscle belly, and may become impinged resulting in neurogenic thoracic outlet syndrome. AS = anterior scalene; IT = inferior trunk; MS = middle scalene; MT = middle trunk; SCM = sternocleidomastoid; ST = superior trunk. The green outlines demarcate the trunks of the brachial plexus.