Radial Neuropathy 41 Anatomy

The radial nerve receives fibers from all three trunks of the brachial plexus (C5-T1 roots). The posterior divisions of the three trunks unite to form the posterior cord, which gives off the radial nerve. The radial nerve exits the lateral wall of the axilla, and travels distally through the proximal arm, just medial to the humerus. Proximally, three sensory nerves: the posterior cutaneous nerve of the arm, the lower lateral cutaneous nerve of the arm, and the posterior cutaneous nerve of the forearm, join the radial nerve, providing cutaneous sensation over the

Table 5

Electrodiagnostic Evaluation of Ulnar Neuropathy at the Wrist"

Nerve conduction studies

1. Ulnar nerve studies

A. Ulnar motor study stimulating at the wrist, below elbow, and above elbow sites; recording from the abductor digiti minimi muscle

B. Ulnar motor study (bilateral) stimulating at the wrist; recording from the first dorsal interosseous muscle

C. Ulnar F-responses

D. Ulnar sensory study stimulating at the wrist; recording from digit 5

E. Dorsal ulnar cutaneous sensory study stimulating forearm; recording from the dorsolateral hand

2. Median nerve studies

A. Median motor study stimulating at the wrist and elbow sites; recording from the abductor pollicis brevis muscle

B. Median F-responses

3. Ulnar-median comparison studies

Lumbrical (2nd)-interosseous (first palmar) comparison study

1. Routine

A. One deep palmar motor muscle (e.g., first dorsal interosseous)

B. One hypothenar branch muscle (e.g., abductor digiti minimi)

2. If testing of any of the routine muscles is abnormal, then additional needle examination should include:

A. At least two nonulnar, lower trunk, C8-T1 muscles (e.g., APB, FPL, and EIP)

B. C8 and T1 paraspinal muscles aFDP, flexor digitorum profounder; FCU, flexor carpi ulnaris; APB, abductor pollicis brevis; FPL, flexor pol-licis longus; EIP, extensor indicis proprius; ECU, extensor carpi ulnaris; EDC, extensor digitorum communis; ECR extensor carpi radialis; FCR, flexor carpi radialis; PT, pronator teres From ref. 1.

Table 6

Nerve Conduction Study Findings in Ulnar Neuropathy at the Wrist"

1. Combined motor and sensory syndrome (Type 1)

Decreased ulnar sensory amplitude. Decreased ulnar motor amplitude with prolonged distal latency. EMG shows denervation of all intrinsic hand muscles.

2. Pure sensory syndrome (Type 2)

Decreased ulnar sensory amplitude. Ulnar motor study will be normal. EMG is normal.

3. Pure motor syndromes

A. Lesion affecting the deep palmar and hypothenar motor branches (Type 3)

Ulnar sensory response is normal. Ulnar motor amplitude is decreased with prolonged distal latency. EMG shows denervation of all intrinsic hand muscles.

B. Lesion affecting the deep palmar motor branch only (Type 4)

Ulnar sensory response is normal. Ulnar motor amplitude is decreased with prolonged distal latency when recording from the first dorsal interosseous muscle. EMG shows denerva-tion of the first dorsal interosseous muscle with sparing of the hypothenar muscles.

C. Lesion affecting only the distal deep palmar motor branch (Type 5)

Ulnar sensory response is normal. Ulnar motor amplitude is decreased with prolonged distal latency when recording from the first dorsal interosseous muscle. EMG shows denerva-tion of the first dorsal interosseous muscle with sparing of the hypothenar muscles.

aFrom ref. 17

posterolateral portions of the arm and a small strip along the middle posterior aspect of the forearm. Muscular branches are provided next to the long, lateral, and medial triceps muscles, as well as the anconeus muscle. Moving distally, the radial nerve wraps around the humerus, traveling in the spiral groove, before giving off additional branches to the supinator, long head of the extensor carpi radialis, and brachioradialis muscles. A few centimeters distal to the lateral epicondyle, the radial nerve divides into the superficial radial sensory nerve and the posterior interosseous nerve. The superficial radial sensory nerve travels distally along the radius providing cutaneous sensation over the dorsolateral hand and proximal portions of the dorsal aspect of the thumb, index, middle, and ring fingers. The posterior interosseous nerve travels through the supinator muscle passing under the arcade of Frohse. The posterior interosseous nerve supplies muscular branches to the short head extensor carpi radialis, extensor digitorum communis, extensor carpi ulnaris, abductor pollicis longus, extensor indicis pro-prius, extensor pollicis longus, and extensor pollicis brevis muscles.

