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Radial Neuropathy


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. In addition, similar to other entrapment neuropathies, neuromuscular ultrasound is often very useful in adding specific anatomic information regarding the location and etiology of a radial neuropathy.

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 ( Figs. 24.1 and 24.2 ). 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 ( Fig. 24.3 ), followed by muscular branches to the three heads of the triceps brachii (medial, long, and lateral) and the anconeus. In some patients, there is evidence that the long head of the triceps may be supplied either by the axillary nerve or the posterior cord directly. 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 . The posterior cutaneous nerve of the forearm accompanies the radial nerve through the spiral groove and remains in the posterior compartment of the arm before becoming subcutaneous approximately 6–7 cm directly proximal to the lateral epicondyle. Descending into the region of the elbow, the main radial nerve then pierces the lateral intermuscular septum to run between the brachialis and brachioradialis muscles. Muscular branches are then given off to the brachioradialis and the long head of the extensor carpi radialis. The radial nerve then enters the radial tunnel, which is the space formed posteriorly by the distal humerus and radiocapitellar joint, the brachialis muscle medially, the brachioradialis muscle anteriorly, and the extensor carpi radialis brevis muscle laterally. The radial tunnel is approximately 5 cm in length and runs between the area where the radial nerve pierces the lateral intermuscular septum to where the deep motor branch enters the proximal edge of the supinator. a

a Some consider the radial tunnel to continue to where the posterior interosseous nerve leaves the distal border of the supinator.

Next, 3–4 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 under the brachioradialis in the forearm and eventually moves subcutaneous 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 ( Fig. 24.4 ). Distally, the nerve is quite superficial, running over the tendon to the extensor pollicis longus, where it can easily be palpated ( Fig. 24.5 ).

Fig. 24.1, Anatomy of the radial nerve.

Fig. 24.2, Anatomy of the radial nerve.

Fig. 24.3, Sensory territories supplied by the radial nerve.

Fig. 24.4, Sensory territory of the superficial radial sensory nerve.

Fig. 24.5, Superficial radial sensory nerve.

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 ( Fig. 24.6 ). 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.

Fig. 24.6, Anatomy of the radial nerve at the elbow.

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 ( Fig. 24.7 ). The following points should help the electromyographer when dealing with potential lesions of the radial nerve in this area:

Fig. 24.7, Anatomy and nomenclature of the radial nerve around the elbow.

Radial Nerve Between the Spiral Groove and the Bifurcation Near the Elbow

  • 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).

  • In some individuals, the main radial nerve will also supply a third muscle, the extensor carpi radialis brevis muscle. b

    b 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.

The Bifurcation Near the Elbow

  • The main radial nerve always bifurcates into superficial and deep branches just distal to the elbow.

Superficial Branch

  • The superficial branch continues as a pure cutaneous sensory branch (the superficial radial sensory branch ).

  • 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. b

Deep Branch

  • The deep radial motor branch first supplies the extensor carpi radialis brevis muscle in some individuals. b

  • It then supplies one or more branches to the supinator muscle before entering the supinator muscle proper.

  • The deep radial motor branch then runs under the Arcade of Frohse (the proximal border of the supinator) and through the supinator muscle.

  • 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:

    • 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 (PIN) 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.

    • 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 PIN 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.

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 ( Fig. 24.8 ). 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.

Fig. 24.8, Radial nerve and the spiral groove.

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

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.

Five potential sites of compression of the deep radial motor branch/posterior interosseous nerve have been reported. These include (1) the medial proximal edge of the extensor carpi radialis brevis muscle; (2) the fibrous tissue anterior to the radiocapitellar joint between the brachialis and brachioradialis muscles; (3) the “Leash of Henry” (recurrent radial vessels that fan over the deep motor branch proximal to the supinator; (4) the Arcade of Frohse; and (5) the distal edge of the supinator muscle. PIN most often 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

This is one of the more controversial and disputed nerve entrapment syndromes. 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, as opposed to patients with a true PIN (see previous discussion), 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 little compelling evidence that this chronic pain syndrome is caused by any nerve entrapment in most patients. 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 PIN, although in the absence of any weakness or other neurologic signs, the EDX study is almost always normal. Nevertheless, follow-up with an ultrasound study of the deep motor branch of the radial nerve/posterior interosseous nerve can be useful to exclude any structural abnormalities of this nerve (see later).

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