A constellation of anatomical variation: middle scalene, wrist extensor, and aortic arch variants share embryological...

During dissection of a 70-year-old male donor, several anatomical variations were observed, highlighted by a bilateral variant middle scalene muscle in the superolateral thoracic wall. The variant scalene muscle was traced from the transverse processes of cervical vertebrae to the fourth rib with a pronounced fascial slip. The elongated middle scalene muscle was thick in girth and abnormally wide at its insertion (56.0 mm), which is hypothesized to reflect compensatory hypertrophy secondary to lung carcinoma. A bilateral wrist extensor variant also was observed as well as an anomalous left vertebral artery from the aortic arch with an abnormal entrance into the vertebral canal. Collectively, these findings represent a constellation of anatomical variations that may be interrelated through altered cervical somite development, providing a fascinating example of how anatomical variations may cluster based on common embryological origin. Clinically, these observations have implications in thoracic outlet syndrome, tendon transfer, and vascular surgery.