Brachial Plexopathy

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 ( Fig. 33.3 ). The biceps and brachioradialis tendon jerks are depressed or absent, but the triceps reflex is spared.

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 ( Fig. 33.4 ). In pure lower trunk plexopathies, there are no reflex abnormalities.

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 (wrist drop and finger drop, 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-innervated muscles and C8–T1 median-innervated 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 ( Fig. 33.5 ). 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 downward, 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.

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, wherein 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 the 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

Neuralgic amyotrophy (NA) is a common although underappreciated disorder. The condition is known by various names, including Parsonage-Turner syndrome , brachial plexitis , idiopathic brachial plexopathy , and brachial amyotrophy , among others. It is included under brachial plexopathy as it was originally thought to be a disorder of the brachial plexus. However, it is now much better appreciated that most often it is a disorder of one or more major nerves in the upper extremity (N.B., there are also rare cases in the lower extremity). Thus, in reality, it is more properly designated as an acute mononeuropathy multiplex. In many but not all cases, the syndrome is preceded by an antecedent event that triggers the immune system, often a viral illness or immunization or, occasionally, surgery, trauma, or unusual muscular effort. Recent outbreaks of hepatitis E have shown a strong relationship to NA. The onset of shoulder pain typically follows within several days to a few weeks. The pain is severe, resistant to analgesics, and often awakens 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 1–2 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. Rare cases are purported to be painless. In these situations, one should strongly consider the possibility of hereditary neuropathy with liability to pressure palsies.

Although some cases may affect the brachial plexus directly, it is more common that one or more upper extremity nerves are involved. Certain nerves are more frequently involved in NA. In order of frequency, they include the suprascapular, long thoracic, anterior interosseous, axillary, musculocutaneous, posterior interosseous, and radial nerves. In some cases, NA affects branches to individual muscles. Note that among these nerves, there is a propensity for “pure motor” nerves. Pure motor nerves actually contain sensory pain fibers to deeper tissues and sensory afferents from muscle (Ia and Ib fibers) but do not contain cutaneous sensory fibers. This propensity for pure motor nerves complicates the interpretation of the electrodiagnostic (EDX) study (see later). In addition, the roots may be involved rarely.

A long thoracic nerve palsy results in characteristic winging of the scapula, due to weakness of the serratus anterior muscle. An anterior interosseous nerve (AIN) 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 NA.

Most episodes of NA are primarily unilateral. On close examination, however, especially with needle EMG, some abnormalities on the contralateral side are not unusual. Likewise, the majority of cases are a one-time event. Recurrent episodes can occur but are much less common. Recurrent episodes of painful brachial neuritis should raise the possibility of hereditary NA, 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).

Regarding etiology, there is a growing consensus that NA may result from an interplay of environmental factors, mechanical factors, and genetic susceptibility. The frequent association with immune triggers strongly implicates environmental factors. In rare cases in which nerve biopsies of individuals with NA are done, epineural perivascular inflammation of T-cells is most often seen, suggesting an autoimmune component. Unusual strenuous or repetitive muscular activity is increasingly recognized as being present prior to the onset of the syndrome. It is postulated that mechanical stretching and compression of nerves may disrupt the blood-nerve barrier and allow the immune system access to peripheral nerve antigens. This hypothesis is further strengthened by the fact that the blood-nerve barrier is located in the perineurium of individual nerve fascicles. Imaging of NA has shown selective involvement of individual nerve fascicles in some cases of NA (see later, in the Ultrasound correlation section). It is not clear why there is a propensity for nerves near the shoulder girdle, but perhaps because the shoulder joint is the most mobile and unstable joint in the body. Presumably, stress and shear on nerves are more likely from normal “wear and tear.”

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 ( Fig. 33.6 ), 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.

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, and 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 ( Fig. 33.7 ). 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 ( Fig. 33.8 ). 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.

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.