In the electromyography (EMG) laboratory, the radial nerve is studied less frequently than the median and ulnar nerves and their respective well-known lesions. Nevertheless, entrapment of the radial nerve does occur, often affecting the main radial nerve either in the upper arm or axilla. Isolated lesions of its terminal divisions in the forearm, the posterior interosseous and superficial radial sensory nerves, also occur. Although radial motor nerve conduction studies are technically demanding, the electrophysiologic evaluation of radial neuropathy usually is able to localize the lesion, assess the underlying pathophysiology, and provide useful information regarding severity and subsequent prognosis.
Anatomy
The radial nerve receives innervation from all three trunks of the brachial plexus and, correspondingly, a contribution from each of the C5–T1 nerve roots ( Figures 21–1 and 21–2 ). After each trunk divides into an anterior and posterior division, the posterior divisions from all three trunks unite to form the posterior cord. The posterior cord gives off the axillary, thoracodorsal, and subscapular nerves before becoming the radial nerve. In the high arm, the radial nerve first gives off the posterior cutaneous nerve of the arm , the lower lateral cutaneous nerve of the arm , and the posterior cutaneous nerve of the forearm ( Figure 21–3 ), followed by muscular branches to the three heads of the triceps brachii (medial, long, and lateral) and the anconeus. The anconeus is a small muscle in the proximal forearm that effectively is an extension of the medial head of the triceps brachii. After giving off these muscular branches, the radial nerve wraps around the posterior humerus in the spiral groove . Descending into the region of the elbow, muscular branches are then given off to the brachioradialis and the long head of the extensor carpi radialis. Next, three to four cm distal to the lateral epicondyle, the radial nerve bifurcates into two separate nerves: one superficial and the other deep. The superficial branch, known as the superficial radial sensory nerve , descends distally into the forearm over the radial bone to supply sensation over the lateral dorsum of the hand as well as part of the thumb and the dorsal proximal phalanges of the index, middle, and ring fingers ( Figure 21–4 ). Distally, the nerve is quite superficial, running over the extensor tendons to the thumb, where it can easily be palpated ( Figure 21–5 ).
The deep branch, known as the deep radial motor branch , first supplies the extensor carpi radialis brevis and the supinator muscles before it enters the supinator muscle under the Arcade of Frohse ( Figure 21–6 ). The Arcade of Frohse is the proximal border of the supinator and in some individuals is quite tendinous. After the nerve enters the supinator, it is known as the posterior interosseous nerve, which then supplies the remaining extensors of the wrist, thumb, and fingers (extensor digitorum communis, extensor carpi ulnaris, abductor pollicis longus, extensor indicis proprius [EIP], extensor pollicis longus, and extensor pollicis brevis). Although the posterior interosseous nerve is thought of as a pure motor nerve (supplying no cutaneous sensation), it does contain sensory fibers that supply deep sensation to the interosseous membrane and joints between the radial and ulna bones.
Nomenclature of the Branches of the Radial Nerve near the Elbow
One of the more confusing aspects of radial nerve anatomy is the inconsistency regarding the nomenclature of the branches of the radial nerve near the elbow used in various anatomic texts and clinical reports ( Figure 21–7 ). The following points should help the electromyographer when dealing with potential lesions of the radial nerve in this area:
Radial Nerve between the Spiral Groove and the Bifurcation near the Elbow
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Distal to the spiral groove but before the elbow, the main radial nerve always supplies two muscles: the brachioradialis and the extensor carpi radialis longus (also known as the long head of the extensor carpi radialis).
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In some individuals, the main radial nerve will also supply a third muscle, the extensor carpi radialis brevis muscle * .
The Bifurcation near the Elbow
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The main radial nerve always bifurcates into superficial and deep branches just distal to the elbow.
Superficial Branch
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The superficial branch continues as a pure cutaneous sensory branch (the superficial radial sensory branch ).
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However, in a small number of individuals, there is an anatomic variation wherein the superficial branch near its origin will supply one muscle, the extensor carpi radialis brevis*.
Deep Branch
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The deep radial motor branch first supplies the extensor carpi radialis brevis muscle in some individuals*.
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It then supplies one or more branches to the supinator muscle before entering the supinator muscle proper.
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The deep radial motor branch then runs under the Arcade of Frohse (the proximal border of the supinator) and through the supinator muscle.
