Radial tunnel syndrome is an uncommon entrapment neuropathy of the radial nerve just below the elbow that is caused by compression of the posterior interosseous branch of the radial nerve by a variety of pathologic processes that have in common their ability to compress the nerve as it travels through the radial tunnel. The most common cause of radial tunnel syndrome is the sharp proximal tendinous margin of the supinator muscle, which is known as the arcade of Frohse (Fig. 58.1). Other pathologic processes that have been implicated in the development of radial tunnel syndrome include anomalous radial recurrent blood vessels that compress the nerve, ganglion cysts, an aberrant aponeurotic band that runs anterior to the radial head, and the sharp tendinous margin of the extensor carpi radialis brevis. These abnormalities may work alone or together to compromise the radial nerve at this anatomic location (Fig. 58.1). Patients suffering from radial tunnel syndrome will experience pain and dysesthesias radiating from the site of compression to the area just below the lateral epicondyle of the humerus. The onset of radial tunnel syndrome can be acute following twisting injuries to the elbow or as a result of direct trauma to the area overlying the radial tunnel. More commonly, the onset of radial tunnel syndrome is insidious and is usually the result of misuse of overuse of the elbow joint and proximal forearm from repetitive pronation and supination. Radial tunnel syndrome has been reported in orchestra conductors, Frisbee players, and swimmers. If this entrapment neuropathy is not treated, pain and functional disability may become more severe, and, ultimately, permanent weakness of the finger extensors and radial deviation of the wrist on extension may result.

Physical findings in patients suffering from radial tunnel syndrome will exhibit weakness of the finger extensors and radial deviation on wrist extension. A Tinel sign will be present at the point where the radial nerve passes through the radial tunnel and the nerve will be tender to palpation. Patients suffering from radial tunnel syndrome exhibit pain on active resisted supination of the forearm and a positive radial tunnel compression test. The radial tunnel compression test is performed by tightly compressing the area over the radial tunnel for 30 seconds (Fig. 58.2). The test is considered positive if the patient experiences dysesthesias in the distribution of the radial nerve and increasing weakness of grip strength.

Radial tunnel syndrome often is misdiagnosed as tennis elbow and can be distinguished from tennis elbow by determining the site of maximal tenderness to palpation. Patients suffering from radial tunnel syndrome will experience maximal tenderness to palpation over the radial nerve 1 inch below the lateral epicondyle, whereas patients suffering from tennis elbow will experience maximal tenderness to palpation that is directly over the lateral epicondyle. Radial tunnel syndrome also should be differentiated from C7 cervical radiculopathy. Furthermore, it should be remembered that cervical radiculopathy and radial nerve entrapment may coexist as the so-called double crush syndrome. The double crush syndrome is seen most commonly with median nerve entrapment at the wrist or with carpal tunnel syndrome but has been reported with the radial nerve.

Electromyography and nerve conduction velocity testing are useful in helping in the differentiation of radial tunnel syndrome from cervical radiculopathy and golfer’s elbow. Plain radiographs, ultrasound imaging, and magnetic resonance imaging are indicated in all patients who present with radial tunnel syndrome in order to rule out occult bony pathology and to identify occult fractures, masses, or tumors that may be responsible for compromise of the radial nerve (Figs. 58.3 and 58.4). In patients suffering from radial tunnel syndrome, magnetic resonance imaging of the radial tunnel will often reveal edema of the supinator muscle (Fig. 58.5) Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood count, uric acid, sedimentation rate, and antinuclear antibody testing. The ultrasound-guided injection technique described below serves as both a diagnostic and therapeutic maneuver.

The key landmark when performing ultrasound-guided radial nerve block at the elbow is the point at which the radial nerve is just below the point within the substance of the brachioradialis muscle where the radial nerve bifurcates (Fig. 58.6). Arising from fibers from the C5 to T1 nerve roots of the posterior cord of the brachial plexus, the radial nerve passes through the axilla lying posterior and inferior to the axillary artery. As the radial nerve exits the axilla, it passes between the medial


and long heads of the triceps muscle and then curves across the posterior aspect of the humerus, giving off a motor branch to the triceps muscle. Continuing its downward path, the radial nerve gives off a number of sensory branches to the upper arm as it travels in the intermuscular septum separating the bellies of the brachialis and brachioradialis muscles. The nerve passes into the substance of the brachioradialis muscle, and at a point just above the lateral epicondyle, the radial nerve divides into deep and superficial branches; the superficial branch continues down the arm along with the radial artery to provide sensory innervation to the dorsum of the wrist and the dorsal aspects of a portion of the thumb and index and middle fingers, and the deep branch crosses the supinator muscle to provide the majority of the motor innervation to the extensors of the forearm. The posterior interosseous nerve is the continuation of the deep branch of the radial nerve, which contains terminal motor fibers. It descends on the interosseous membrane, in front of the extensor pollicis longus, to the dorsal aspect of the carpal bones where it sends off fibers to supply the finger and thumb extensors, extensor carpi ulnaris, and the abductor pollicis longus region.