When C5 takes the road less traveled: an intramuscular course through anterior scalene in a prefixed, pass-through brachial plexus

Moultrie, GA

BACKGROUND: The brachial plexus (BP), formed by the ventral rami of C5-T1, provides sensory and motor innervation to the upper extremities. Classically, the BP roots pass between the anterior and middle scalene muscles within the interscalene triangle. However, anatomical variation is common due to the complex arrangement of cervical nerve roots and branches. Variants, such as a nerve root penetrating the anterior scalene muscle, may predispose individuals to nerve compression and may complicate procedures such as interscalene nerve blocks. Recognition of these variations is important for surgical, anesthetic, and imaging procedures involving the neck and shoulder. This report describes a cadaveric BP variation involving a C5 nerve root piercing the anterior scalene muscle with a communicating branch between C4 and C5.

METHODS: Dissection was performed during routine anatomical education at the Philadelphia College of Osteopathic Medicine-South Georgia. Standard blunt and sharp dissection techniques were used to expose the lateral cervical region, including the anterior and middle scalene muscles and the BP roots. The nerve roots were traced distally to observe formation of the superior trunk. Anatomical relationships were documented under direct visualization and digital photography.

RESULTS: The variation was identified in a 77-year-old cadaver donated to the PCOM South Georgia Body Donor Program. Donor records indicated a cause of death of malignant melanoma and a medical history of essential hypertension, coronary artery disease, and atrial fibrillation. Dissection of the right cervical region revealed an anastomotic connection between the C4 and C5 ventral rami proximal to the superior trunk, suggesting a prefixed contribution to the BP. Additionally, the C5 nerve root was observed piercing the anterior scalene muscle rather than passing through the interscalene triangle before joining C6 to form the superior trunk.

CONCLUSION: This case demonstrates two brachial plexus variations: a C4-c5 communicating branch and a C5 nerve root piercing the anterior scalene muscle. Such variants may increase susceptibility to nerve compression and may affect the accuracy of interscalene brachial plexus blocks. Awareness of these anatomical differences is important for clinicians performing procedures and interpreting imaging in the cervical region. Although limited to a single cadaveric observation, this report highlights the clinical relevance of recognizing brachial plexus anatomical variability.