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After leaving the supinator muscle, branches are given off that supply the extensor muscles to the thumb and fingers as well as the abductor pollicis longus and extensor carpi ulnaris. The inconsistency in the nomenclature regarding these nerve branches involves where the posterior interosseous nerve begins, and whether the posterior interosseous nerve and the deep radial motor branch are one and the same nerve:
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In some textbooks and many clinical reports, the entire deep radial motor branch is known as the posterior interosseous nerve , with the two names used interchangeably. Thus, using this anatomic definition, a complete posterior interosseous neuropathy would include the supinator and the extensor carpi radialis brevis muscles, as well as the extensors to the thumb and fingers, and the abductor pollicis longus and extensor carpi ulnaris.
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In most anatomic texts, however, only the segment of the deep branch between the bifurcation of the main radial nerve at the elbow to where the nerve enters the supinator muscle at the Arcade of Frohse is known as the deep radial motor branch . The posterior interosseous nerve is then the continuation of the deep radial motor branch after it enters the supinator . In the remainder of this text, we will use this latter anatomic definition. Thus, with this anatomic definition, a complete posterior interosseous neuropathy would spare the supinator and the extensor carpi radialis brevis muscles. As the most common entrapment site of the posterior interosseous nerve is at the Arcade of Frohse, the use of this anatomic convention fits the common clinical syndromes most appropriately as well.
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* Thus, the innervation to the extensor carpi radialis brevis has several normal variations: from the main radial nerve, the superficial radial nerve, and the deep radial motor branch of the radial nerve.
Clinical
Radial neuropathies can be divided into those caused by lesions at the spiral groove, lesions in the axilla, and isolated lesions of the posterior interosseous and superficial radial sensory nerves. These lesions usually can be differentiated by clinical findings.
Radial Neuropathy at the Spiral Groove
The most common radial neuropathy occurs at the spiral groove. Here, the nerve lies juxtaposed to the humerus and is quite susceptible to compression, especially following prolonged immobilization ( Figure 21–8 ). One of the times this characteristically occurs is when a person has draped an arm over a chair or bench during a deep sleep or while intoxicated (‘Saturday night palsy’). The subsequent prolonged immobility results in compression and demyelination of the radial nerve. Other cases may occur after strenuous muscular effort, fracture of the humerus, or infarction from vasculitis. Clinically, marked wrist drop and finger drop develop (due to weakness of the EIP, extensor digitorum communis, extensor carpi ulnaris, and long head of the extensor carpi radialis), along with mild weakness of supination (due to weakness of the supinator muscle) and elbow flexion (due to weakness of the brachioradialis). Notably, elbow extension (triceps brachii) is spared. Sensory disturbance is present in the distribution of the superficial radial sensory nerve, consisting of altered sensation over the lateral dorsum of the hand, part of the thumb, and the dorsal proximal phalanges of the index, middle, and ring fingers.
In isolated radial neuropathy at the spiral groove, median- and ulnar-innervated muscles are normal. However, tested in a wrist drop and finger drop posture, finger abduction may appear weak, giving the mistaken impression of ulnar nerve dysfunction. To prevent this error, one should test the patient’s finger abduction (ulnar-innervated function) with the fingers and wrist passively extended to a neutral wrist position. This often can be accomplished by placing the hand on a flat surface.
Radial Neuropathy in the Axilla
Radial neuropathy may occur in the axilla from prolonged compression. For instance, this is often seen in patients on crutches who use them inappropriately, applying prolonged pressure to the axilla. The clinical deficit is similar to that seen in radial neuropathy at the spiral groove, with the notable exception of additional weakness of arm extension (triceps brachii) and sensory disturbance extending into the posterior forearm and arm (posterior cutaneous nerves of the forearm and arm). Radial neuropathy in the axilla is differentiated from even more proximal posterior cord lesions by normal strength of the deltoid (axillary nerve) and latissimus dorsi (thoracodorsal nerve).
Posterior Interosseous Neuropathy
Posterior interosseous neuropathy (PIN) clinically resembles entrapment of the radial nerve at the spiral groove at first glance. In both conditions, patients present with wrist drop and finger drop with sparing of elbow extension. However, with closer inspection, several important differences easily separate the two. In PIN, there is sparing of radial-innervated muscles above the takeoff of the posterior interosseous nerve (i.e., brachioradialis, long and short heads of the extensor carpi radialis, triceps). Thus, a patient with PIN still may be able to extend the wrist, but weakly, with a radial deviation. This is due to the relative preservation of the extensor carpi radialis longus and brevis that arise proximal to the posterior interosseous nerve, with a weak extensor carpi ulnaris. In addition, of course, are the sensory findings. In PIN, there is no cutaneous sensory loss. However, there may be pain in the forearm from involvement of the deep sensory fibers of the posterior interosseous nerve that supply the interosseous membrane and joint capsules.
