Panplexus

A complete brachial plexopathy results in weakness, sensory loss, and decreased or absent reflexes in the entire arm. Provided the roots remain intact, the serratus anterior and rhomboids usually are the only muscles spared because they are innervated by nerves that come directly off the roots, proximal to the plexus. The assessment of these two muscles is key, both clinically and electrically, in differentiating a severe lesion at the level of the plexus from one originating at the roots.

Upper Trunk Plexopathy

The upper trunk is formed from the C5–C6 roots. Thus, upper trunk lesions result in weakness of nearly all muscles with C5–C6 innervation. Most affected are the deltoid, biceps, brachioradialis, supraspinatus and infraspinatus muscles. Muscles that receive partial upper trunk innervation, such as the pronator teres (C6–C7) and triceps (C6–C7–C8), may be partially affected. Sensory loss involves the lateral arm, lateral forearm, lateral hand, and thumb. This territory corresponds to the sensory distributions of the axillary and lateral antebrachial cutaneous nerves, as well as the median and radial sensory branches to the thumb and index finger (Figure 30–3). The biceps and brachioradialis tendon jerks are depressed or absent, but the triceps reflex is spared.

FIGURE 30–3 Upper trunk brachial plexopathy sensory loss.

This territory corresponds to the sensory distributions of the axillary nerve, lower lateral cutaneous nerve of the arm (a.k.a. the lateral brachial cutaneous nerve), and lateral cutaneous nerve of the forearm (a.k.a. the lateral antebrachial cutaneous nerve), as well as the median and radial sensory branches to the index finger and thumb.

(Adapted with permission from Haymaker, W., Woodhall, B., 1953. Peripheral nerve injuries. WB Saunders, Philadelphia.)

Middle Trunk Plexopathy

Middle trunk lesions are very rare. Because the middle trunk is formed directly from the C7 root, middle trunk lesions mimic C7 radiculopathies. Weakness involves primarily the triceps, flexor carpi radialis, and pronator teres muscles. Sensory abnormalities predominantly affect the middle finger, and less so the index and ring fingers (sensory branches of the median nerve) and posterior forearm (posterior cutaneous nerve of the forearm). Only the triceps reflex is abnormal on reflex testing.

Lower Trunk Plexopathy

The lower trunk is formed from the C8–T1 roots. The entire ulnar nerve, the medial brachial cutaneous nerve, and the medial antebrachial cutaneous nerve are ultimately supplied from fibers passing through the lower trunk. In addition, both the median and radial nerves receive partial motor innervation from the lower trunk. Accordingly, lower trunk lesions involve all ulnar muscles, in addition to median C8–T1-innervated muscles (e.g., abductor pollicis brevis [APB], flexor pollicis longus, flexor digitorum profundus) and radial C8-innervated muscles (e.g., extensor indicis proprius [EIP], extensor pollicis brevis). Sensory loss involves the medial arm, medial forearm, medial hand, and fourth and fifth fingers. This territory corresponds to the distribution of the medial brachial cutaneous, medial antebrachial cutaneous, ulnar sensory, and dorsal ulnar cutaneous sensory nerves (Figure 30–4). In pure lower trunk plexopathies, there are no reflex abnormalities.

FIGURE 30–4 Lower trunk brachial plexopathy sensory loss.

This territory corresponds to the distribution of the medial cutaneous nerve of the arm (a.k.a. the medial brachial cutaneous nerve), medial cutaneous nerve of the forearm (a.k.a. the medial antebrachial cutaneous nerve), ulnar sensory, and dorsal ulnar cutaneous sensory nerves.

(Adapted with permission from Haymaker, W., Woodhall, B., 1953. Peripheral nerve injuries. WB Saunders, Philadelphia.)

Lateral Cord Plexopathy

The entire musculocutaneous nerve and the C6–C7 portion of the median nerve are derived from the lateral cord. Accordingly, lateral cord lesions result in median weakness of arm pronation (pronator teres) and wrist flexion (flexor carpi radialis) and musculocutaneous weakness of elbow flexion (biceps). Sensory loss involves the lateral forearm, lateral hand, and first three fingers. This territory corresponds to the distribution of the lateral antebrachial cutaneous and median sensory nerves. On reflex testing, the biceps reflex is abnormal, but the triceps and brachioradialis reflexes are preserved.

