Anatomical relationships between the proximal brachial plexus and scalene musculature identified in the cadaveric part of this study: (A) classic anatomical relationship between the brachial plexus and anterior scalene muscle. Superior, middle, and inferior trunks of the
brachial plexus travel with the subclavian artery through the interscalene gap, between the anterior and middle scalene muscles; (B) Superior piercing variant: The superior trunk of the brachial plexus pierces the anterior scalene muscle; (C) Multiple piercing variant: The superior and middle trunks of the brachial plexus pierce the anterior scalene muscle. Note: In (B) and (C), the scalene muscle belly would impinge on the nerves causing a predisposition for neurogenic thoracic outlet syndrome. AS = anterior scalene; C5 = anterior ramus of C5; C6 = anterior ramus of C6; IT = inferior trunk; MS = middle scalene; MT = middle trunk; SA = subclavian artery; ST = superior trunk.