4.1.1. Radial Neuropathy at the Axilla

This entrapment results from prolonged compression of the nerve as it courses through the axilla. A common presentation is the patient on crutches, who uses them incorrectly, thereby, applying prolonged pressure to the axilla. Because the lesion occurs proximal to muscular branches supplying the triceps muscle group, the clinical presentation is similar to radial neuropathy at the spiral groove, with the addition of triceps muscle weakness. Additionally, sensory disturbance extending into the posterior arm and forearm caused by compression of the posterior cutaneous sensory nerves of the forearm and arm is commonly seen.

4.1.2. Radial Neuropathy at the Spiral Groove

This is the most common site of compression of the radial nerve. This commonly occurs when a person has draped an arm over a chair or bench during deep sleep or intoxication ("Saturday Night Palsy"). Other cases may occur after strenuous muscular effort or fracture of the humerus. Patients with this particular entrapment typically present with wrist and finger drop in combination with decreased sensation over the posterolateral hand in the distribution of the superficial radial sensory nerve. Patients typically have weakness of supination and elbow flexion. However, elbow extension (triceps muscle) will be spared.

4.1.3. Posterior Interosseous Neuropathy

In this condition, patients also present with wrist drop. However, there are several distinct features of this particular entrapment that distinguish it from lesions at the spiral groove. In a posterior interosseous neuropathy (PIN), there is sparing of radial-innervated muscles proximal to the takeoff of the posterior interosseous nerve (triceps, anconeus, brachioradialis, and long head of the extensor carpi radialis muscles). Entrapment usually occurs at the Arcade of Frohse. When the patient extends the wrist, they may do so weakly, and with radial deviation; this occurs because the extensor carpi ulnaris is weak but the extensor carpi radialis is preserved. These patients typically do not experience cutaneous sensory deficits. Patients, however, may complain of forearm pain, which results from dysfunction of the deep sensory fibers of the posterior interosseous nerve that supplies the interosseous membrane and joint capsule.

4.1.4. Superficial Radial Sensory Neuropathy

In the forearm, the superficial radial sensory nerve travels subcutaneously next to the radius. Its superficial location makes it quite susceptible to compression. Sensory disturbances occur over the dorsolateral surface of the hand, and dorsal, proximal, portions of the

Table 7

Electrodiagnostic Evaluation for Evaluating Radial Neuropathy"

Nerve conduction studies

1. Radial nerve studies

A. Radial motor study stimulating at the forearm, elbow, below spiral groove, and above spiral groove sites; recording from extensor indicis proprius muscle

B. Superficial radial sensory study. Stimulating at the forearm; recording over the extensor tendons of the thumb. Bilateral studies are recommended

2. Median nerve studies

A. Median motor study stimulating at the wrist and below elbow sites; recording from the abductor pollicis brevis muscle

B. Median sensory study stimulating at the wrist; recording from the first digit

C. Median F-responses

3. Ulnar nerve studies

A. Ulnar motor study, stimulating at the wrist, below elbow, and above elbow sites; recording from the abductor digiti minimi muscle

B. Ulnar F-responses

C. Ulnar sensory study stimulating at the wrist; recording from the fifth digit

1. At least two PIN-innervated muscles (e.g., EIP, ECU, and EDC)

2. At least two radial-innervated muscles proximal to the PIN, but distal to the spiral groove (e.g., brachioradialis and long-head ECR)

