Radial Nerve Entrapment 

Radial nerve compression or injury may occur at any point along the anatomic course of the nerve and may have varied etiologies. The most frequent site of compression is in the proximal forearm in the area of the supinator muscle and involves the posterior interosseous branch. However, problems can occur proximally in relation to fractures of the humerus at the junction of the middle and proximal thirds, as well as distally on the radial aspect of the wrist.^{[1, 2, 3, 4, 5, 6]}

Recent studies

Toros et al prospectively studied 26 patients with sensory or motor deficiency of a peripheral nerve caused by trauma or entrapment using ultrasonography. They confirmed that high-resolution ultrasound can show the exact location, extent, and type of lesion, yielding important information that might not be obtained by other modalities. In the study, ultrasonography provided reliable visualization of injured nerves in all patients. Axonal swelling and hypoechogenicity of the nerve was diagnosed in 15 cases, loss of continuity of a nerve bundle in 17, formation of a neuroma of a stump in 6, and partial laceration of a nerve with loss of the normal fascicular pattern in 5. The ultrasound findings were confirmed at operation in the 19 patients who underwent surgery.^[7]

Clavert et al dissected 30 cadaveric upper limbs to define the radial nerve anatomic landmarks and todetermine the relationship of the radial nerve main trunk and branches to the peripheral osseous and muscular structures in the anterior aspect of the elbow joint, so as to identify likely causes of compressive neuropathy. No radial compressive neuropathy was found at the level of the supinator arch, and no adhesions were identified between the radial nerve and the joint capsule. In 4 cases, dense fibrous tissue surrounded the radial nerve supply to the extensor carpi radialis brevis. Neither fibrous structures nor adhesions of the deep branch of the radial nerve were observed along its course through the supinator muscle. The fibrous arch of the supinator muscle arose in a semicircular manner and was tendinous in 87% of extremities and membranous in 13%. Average length of the Frohse arcade was 25.9 mm, and the angle formed by the radial shaft and supinator arch was 23 degrees.^[8]

Lo et al compared ultrasonography with electrophysiologic testing in the diagnosis of entrapment neuropathy of the radial nerve at the spiral groove, which is relatively common but technically challenging to localize. The study included 32 normal controls, to obtain ultrasound parameters of the radial nerve, and 10 patients with suspected radial neuropathy, in whom ultrasound and electrophysiologic techniques were used. Ultrasonography correctly identified all 6 patients with radial neuropathy; the 4 patients with other diagnoses did not show ultrasound abnormalities. Ultrasonography also required significantly less time than electrophysiologic testing.^[9]

Noaman et el studied 36 patients who underwent early surgical exploration of radial nerve injury associated with fracture of the shaft of the humerus. A narrow dynamic compression plate was generally used for fixation. Compression at the lateral intermuscular septum was present in 19 cases, entrapment in the fracture site in 9 cases, and loss of continuity in 8 cases. Neurolysis was required in 20 cases, epineurorrhaphy in 9 cases, interfascicular nerve grafts in 5, and first-intention tendon transfer in 2. Mean follow-up was 8.2 years. Outcome was rated good to excellent in 28 patients, fair in 1, and poor (failure) in 3. First-intention tendon transfers were performed in 2 patients, and 2 patients were lost to follow-up. Mean delay to recovery was 7 months after neurolysis and nerve repair and 15 months after nerve grafts. Fracture was united in all cases, and the mean time of union was 5 months.^[10]

Compression or scarring of the radial nerve at different points along its course may cause denervation of extensor/supinator muscles and numbness or paresthesias in the distribution of the radial sensory nerve (RSN). The result can be pain, weakness, and dysfunction.^[2]

Frequency

Among the problems associated with the 3 major nerves in the upper extremity, radial nerve entrapment is the least common. Carpal tunnel syndrome (median nerve compression at the wrist) and cubital tunnel syndrome (ulnar nerve compression at the elbow) are much more frequent.^{[11, 12]}

