A brachial plexus variation characterized by the absence of the superior trunk

The brachial plexus is formed by the union of the ventral rami of the inferior four cervical (C5-C8) and first thoracic (T1) nerves (Figure 1). The C5 nerve usually receives a small contribution from the fourth-cervical nerve and the T1 nerve normally receives one from the second thoracic nerve. The brachial plexus supplies cutaneous and muscular innervation to the upper limb(s) and any injury at this level can lead to significant disability. The plexus extends from the inferior lateral portion of the neck downward and laterally over the first rib, posterior to the clavicle, and enters the axilla. Upon exit from the intervertebral foramina, the ventral rami of C5 and C6 cervical nerves unite to form the superior trunk. That of C7 nerve runs alone as the middle trunk and those of C8 and T1 nerves unite to form the inferior trunk; each of the trunks then bifurcates into anterior and posterior divisions as they pass beneath the clavicle. The anterior divisions of the superior and middle trunk form the lateral cord. The anterior division of the inferior trunk continues as the medial cord. The posterior divisions of all three trunks unite to form the posterior cord. From the cords arise the terminal branches of the brachial plexus including the musculocutaneous, median, ulnar, axillary, and radial nerves [1]. Anomalies of the cords and terminal branches of the brachial plexus are relatively common and have been well documented [2], however, anomalies of the roots and trunks are comparatively rare and have been reported in only a few cases. As the brachial plexus supplies cutaneous and muscular innervation to the upper limb, its anatomical variations have important clinical implications. Therefore, it is important for the anatomist, surgeon, anesthesiologist, and radiologist to be aware of anatomical variations that deviate from the classic anatomy.