PIN usually occurs as an entrapment neuropathy under the tendinous Arcade of Frohse. Rarely, other mass lesions (e.g., ganglion cysts, tumors) result in PIN.
Radial Tunnel Syndrome
In radial tunnel syndrome, patients are reported to have isolated pain and tenderness in the extensor forearm, not unlike persistent tennis elbow, thought to result from compression of the posterior interosseous nerve near its origin. However, this is one of the more controversial and disputed nerve entrapment syndromes. As opposed to patients with a true posterior interosseous neuropathy (see above), these patients typically have no objective neurologic signs on examination, and accordingly have normal EDX studies. They are said to have increased pain with maneuvers that contract the extensor carpi radialis or the supinator (e.g., resisted extension of the middle finger or resisted supination, respectively). However, there is no compelling evidence that this chronic pain syndrome is caused by any nerve entrapment. Nevertheless, this syndrome is important to know of, as it is not unusual for a patient to be referred to the EMG laboratory for evaluation of “radial tunnel syndrome.” In such cases, the focus of the EDX is to look for any objective evidence of a posterior interosseous neuropathy, although in the absence of any weakness or other neurological signs, the EDX study is almost always normal.
Superficial Radial Sensory Neuropathy
The superficial radial sensory nerve is derived from the main radial nerve in the region of the elbow. In the distal third of the forearm, it runs subcutaneously next to the radius. Its superficial location next to bone makes it extremely susceptible to compression, a syndrome coined “Cheiralgia Paresthetica” which translates from the Greek as cheir + algos , meaning pain in the hand. Tight-fitting bands, watches, or bracelets may result in compression of the superficial radial nerve. Handcuffs, especially when excessively tight, also characteristically result in a superficial radial neuropathy. Because the superficial radial sensory nerve is purely sensory, no weakness develops. A characteristic patch of altered sensation develops over the lateral dorsum of the hand, part of the thumb, and the dorsal proximal phalanges of the index, middle, and ring fingers.
Differential Diagnosis
The differential diagnosis of wrist drop, aside from a radial neuropathy at the spiral groove, axilla, and PIN, includes unusual presentations of C7–C8 radiculopathy, brachial plexus lesions, and central causes ( Box 21–1 ). Because most muscles that extend the wrist and fingers are innervated by the C7 nerve root, C7 radiculopathy may rarely present solely with a wrist drop and finger drop, with relative sparing of non-radial C7-innervated muscles. However, several key clinical features help differentiate a C7 radiculopathy from a radial neuropathy, PIN, brachial plexopathy, or central lesion ( Table 21–1 ). Radial neuropathy at the spiral groove or axilla should result in weakness of the brachioradialis, a C5–C6-innervated muscle, which should not be weak in a lesion of the C7 nerve root. On the other hand, radial neuropathy at the spiral groove and PIN should spare the triceps, which would be expected to be weak in a C7 radiculopathy. If a C7 radiculopathy is severe enough to cause muscle weakness, other non-radial C7-innervated muscles also should be weak (e.g., pronator teres, flexor carpi radialis), leading to weakness of arm pronation and wrist flexion. However, in rare situations, non-radial C7-innervated muscles may be relatively spared, making the clinical differentiation quite difficult.
Posterior interosseous nerve
Radial nerve at the spiral groove
Radial nerve in the axilla
Posterior cord of the brachial plexus
C7 root
Central nervous system
| Posterior Interosseous Neuropathy | Radial Nerve: Spiral Groove | Radial nerve: Axilla | Posterior Cord | C7 | |
|---|---|---|---|---|---|
| Wrist drop or finger drop | X | X | X | X | X |
| Radial deviation on wrist extension | X | ||||
| Weakness of supination (mild) | X | X | X | ||
| Weakness of elbow flexion (mild) | X | X | X | ||
| Diminished brachioradialis tendon reflex | X | X | X | ||
| Weakness of elbow extension | X | X | X | ||
| Diminished triceps tendon reflex | X | X | X | ||
| Weakness of shoulder abduction | X | ||||
| Sensory loss in lateral dorsal hand | X | X | X | X (equivocal) | |
| Sensory loss in posterior arm or forearm | X | X | X (equivocal) | ||
| Weakness of wrist flexion | X |
Although lesions of the posterior cord of the brachial plexus result in weakness of radial-innervated muscles, the deltoid (axillary nerve) and latissimus dorsi (thoracodorsal nerve) should also be weak. Central lesions may result in a wrist drop and finger drop. The typical upper motor neuron posture results in flexion of the wrist and fingers, which in the acute phase or when the lesion is mild may superficially resemble a radial neuropathy. Central lesions are identified by increased muscle tone and deep tendon reflexes (unless acute), slowness of movement, associated findings in the lower face and leg, and altered sensation beyond the radial distribution.