Radial nerve palsy

Radial nerve palsy in the arm most commonly is caused by fracture of the humerus, especially in the middle third (Holstein-Lewis fracture) or at the junction of the middle and distal thirds. The nerve may be compressed by the lateral intermuscular septum. This palsy may occur acutely at the time of the injury, secondary to fracture manipulation, or from a healing callus. Other less common causes of radial nerve palsy in the arm include compression at the fibrous arch of the lateral head of the triceps muscle and compression by an accessory subscapularis-teres-latissimus muscle.^{[13, 14, 15, 16, 17]}

Radial tunnel syndrome

This diagnosis is highly controversial and is thought to be a result of overuse. Some authors believe radial tunnel syndrome may represent an early posterior interosseous nerve syndrome.^[3] Sites of compression include the fibrous bands attached to the radiocapitellar joint, radial recurrent vessels, the tendinous origin of the extensor carpi radialis brevis, the tendinous origin of the supinator (ie, arcade of Frohse), and fibrous thickenings within and at the distal margin of the supinator.^{[18, 19, 20]}

Posterior interosseous nerve syndrome

The etiology of posterior interosseous nerve syndrome is similar to that of radial tunnel syndrome. Compression is thought to occur after takeoff of the branches to the radial wrist extensors and the radial sensory nerve (RSN). After emerging from the supinator, the nerve may be compressed before it bifurcates into medial and lateral branches, causing a complete paralysis of the digital extensors and dorsoradial deviation of the wrist secondary to paralysis of the extensor carpi ulnaris. If compression occurs after the nerve bifurcates, selective paralysis of muscles occurs, depending on which branch is involved. Compression of the medial branch causes paralysis of the extensor carpi ulnaris, extensor digiti quinti, and extensor digitorum communis. Compression of the lateral branch causes paralysis of the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis proprius. Most commonly, entrapment occurs at the proximal edge of the supinator.

Other possible etiologies for posterior interosseous nerve dysfunction include trauma (Monteggia fractures), synovitis (rheumatoid), tumors, and iatrogenic injuries.

Wartenberg syndrome, described in 1932, is essentially entrapment of the superficial sensory branch of the radial nerve.

Many factors may contribute to the development of Wartenberg syndrome. In patients with de Quervain tenosynovitis, secondary irritation of the RSN is frequent. Other common causes include postsurgical injury, external compression, and trauma.

The anatomic site of compression corresponds to the transit of the nerve from its submuscular position beneath the brachioradialis to its subcutaneous position on the extensor carpi radialis longus. Especially with pronation, these 2 muscles can create a scissorlike effect compressing the RSN.

Nerve injury secondary to compression or traction depends on intensity and duration.^[4]

Seddon has classified nerve injuries into 3 categories^[21] :

• The first, neuropraxia, is a transient episode of motor paralysis with little or no sensory or autonomic dysfunction. No disruption of the nerve or its sheath occurs. With removal of the compressing force, recovery should be complete.
• The second, axonotmesis, is a more severe nerve injury with disruption of the axon but with maintenance of the Schwann sheath. Motor, sensory, and autonomic paralysis results. Recovery can occur if the compressing force is removed in a timely fashion and if the axon regenerates.
• The third, neurotmesis is the most serious injury. The nerve and its sheath are disrupted. Although recovery may occur, it is never complete, secondary to loss of nerve continuity.

Sunderland has classified nerve injury into 5 categories^[4] :

• The first is similar to neuropraxia.
• The second is similar to axonotmesis.
• The third, fourth, and fifth degrees correspond to varying degrees of neurotmesis.

Radial nerve palsy

Radial nerve palsy in the middle third of the arm is characterized by palsy or paralysis of all extensors of the wrist and digits, as well as the forearm supinators. Very proximal lesions also may affect the triceps. Numbness occurs on the dorsoradial aspect of the hand and the dorsal aspect of the radial 3 ½ digits. Sensation over the distal and lateral forearm is supplied by the lateral antebrachial cutaneous nerve and therefore is preserved.^{[13, 14, 15, 16, 17]}

Radial tunnel syndrome

Radial tunnel syndrome is characterized by pain over the anterolateral proximal forearm in the region of the radial neck. This syndrome often appears in individuals whose work requires repetitive elbow extension or forearm rotation. The maximum tenderness is located 4 fingerbreadths distal to the lateral epicondyle, as compared with lateral epicondylitis, in which maximum tenderness is usually directly over the epicondyle. Symptoms are intensified by extending the elbow and pronating the forearm. In addition, resisted active supination and extension of the long finger cause pain. Weakness and numbness usually are not demonstrated.^{[18, 19, 20]}

Posterior interosseous nerve syndrome

Patients with posterior interosseous nerve syndrome present with weakness or paralysis of the wrist and digital extensors. Pain may be present, but it usually is not a primary symptom. Attempts at active wrist extension often result in weak dorsoradial deviation due to preservation of the radial wrist extensors but involvement of the extensor carpi ulnaris and extensor digitorum communis. These patients do not have a sensory deficit.