3. At least one radial-innervated muscle proximal to the spiral groove (e.g., triceps)

4. At least one nonradial, posterior cord-innervated muscle (e.g., deltoid)

5. At least two nonradial C7-innervated muscles (e.g., FCR, PT, FPL, and cervical paraspinal muscles)

aPIN, posterior interosseous neuropathy; EIP, extensor indicis proprius; ECU, extensor carpi ulnaris; EDC, extensor digitorum communis; ECR, extensor carpi radialis; FCR, flexor carpi radialis; PT, pronator teres; FPL, flexor pollicis longus. From ref. 1.

fingers. Various objects, such as tight fitting bands, watches, bracelets, or handcuffs may lead to a superficial radial neuropathy. Because this is a pure sensory neuropathy, these patients do not develop weakness.

4.1.4.1. Differential Diagnosis of Wrist Drop

The differential diagnosis of a wrist drop should include the various radial nerve lesions that have been discussed. In addition, lesions that are more proximal, such as a posterior cord brachial plexopathy, a C7-C8 radiculopathy, or even a central lesion, should be considered. A careful clinical examination can be invaluable in localizing the lesion causing wrist drop.

4.1.4.2. Electrophysiology

The electrodiagnostic study should identify the presence of a radial neuropathy and properly localize the level of dysfunction. The radial motor study should be performed and compared with the contralateral side. A protocol outlining electrodiagnostic recommendations for evaluating radial neuropathy is outlined in Table 7.

Needle examination should focus on localizing the lesion level. Typically, one should examine at least: two PIN-innervated muscles (e.g., extensor indicis proprius, extensor carpi ulnaris, and extensor digitorum communis muscles); two radial-innervated muscles that are proximal to the PIN but distal to the spiral groove (e.g., long head of extensor carpi radialis

Table 8

Electrodiagnostic Findings in Radial Neuropathy"

1. Posterior interosseous neuropathy

A. Nerve conduction studies—superficial radial sensory response is normal. Radial motor study may show low-amplitude response (if axonal) or conduction block at the elbow (if demyeli-nating)

B. EMG—denervation in the extensor indicis proprius, extensor digitorum communis, and extensor carpi ulnaris muscles

2. Radial neuropathy at the spiral groove

A. Nerve conduction studies—superficial radial sensory response is low (if axonal). Radial motor study may show low-amplitude response (if axonal) or conduction block at the spiral groove (if demyelinating)

B. EMG—denervation as in PIN plus long head extensor carpi radialis, brachioradialis, and supinator muscles

3. Radial neuropathy at the axilla

A. Nerve conduction studies—superficial radial sensory response is low (if axonal). Radial motor study may show low-amplitude response (if axonal)

B. EMG—denervation as in spiral groove, plus triceps muscle

4. Posterior cord brachial plexopathy

A. Nerve conduction studies—superficial radial sensory response is low (if axonal). Radial motor study may show low-amplitude response (if axonal)

B. EMG—denervation as in axilla, plus deltoid, and latissimus dorsi muscles

5. C7 Radiculopathy

A. Nerve conduction studies—radial motor study may show low-amplitude response (if axonal)

B. EMG—denervation as in axilla, plus flexor carpi radialis and cervical paraspinal muscles, but sparing the brachioradialis and supinator muscles aPIN, posterior interosseous neuropathy; From ref. 1.

and brachioradialis); two nonradial nerve, C7-innervated muscles (e.g., pronator teres, flexor pollicis longus, flexor carpi radialis, and cervical paraspinal muscles); one radial-innervated muscle proximal to the spiral groove (e.g., triceps muscle); and one nonradial, posterior cord-innervated muscle (e.g., deltoid). Table 8 summarizes the different electrophysiological abnormalities that are encountered in the various radial nerve lesions discussed.

4.2. Other Focal Mononeuropathies of the Upper Extremity 4.2.1. Suprascapular Neuropathy

This entrapment most commonly occurs at the suprascapular notch, under the transverse scapular ligament. Less frequently, the nerve can be entrapped distally at the spinoglenoid notch. The most common symptom is shoulder pain. Shoulder pain is typically deep and boring, and occurs along the superior aspect of the scapula, with radiation into the shoulder. The pain may be exacerbated by adduction of the extended arm.

