The brachial plexus arises from the inferior portion of the cervical spinal cord enlargement. It is formed by the ventral primary rami of spinal nerves C5 through C8 and most of T1. The network of nerves that form the brachial plexus is divided anatomically from proximal (medial) to distal (lateral) into roots, trunks, divisions, cords, and terminal branches (mnemonic: “R. Travis drinks cold Texas beer”). The roots unite to form superior, middle, and inferior trunks. The suprascapular nerve and the nerve to the subclavius muscle arise from the superior trunk. Each trunk is divided into anterior and posterior divisions, which will innervate musculature of the anterior and posterior compartments, respectively. The anterior divisions of the superior and middle trunks unite to form the lateral cord, which branches off to the lateral pectoral nerve. The anterior division of the inferior trunk continues distally as the medial cord, whose branches are the medial pectoral, medial brachial cutaneous, and medial antebrachial cutaneous nerves. The posterior divisions of all three trunks unite to form the posterior cord, and its branches are the upper and lower subscapular and thoracodorsal nerves. The three cords are named according to their relation to the axillary artery, which passes through the plexus at this level. The terminal branches of the brachial plexus are the axillary, musculocutaneous, median, ulnar, and radial nerves. The axillary nerve (C5 and C6) arises from the posterior cord and courses posteriorly around the surgical neck of the humerus, where it is at risk for injury. The posterior circumflex humeral artery accompanies the nerve in this course. The axillary nerve supplies the deltoid and teres minor muscles, is sensory to the skin over the lower portion of the deltoid, and is optimally tested on the “shoulder patch” portion of the upper arm. Axillary nerve injury, such as that due to fracture at the surgical neck of the humerus, results in an inability to abduct the arm at the shoulder to a horizontal position and in sensory loss in the shoulder patch area . The musculocutaneous nerve (C5–C7) is the continuation of the lateral cord. It courses distally through the coracobrachialis muscle to innervate it in addition to the biceps brachii and brachialis muscles. The lateral antebrachial cutaneous nerve to the skin of the lateral forearm represents the terminal continuation of this nerve. Damage to the musculocutaneous nerve causes weakness in supination and flexion of the shoulder and elbow. The upper portion of the brachial plexus arises from spinal cord segments C5 and C6; forms the superior trunk; and makes major contributions to the axillary, musculocutaneous, lateral pectoral, and suprascapular nerves and the nerve to the subclavius muscle. Injury to the upper plexus typically occurs with an increase in the angle between the shoulder and the neck. This can occur in a newborn during an obstetrical delivery or in adults as the result of a fall on the shoulder and side of the head and neck, which produces a widened angle. The resultant muscle paralysis due to such an injury may be understood more easily in an adult with such an injury. The upper extremity hangs limp by the side because the deltoid and supraspinatus (abductors of the arm) are paralyzed as a result of injury of the axillary and suprascapular nerves, respectively. In addition, the anterior deltoid, biceps brachii, and coracobrachialis (flexors of the arm) are paralyzed due to injury of the axillary and musculocutaneous nerves. The elbow is extended and the hand is pronated because of paralysis of the biceps brachii and brachialis muscles, both of which are innervated by the musculocutaneous nerve. The extremity is medially rotated because of paralysis of the teres minor and infraspinatus muscles (lateral rotators of the arm) and injury to the axillary and suprascapular nerves. The palm of the hand is turned posteriorly in the “waiter’s tip” sign. There is loss of sensation along the lateral aspect of the upper extremity, which corresponds to the dermatome at C5 and C6.
The upper brachial plexus injury is known as Erb’s or Duchenne-Erb palsy. The ulnar nerve (C8 and T1) is a continuation of the medial cord, which enters the posterior compartment through the medial intermuscular septum and passes distally to enter the forearm by curving posteriorly to the medial epicondyle. Here it is superficial and at risk for injury. It enters the anterior compartment of the forearm, where it innervates the flexor carpi ulnaris and the bellies of the flexor digitorum profundus to the ring and little fingers. The ulnar nerve enters the hand through a canal (Guyon canal) superficial to the flexor retinaculum. The nerve supplies all the intrinsic muscles of the hand except for the three thenar muscles and the lumbricals of the index and middle fingers. It is sensory to the medial border of the hand, the little finger, and the medial aspect of the ring finger. Damage to the ulnar nerve in the upper forearm causes lateral (radial) deviation of the hand, with weakness in flexion and adduction of the hand at the wrist and loss of flexion at the distal interphalangeal joint of the ring and little fingers. Damage to the ulnar nerve in the upper forearm or at the wrist also results in loss of abduction and adduction of the index, middle, ring, and little fingers due to paralysis of the interossei muscles.
A “claw hand” deformity results, and with longstanding damage, atrophy of the interosseous muscles occurs. Injury to the lower brachial plexus, known as Klumpke palsy, occurs by a similar mechanism, that is, an abnormal widening of the angle between the upper extremity and the thorax. This may occur at obstetrical delivery by traction on the fetal head or when an individual reaches out to interrupt a fall. The roots from C8 and T1 and/or the inferior trunk are stretched or torn. Spinal cord segments C8 and T1 form the ulnar nerve and a significant portion of the median nerve. Most of the muscles of the anterior forearm are innervated by the median nerve and will display weakness. Most of the muscles of the hand are innervated by the ulnar nerve. There will be loss of sensation along the median aspect of the arm, forearm, hypothenar eminence, and little finger (C8 and T1 dermatome).