Posterior Cord Plexopathy

The radial, axillary, and thoracodorsal nerves are derived from the posterior cord. Accordingly, posterior cord lesions result in complete radial palsies (wristdrop and fingerdrop, arm extension weakness) in addition to weakness of shoulder abduction (deltoid) and adduction (latissimus dorsi). Sensory loss involves the lateral arm, posterior arm and forearm, and radial dorsal hand. This territory corresponds to the sensory distribution of the radial (superficial radial, posterior cutaneous nerve of the forearm) and axillary nerves. On reflex testing, the triceps and brachioradialis reflexes are abnormal.

Medial Cord Plexopathy

The medial cord is the direct continuation of the anterior division of the lower trunk. Thus, medial cord lesions are nearly identical to lower trunk plexopathies, except for intact radial C8 fibers, which pass through the posterior division of the lower trunk and then through the posterior cord. Medial cord lesions result in weakness of all ulnar muscles and C8–T1 median muscles (APB, flexor pollicis longus, flexor digitorum profundus – median). Notably, finger extensors, especially to the index finger (radial innervated), are spared. Sensory loss is identical to that seen in lower trunk lesions, involving the medial arm, medial forearm, medial hand, and fourth and fifth fingers.

Traumatic Brachial Plexopathy

Traumatic injuries are the most common cause of brachial plexopathies. Most frequently, traumatic brachial plexopathies are the result of automobile, motorcycle, or bicycle accidents. Penetrating knife or gunshot wounds may injure the brachial plexus. Traumatic brachial plexopathies may occur in newborns, usually as a result of traction during delivery.

Most traumatic plexopathies are the result of traction and stretch injuries. Injuries in which the head is pushed away from the shoulder (e.g., the head and shoulder striking the pavement when a person is thrown from a moving vehicle) typically result in upper plexopathies, affecting the C5–C6 fibers (Figure 30–5). Such injuries result in characteristic weakness of shoulder abduction, elbow flexion, and arm supination, known as Erb’s palsy. This is also the most common type of brachial plexopathy seen in newborns, presumably as a result of the head being delivered down, away from the shoulder. The most common risk factor for an Erb’s palsy in a newborn is shoulder dystocia in a large infant. In contrast, injuries in which the arm and shoulder are pulled up typically result in lower plexopathies, affecting the C8–T1 fibers. Severe hand weakness, known as Klumpke’s palsy, characteristically occurs in these latter injuries, with preservation of upper arm and shoulder girdle muscles. One of the most common scenarios in which this occurs is when an individual (often unconscious) is dragged by one arm.

FIGURE 30–5 Traumatic upper trunk plexopathies.

Most traumatic plexopathies are the result of traction and stretch injuries. Although the entire brachial plexus can be injured, the upper trunk is most commonly injured when the head is pushed away from the shoulder, as might occur when the head and shoulder strike the pavement after a person is thrown from a moving vehicle.

It is important to understand that severe traction injuries may result in damage to the roots as well as the plexus. A traction injury can cause frank root avulsion, in which the roots are physically separated from the spinal cord. This is the most serious type of injury, with no chance for recovery. Nerve conduction studies and needle EMG are useful in differentiating root avulsion from plexus lesions, or lesions that involve both the roots and the plexus.

Neoplasms and Other Mass Lesions

Brachial plexopathy may result from local tumor invasion. For example, Pancoast tumors of the lung may spread and invade the plexus directly. More commonly, tumors metastasize to nearby lymph nodes, where they grow and compress the plexus. Lymphomas, breast cancer, and lung cancer are the most frequent causes. Lymphomas and leukemia can also infiltrate nerve directly, in the absence of a mass lesion. Rarely, primary nerve sheath tumors (e.g., schwannomas, neurofibromas, or neurofibrosarcomas) may affect the brachial plexus. In unusual cases, non-neoplastic mass lesions, such as hematomas and unusual vascular anomalies (e.g., aneurysm, arteriovenous malformation), can compress the brachial plexus.

Characteristically, neoplastic brachial plexopathy results in a slowly progressive syndrome often associated with prominent pain. In some cases, it may be difficult or impossible to distinguish these lesions clinically from more proximal lesions of the cervical nerve roots. Nerve conduction studies and needle EMG often are very useful in distinguishing brachial plexus from cervical root lesions in these cases.

Neuralgic Amyotrophy (Brachial Plexitis)

Neuralgic amyotrophy is a common although underappreciated disorder that frequently affects individual upper extremity nerves or the brachial plexus. The condition is known by various names, including Parsonage–Turner syndrome, brachial plexitis, idiopathic brachial plexopathy, and brachial amyotrophy. In many but not all cases, the syndrome is preceded by an antecedent event that triggers the immune system, usually a viral illness or immunization or, occasionally, surgery. The onset of shoulder pain typically follows within several days to a few weeks. The pain is severe, often awakening the patient from sleep. Early on, muscle weakness may be difficult to detect on examination because of the prominent pain. However, as the pain subsides, typically after 1 to 2 weeks, significant underlying weakness becomes apparent. Muscle atrophy follows. Although paresthesias and sensory loss may also be present, it is not unusual to find only mild or minimal sensory abnormalities on examination.