Patients may demonstrate weakness of shoulder abduction (supraspinatus) and external rotation (infraspinatus). Atrophy may be present in cases that are more severe. If the nerve is entrapped distally at the spinoglenoid notch, the deficit is limited to the infraspinatus muscle only. In this case, pain is usually absent because the deep sensory fibers to the shoulder joint arise proximal to the lesion.

Various conditions that should be excluded include C5-C6 radiculopathy, upper trunk brachial plexopathy, rotator cuff injury, and other orthopedic conditions of the shoulder.

In rare cases, a focal suprascapular neuropathy may be the primary manifestation of a more diffuse autoimmune-mediated brachial plexopathy (neuralgic amyotrophy).

Electrodiagnostic studies should identify the involvement of suprascapular-innervated muscles, and exclude other causes. Routine nerve conduction studies of the upper extremity should be normal. Motor studies of the suprascapular nerve can be performed, but require careful technique, and, therefore, are not commonly performed. Compound muscle action potentials may be recorded using a monopolar needle electrode placed in the spinatii muscles while stimulating over Erb's point. Needle examination is very useful, and will help identify abnormalities limited to the spinatii muscles, thus, supporting the diagnosis of suprascapular neuropathy. An MRI of the shoulder is usually recommended to exclude a ganglion cyst causing compression of the nerve.

4.2.2. Axillary Neuropathy

This neuropathy typically occurs in the setting of trauma (dislocation of the shoulder or fracture of the humerus). Clinically, patients have a well-demarcated "patch" of numbness along the lateral aspect of the shoulder, and will have weakness of shoulder abduction (deltoid) and external rotation (teres minor). Differential diagnosis is similar to that of a suprascapular neuropathy. Routine nerve conduction studies are, again, normal. Motor study of the axillary nerve can be performed but, again, requires careful technique and experience. The needle examination usually demonstrates abnormalities in the deltoid and teres minor muscles only.

4.2.3. Long Thoracic Neuropathy

The long thoracic nerve arises directly from the C5, C6, and C7 nerve roots proximal to the brachial plexus. The nerve supplies the serratus anterior muscle exclusively. Most commonly, this neuropathy is seen in patients with neuralgic amyotrophy. Dysfunction of the long thoracic nerve can result from traumatic injuries that cause widespread damage to multiple cervical nerve roots. Isolated cases of long thoracic neuropathy, although rare, are observed. Patients will exhibit scapular winging with their arms outstretched. Electrodiagnostic studies should be performed to determine if a more widespread process exists.

4.2.4. Musculocutaneous Neuropathy

An isolated musculocutaneous neuropathy is rare. Most commonly, it occurs as part of a widespread traumatic lesion involving the shoulder and arm. Patients will have weakness of elbow flexion, sensory loss over the lateral forearm (lateral antebrachial cutaneous nerve), and an absent biceps reflex. The electrodiagnostic study should localize the lesion to this nerve, and exclude a brachial plexopathy or cervical radiculopathy. The lateral antebrachial cutaneous sensory study should be abnormal as compared with the contralateral side. Motor study of the musculocutaneous nerve can be performed, but careful technical skill is required to ensure accuracy of the result. Needle examination should demonstrate abnormalities in the biceps, brachialis, and coracobrachialis muscles.

4.2.5. Spinal Accessory Neuropathy

Isolated lesions of this nerve occur in the region of the posterior cervical triangle and result in isolated weakness of the trapezius muscle. Stretch injuries may occur, but usually this neuropathy results after local surgical procedures. Patients may have shoulder drop caused by weakness of the trapezius muscle. If the lesion is more proximal, there may be weakness of the sternoclei-domastoid muscle as well. The needle examination provides the greatest usefulness with this type of entrapment syndrome. Routine nerve conduction studies of the upper extremity should be normal. A careful needle examination should exclude brachial plexus dysfunction.

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Responses

  • susanna
    When having a emg nerve conduction test what is decreased axillary amplitude bilaterally?
    6 years ago

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