Upper Extremity Peripheral Nerve Blocks: Brachial Plexus Anatomy

UPPER EXTREMITY PERIPHERAL NERVE BLOCKS

Brachial Plexus Anatomy

The brachial plexus is formed by the union of the anterior primary divisions (ventral rami) of the fifth through the eighth cervical nerves and the first thoracic nerves. Contributions from C4 and T2 are often minor or absent. As the nerve roots leave the intervertebral foramina, they con-verge, forming trunks, divisions, cords, branches, and then finally terminal nerves. The three distinct trunks formed between the anterior and middle scalene muscles are termed superior, middle, and inferior based on their vertical orientation. As the trunks pass over the lateral border of the first rib

and under the clavicle, each trunk divides into anterior and posterior divisions. As the brachial plexus emerges below the clavicle, the fibers com-bine again to form three cords that are named according to their relationship to the axillary artery: lateral, medial, and posterior. At the lateral border of the pectoralis minor muscle, each cord gives off a large branch before ending as a major terminal nerve. The lateral cord gives off the lateral branch of the median nerve and terminates as the musculocutaneous nerve; the medial cord gives off the medial branch of the median nerve and termi-nates as the ulnar nerve; and the posterior cord gives off the axillary nerve and terminates as the radial nerve. Local anesthetic may be depos-ited at any point along the brachial plexus,depending on the desired block effects (Figure 46–7): interscalene for shoulder and proxi-mal humerus surgical procedures; and supracla-vicular, infraclavicular, and axillary for surgeries distal to the mid-humerus.