In some cases, all or part of the brachial plexus may be affected. In others, individual upper extremity nerves, including nerves that come directly off the roots, may be affected in isolation, in a pattern that more resembles a mononeuropathy multiplex. Certain nerves, especially the long thoracic and anterior interosseous nerves, are frequently involved in neuralgic amyotrophy. A long thoracic nerve palsy results in characteristic winging of the scapula, due to weakness of the serratus anterior muscle. An anterior interosseous nerve palsy is recognized principally by weakness of the long flexors of the thumb and index finger (flexor pollicis longus and flexor digitorum profundus – median): the patient is unable to make an “OK” sign. In some cases, involvement of the phrenic nerve has been reported, either in isolation or in conjunction with other mononeuropathies. Exceptionally, lower cranial neuropathies (IX–XII) have accompanied otherwise classic presentations of neuralgic amyotrophy.

Most episodes of neuralgic amyotrophy are primarily unilateral. On close examination, however, especially with needle EMG, some abnormalities on the contralateral side are not unusual. Likewise, most cases are a one-time event. Recurrent episodes can occur but are distinctly uncommon. Recurrent episodes of painful brachial neuritis should raise the possibility of hereditary neuralgic amyotrophy, a rare, dominantly inherited disorder associated with mutations in the SEPT9 (septin-9) gene on chromosome 17q25 that has a similar clinical presentation to the idiopathic cases. Minor dysmorphic features may be present on physical examination of these patients (i.e., hypotelorism, short stature, cleft palate, epicanthal folds, ring-shaped skin creases on limbs and neck, partial syndactyly).

Postoperative Brachial Plexopathy

Brachial plexopathy is the most common peripheral nervous system complication occurring after coronary artery bypass and other similar chest surgery. These lesions are thought to result from stretch injury following chest wall retraction or occur secondary to compression from hematomas associated with internal jugular catheters. Nearly all involve principally the lower trunk or medial cord of the plexus.

In lesions of the lower trunk, patients note sensory disturbance in the fourth and fifth fingers (ulnar distribution), which may continue up the medial forearm and arm (medial brachial and medial antebrachial cutaneous nerves). Weakness involves all C8–T1 muscles, including median and ulnar hand intrinsics, all forearm long finger flexors (Figure 30–6), and, less so, the finger extensors (principally the extensors to the thumb and index finger). In some cases, pain may be a prominent symptom. Because the presumed injury is secondary to stretch and compression, without any tearing or shearing of nerve and basement membrane, most patients make a good recovery over several months. Rarely, patients may not recover completely; occasionally, patients are left with chronic pain that is difficult to treat.

FIGURE 30–6 Postoperative brachial plexopathy.

Postoperative brachial plexopathies characteristically affect C8–T1 fibers that travel through the lower plexus. Weakness of intrinsic hand muscles and the long finger flexors results. In the patient shown here, weakness of the long finger flexors on the left is recognized as the inability to make a grip and fully flex the fingers and thumb at the distal interphalangeal joints.

Delayed Radiation Injury

Radiation may result in a progressive brachial plexopathy, typically presenting years after the radiation exposure. Radiation ports often include the region of the brachial plexus, especially in the treatment of lymphomas and breast, lung, and neck cancers. The risk of radiation-induced plexopathy increases with the dose of radiation; it is more common after doses of more than 5700 rads.

When a patient with a prior history of malignancy who has been treated with radiation develops a slowly progressive brachial plexus lesion, the differential diagnosis usually rests between radiation-induced brachial plexopathy and direct invasion from recurrent tumor. Several clinical and electrophysiologic findings may be of help in distinguishing between the two. First, pain is an earlier and more prominent finding in direct neoplastic invasion. Likewise, the presence of a Horner’s syndrome is much more common in direct neoplastic invasion. In contrast, sensory symptoms (i.e., paresthesias and numbness) appear more commonly and earlier in cases of radiation damage. In addition, patients with radiation-induced plexopathy usually are symptomatic for a much longer time, often many years, before coming to medical attention.

