Brachial Plexus Injuries and Reanimation

Roots

The traditional brachial plexus has contributions from cervical roots C5–C8 and thoracic root T1. Anatomical variations of this included a prefixed plexus (additional contribution from C4) and a postfixed plexus (additional contribution from T2). The spinal roots pass between the anterior and middle scalene muscles. There are three nerve branches exiting from the root level. The dorsal scapular nerve arises from the C5 nerve root and supplies the rhomboid muscles and the levator scapulae muscle. The long thoracic nerve arises from roots C5, C6, and C7 and supplies the serratus anterior muscle, which stabilizes the scapula. The nerve to subclavius arises from the distal C5 and C6 nerve roots. This nerve supplies the subclavius muscle, which is unable to be tested on clinical examination.

Trunks

In the inferior portion of the posterior cervical triangle, the roots combine to form trunks. The upper trunk is formed from C5 and C6. The middle trunk is a continuation of the C7 root. The lower trunk is formed from C8 and T1. There is a single branch off the upper trunk, the suprascapular nerve, that is critical for shoulder function. It departs from the upper trunk, travels deep to the trapezius muscle and runs through the suprascapular notch of the scapula to supply the supraspinatus and infraspinatus muscles.

Divisions

Passing deep to the clavicle, each trunk divides into an anterior and posterior division. There are no extraneous branches at the level of the divisions.

Cords

The divisions pass deep to the pectoralis minor muscle and combine in a predictable fashion to form the cords. The cords are named in relation to their anatomical location around the axillary artery. The anterior divisions of the upper and middle trunks combine to form the lateral cord. The medial cord is the continuation of the anterior division of the lower trunk. All of the posterior divisions combine to form the posterior cord.

Branches

There are several branches that arise from the cords. The lateral cord gives off the lateral pectoral nerve, which innervates the clavicular head of the pectoralis major. The medial cord gives off the medial pectoral nerve, which innervates the sternocostal head of the pectoralis major muscle. The medial brachial cutaneous and medial antebrachial cutaneous nerves also branch from the medial cord supplying sensation to the medial aspect of the upper arm and forearm. More distally, the lateral and medial cords each contribute to the median nerve, supplying the sensory and motor functions of the nerve respectively. This relationship continues distally and is useful for understanding the internal topography of the median nerve, where motor fascicles are located on the medial side of the nerve and sensory fascicles remain more lateral. Following its contribution to the median nerve, the lateral cord becomes the musculocutaneus nerve. This nerve innervates the coracobrachialis, biceps brachii, and brachialis. The terminal portion of this nerve is the lateral cutaneous nerve of the forearm. After contributing to the median nerve, the medial cord becomes the ulnar nerve, which, after innervating the flexor carpi ulnaris and the small and ring flexor digitorum profundus muscles, is primarily responsible for the intrinsic musculature of the hand.

The internal topography of the ulnar nerve can best be remembered as sensory–motor–sensory, where the most medial portion of the nerve will eventually contribute to the dorsal cutaneous ulnar branch, exiting the main portion of the ulnar nerve approximately 9 cm proximal to the wrist crease and providing sensation to the dorsal ulnar aspect of the hand. After this division, the more medial 40% of the ulnar nerve will pass through Guyon’s canal to become the deep motor branch and the more radial 60% will contribute to the superficial sensory branch ( Fig. 53.2 ).

The posterior cord gives off the upper subscapular nerve, the thoracodorsal nerve, and the lower subscapular nerve. These branches innervate the superior portion of the subscapularis muscle, the latissimus dorsi muscle, and the lower subscapularis/teres major muscles, respectively. The two most terminal branches of the posterior cord are the axillary nerve and the radial nerve. The axillary nerve travels through the quadrangular space (bounded by the humerus, the long head of triceps brachii, teres minor, and teres major) with the posterior circumflex humeral artery. It innervates the deltoid and the teres minor, but also provides sensation to a patch of skin overlying the deltoid, sometimes known as the “soldier’s patch.” This sensory component is located at the most inferior portion of the axillary nerve. The radial nerve travels through the triangular interval (bounded by the teres major and the lateral and long heads of triceps brachii) with the profunda brachii artery. Wrapping around the spiral groove, the radial nerve goes on to supply the elbow, wrist, finger, and thumb extensors.

Traumatic

The majority of brachial plexopathies secondary to trauma occur in young males, with up to 90% of cases comprising this patient population. Traumatic injuries to the brachial plexus can be divided into open and closed injuries. Open injuries are those secondary to penetrating injury, such as knives or other sharp objects. These necessitate immediate surgical exploration. The majority of traumatic brachial plexus injuries are closed, and these can be composed of crush injuries or traction injuries. Crush injuries tend to occur after motor vehicle accidents, whereas traction injuries are more likely to be related to motorcycle accidents, skiing, falls, and sports injuries. In traction injuries, displacement of the upper extremity inferiorly combined with neck flexion contralaterally causes an increase in the neck–shoulder angle, which results in injury to the upper roots and trunks. This commonly occurs in both high-velocity injuries such as motorcycle collisions and also in lower-velocity accidents such as sports injuries. The opposite can occur with strong upward force or traction on the upper extremity increasing the scapula–humoral angle and causing injury to the lower plexus. The brachial plexus is susceptible to these injuries due to limited mobility secondary to prevertebral fascia and attachment of the cervical roots to the vertebrae. Traction injuries are also responsible for obstetrical brachial plexopathies, most commonly following shoulder dystocia. Risk factors for obstetrical plexus injury include high birth weight, fetal acidosis, posterior arm involvement, and narrow pelvis.

Iatrogenic

During long operations, positioning of the upper extremity can contribute to a traction palsy. Most commonly these injuries are neuropraxic in nature and will recover spontaneously; occasionally, however, a more severe injury will occur. There are patient factors that contribute to the propensity for developing these injuries, including diabetes, hypothyroidism, prior injury, hereditary neuropathy with liability to pressure palsy (HNPP), and obesity.

Compression

Compression can be secondary to structures adjacent to the plexus in an enclosed space such as malignancy, callous formation, hematoma or pseudoaneurysm. Anatomical variants may be responsible for compression primarily or may increase the propensity for trauma to the brachial plexus. These variants include cervical ribs, prominent transverse processes, and congenital fibrous bands. Compression often occurs in the costoclavicular space as the brachial plexus travels between the clavicle and the first rib. Traumatic injuries can result in swelling or hematoma that cause compression of the brachial plexus. Occupational injuries where a heavy object lands on the shoulder or neck are an example.

Neuralgic Amyotrophy

Brachial neuritis, or Parsonage–Turner syndrome, is a brachial plexopathy of unclear etiology that most commonly involves the lower brachial plexus; the axillary nerve is implicated in up to 70% of patients. Most commonly, these nontraumatic plexopathies present with a history of severe neuropathic pain that resolves and then is followed by patchy weakness. Possible contributory factors are viral disease, infection, immunization, trauma, surgery, and autoimmune mechanisms. The majority of these plexopathies will have a full spontaneous recovery within approximately 3 years, however the prognosis is more guarded in the setting of other medical comorbidities such as diabetes.