Electrophysiologic Evaluation
In the evaluation of a patient with a wrist drop, the role of nerve conduction studies and EMG is to identify a potential radial neuropathy, assess its location and severity, and, by defining the underlying pathophysiology, establish a prognosis ( Table 21–2 ).
| Posterior Interosseous Neuropathy | Radial Nerve: Spiral Groove | Radial Nerve: Axilla | Posterior Cord | C7 | |
|---|---|---|---|---|---|
| EMG Findings | |||||
| Extensor indicis proprius | X | X | X | X | X |
| Extensor digitorum communis | X | X | X | X | X |
| Extensor carpi ulnaris | X | X | X | X | X |
| Extensor carpi radialis-long head | X | X | X | X | |
| Brachioradialis | X | X | X | ||
| Supinator | X | X | X | ||
| Anconeus | X | X | X | ||
| Triceps | X | X | X | ||
| Deltoid | X | ||||
| Latissimus dorsi | X | X | |||
| Flexor carpi radialis, pronator teres | X | ||||
| Cervical paraspinal muscles | X | ||||
| Nerve Conduction Study Findings | |||||
| Abnormal radial SNAP (if axonal) | X | X | X | ||
| Low radial CMAP (if axonal) | X | X | X | X | X |
| Conduction block at spiral groove (if demyelinating) | X | ||||
| Conduction block between forearm and elbow (if demyelinating) | X |
Nerve Conduction Studies
The most important nerve conduction study in assessing a wrist drop is the radial motor study ( Box 21–2 ). A radial compound muscle action potential (CMAP) can be recorded over the EIP muscle, placing the active electrode two fingerbreadths proximal to the ulnar styloid with a reference electrode placed over the ulnar styloid ( Figure 21–9 ). The radial nerve can be stimulated in the forearm, at the elbow (in the groove between the biceps and brachioradialis muscles), and below and above the spiral groove. The normal CMAP recorded from the EIP typically is 2 to 5 mV. Comparing the CMAP amplitude to that on the contralateral asymptomatic side is always important. Any axonal loss will result in a decreased distal CMAP amplitude after 3 to 5 days, when wallerian degeneration for motor fibers has occurred. In fact, the best way to assess the degree of axonal loss is to compare the CMAP amplitudes between the involved side and the contralateral side.
Routine studies:
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Radial motor study recording extensor indicis proprius, stimulating forearm, elbow, below spiral groove, and above spiral groove; bilateral studies
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Ulnar motor study recording abductor digiti minimi, stimulating wrist, below groove, and above groove in the flexed elbow position
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Median motor study recording abductor pollicis brevis, stimulating wrist and antecubital fossa
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Median and ulnar F responses
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Superficial radial sensory study recording over the extensor tendons to thumb, stimulating forearm; bilateral studies
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Ulnar sensory study recording digit 5, stimulating wrist
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Median sensory study recording digit 2 or 3, stimulating wrist
The following patterns may result:
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Posterior interosseous neuropathy (axonal loss lesion) : Normal superficial radial SNAP, low amplitude distal radial CMAP.
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Posterior interosseous neuropathy (demyelinating lesion) : Normal superficial radial SNAP, normal amplitude distal radial CMAP with motor conduction block between forearm and elbow.
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Posterior interosseous neuropathy (mixed axonal loss and demyelinating lesion) : Normal superficial radial SNAP, low amplitude distal radial CMAP with motor conduction block between forearm and elbow.
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Radial neuropathy at the spiral groove (axonal loss lesion) : Reduced superficial radial SNAP, low-amplitude distal radial CMAP. No conduction block across spiral groove.
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Radial neuropathy at the spiral groove (demyelinating lesion) : Normal superficial radial SNAP, normal amplitude distal radial CMAP with conduction block across spiral groove.
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Radial neuropathy at the spiral groove (mixed axonal loss and demyelinating lesion) : Reduced superficial radial SNAP, low amplitude distal radial CMAP with conduction block across spiral groove.
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Radial neuropathy at the axilla (axonal loss lesion) : Reduced superficial radial SNAP, low amplitude distal radial CMAP.
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Radial neuropathy at the axilla (demyelinating lesion) : Normal superficial radial SNAP, normal amplitude distal radial CMAP with normal motor study to above spiral groove.
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Superficial radial sensory neuropathy : Reduced superficial radial SNAP, normal radial motor study.
CMAP, compound muscle action potential; SNAP, sensory nerve action potential.
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