On electrophysiologic testing, the presence of myokymic discharges and fasciculations is especially helpful in differentiating radiation-induced from neoplastic plexopathy. Myokymic discharges are characteristic of radiation-induced brachial plexopathy. They may be seen clinically but are more often appreciated on needle EMG. Although conduction block across the brachial plexus has been described in patients with radiation plexitis, it is a nonspecific finding that has also been reported, although less frequently, in plexopathy associated with neoplasm. Other findings on nerve conduction studies and EMG, including the region of the plexus involved, and the presence of clinical weakness are generally not helpful in differentiating radiation-induced from direct neoplastic brachial plexopathy.

Thoracic Outlet Syndrome

The term thoracic outlet refers to the exit of the brachial plexus and the major arteries and veins from the shoulder and axilla into the arm. Several types of thoracic outlet syndrome (TOS) occur, depending on which structure is entrapped. Impingement of the subclavian and axillary vessels may result in vascular TOS. Entrapment of the brachial plexus itself results in true neurogenic TOS.

In the past, the diagnosis of neurogenic TOS was made frequently, and many patients underwent surgical procedures to decompress the thoracic outlet. These procedures included removal of cervical ribs, first rib resections, lysis of fibrous bands, as well as sectioning of some of the scalene muscles. However, impingement of the cervical nerve roots at the intervertebral foramina and the common entrapment neuropathies in the arm were not well appreciated at that time. It has since become apparent that true neurogenic TOS is quite rare. Most patients diagnosed with TOS in the past actually had either a cervical radiculopathy or an entrapment of either the ulnar nerve at the elbow or the median nerve at the wrist.

Most cases of true neurogenic TOS are caused by a fibrous band that runs from a rudimentary cervical rib to the first thoracic rib, entrapping the lower trunk of the brachial plexus (Figure 30–7). Accordingly, sensory and motor loss develops in the C8–T1 distribution. Anatomically, the fibrous band most often preferentially affects the T1 fibers. This results in a characteristic pattern of signs and symptoms, including prominent wasting and weakness of the thenar and, less prominently, the hypothenar muscles (Figure 30–8). The explanation for the relative vulnerability of the thenar muscles is not completely clear, but it may be that the thenar muscles are more T1 innervated, whereas the hypothenar muscles receive more C8 innervation.

FIGURE 30–7 Neurogenic thoracic outlet syndrome anatomy.

Most cases of true neurogenic thoracic outlet syndrome are caused by a fibrous band that runs from a rudimentary cervical rib to the first thoracic rib (arrow), entrapping the lower trunk of the brachial plexus. C, clavicle; FTR, first thoracic rib.

(Adapted from Levin, K.H., Wilbourn, A.J., Maggiano, H.J., 1998. Cervical rib and median sternotomy-related brachial plexopathies: a reassessment. Neurology 50, 1407–1413, with permission.)

FIGURE 30–8 Hand atrophy and posture in a patient with neurogenic thoracic outlet syndrome.

Neurogenic thoracic outlet syndrome preferentially affects the T1 fibers. This results in a characteristic pattern of wasting and weakness of the thenar and, less prominently, the hypothenar muscles (left hand). The explanation for the relative vulnerability of the thenar muscles is likely that they are more T1 innervated, whereas the hypothenar muscles receive more C8 innervation.

In addition to the median and ulnar intrinsic hand muscles, the long flexors to the fingers (i.e., flexor digitorum profundus) and thumb (flexor pollicis longus) also are C8–T1 innervated and may be affected. Radial C8 weakness (e.g., EIP) can occur but is less common. Paresthesias and sensory loss affect the fourth and fifth fingers, medial hand, and medial forearm. These sensory changes are in the distribution of the ulnar and the medial antebrachial cutaneous sensory nerves, both of which pass through the lower trunk of the brachial plexus.

Neurogenic TOS is most often confused clinically with the more common ulnar neuropathy at the elbow or C8–T1 radiculopathy. Several pieces of clinical information are helpful in differentiating among these conditions. A history of neck pain with radiation down the arm, provoked by neck movement, strongly favors the diagnosis of radiculopathy. Local tenderness and pain around the elbow commonly accompany ulnar neuropathy at the elbow. In all three conditions, atrophy and weakness may affect both the thenar and hypothenar muscles. With ulnar neuropathy at the elbow, however, thumb abduction will be spared (median innervated). In neurogenic TOS, thumb abduction not only is involved but is often preferentially affected. In a C8–T1 radiculopathy, thumb abduction may be weak but is not out of proportion to weakness of the other C8–T1-innervated muscles. On sensory testing, abnormalities are restricted to the fifth and medial fourth fingers and medial hand in ulnar neuropathy at the elbow. In both neurogenic TOS and C8–T1 radiculopathy, sensory disturbance extends more proximally into the medial forearm, in the distribution of the medial antebrachial cutaneous sensory nerve.