Rarely, compression of the posterior interosseous nerve may occur after bifurcation into medial and lateral branches. Selective medial branch involvement causes paralysis of the extensor carpi ulnaris, extensor digiti quinti, and extensor digitorum communis. With compression of the lateral branch, paralysis of the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis proprius is noted.

Wartenberg syndrome

Patients with the diagnosis of Wartenberg syndrome complain of pain over the distal radial forearm associated with paresthesias over the dorsal radial hand. They frequently report symptom magnification with wrist movement or when tightly pinching the thumb and index digit. These individuals demonstrate a positive Tinel sign over the RSN and local tenderness. Hyperpronation of the forearm can cause a positive Tinel sign. A high percentage of these patients reveal physical examination findings consistent with de Quervain tenosynovitis.

Radial nerve palsy

Immediate exploration of a palsied nerve following a closed fracture of the humerus is contraindicated. A period of expectancy is indicated for 6-12 weeks to allow swelling and the palsy to subside. With a palsy developing after a closed manipulation, a further gentle remanipulation is carried out. Open exploration is indicated if there is no relief of the palsy or if it is felt that the nerve may be entrapped between the fracture fragments. In an open fracture or with a gunshot wound to the humerus with an associated palsy, exploration of the nerve at the time of debridement, as well as possible fixation, is the treatment of choice.^{[13, 14, 15, 16, 17]}

Radial tunnel syndrome

In radial tunnel syndrome, prolonged conservative treatment is indicated if the only symptom is pain. If pain does not resolve after 12 weeks, surgery may be indicated.^{[3, 18, 19, 20]}

Posterior interosseous nerve syndrome

In posterior interosseous nerve syndrome, institute conservative treatment for 6-12 weeks. Surgery is indicated if no improvement occurs or paralysis increases.

Wartenberg syndrome

Wartenberg syndrome is best treated nonoperatively. Local application of steroids or iontophoresis is used. Nerve decompression is indicated only in resistant cases.

The radial nerve is the largest branch of the brachial plexus and is the continuation of the posterior cord, with nerve fibers from C6, C7, C8, and, occasionally, T1. The radial nerve innervates the extensor and supinator musculature located in the arm and forearm and provides distal sensation. Its course carries it across the latissimus dorsi deep to the axillary artery. It passes the inferior border of the teres major, winds around the medial side of the humerus, and enters the triceps muscle between the long and medial heads. It follows the spiral groove of the humerus, piercing the lateral intermuscular septum (10 cm proximal to the lateral epicondyle) from posterior to anterior, and runs between the brachialis and brachioradialis to lie anterior to the lateral condyle of the humerus.^[5]

Branches to the brachioradialis and extensor carpi radialis longus are given off just proximal to the elbow. The anconeus receives a branch from the radial nerve as well. The nerve then divides into a superficial branch and a deep branch. The extensor carpi radialis brevis may receive its innervation either from the radial nerve proper or from the posterior interosseous nerve. The superficial branch, which is purely sensory, runs under cover of the brachioradialis in the forearm. Eight centimeters proximal to the tip of the radial styloid, the nerve pierces the fascia medial to the brachioradialis to lie dorsal to the extensor tendons. It divides into a medial branch and a lateral branch to innervate the radial wrist (with some variable overlap from the lateral antebrachial cutaneous nerve), dorsal radial hand, and dorsum of the radial 3.5 digits (to approximately the middle phalanx level).

The deep branch of the radial nerve, the posterior interosseous nerve, winds to the dorsum of the forearm, around the lateral side of the radius, and through the muscle fibers of the supinator. It then divides into medial and lateral branches, each of which supplies different extensor muscles.^[22]