Wrist Drop Radial Nerve Connective Tissue Disease

• Beware of cervical radicu-lopathy presenting as posterior interosseous syndrome. No sensory symptoms in posterior interosseous syndrome.

• Beware tendon rupture in rheumatoid patients presenting as posterior interosseous nerve syndrome. Treatment is entirely different.

• Revisions usually secondary to incomplete release of nerve through supinator. Compression of posterior interosseous nerve has been reported at distal edge of supinator.

The right forearm measured 2 cm larger than the left in circumference and measured 5 cm distal to the lateral epicondyle. A palpable mass was noted over the dorsal proximal forearm, and the patient was tender to palpation over the radial nerve as it passed through the mobile muscle mass at and just distal to the radial head. Slight tenderness to palpation was noted over the same area to the left forearm but not as severe. Palpation directly over the lateral epicondyle caused mild discomfort, less severe than over the course of the nerve. Full passive flexion of the wrist and fingers with the elbow in extension reproduced the pain. Resisted supination with the extended arm similarly reproduced the pain symptoms. Radial deviation was observed with wrist extension. The patient was able to fully extend the interphalangeal joints, but he could not fully extend the metacarpophalangeal joints beyond 45 degrees.

Diagnostic Studies

Anteroposterior, lateral, and oblique radiographs of the right elbow did not show any bony abnormalities. An abnormal soft tissue shadow was noted on the lateral radiograph dorsally. Magnetic resonance imaging of the proximal forearm revealed

Brachial Biceps Lipoma
Figure 17—1. Magnetic resonance image demonstrating a large lipoma overlying the proximal radius. Note the distended supinator muscle.

a mass consistent with a lipoma involving the supinator muscle (Fig. 17—1). Electromyographic and nerve conduction studies of the bilateral upper extremities demonstrated denervation of the extensor digitorum communis and extensor carpi ulnaris. The abductor pollicis longus and extensors pollicis longus and brevis were spared.

Differential Diagnosis

Radial tunnel syndrome Cervical radiculopathy Lead poisoning

Systemic connective tissue disease Hysterical wrist drop Posterior interosseous syndrome

Diagnosis

Posterior Interosseous Syndrome

Radial tunnel syndrome involves compression of the posterior interosseous nerve branch of the radial nerve. Radial tunnel syndrome is defined when there is an absence of muscular involvement and the diagnosis of posterior interosseous syndrome is defined when there is muscular weakness and atrophy in the efferent distribution. The posterior interosseous nerve is not a purely motor nerve. It carries gamma-afferent fibers from the muscles it innervates. This can be demonstrated by the discomfort experienced by squeezing any muscle belly. The posterior interosseous nerve also supplies sensory afferent fibers to the radiocarpal, intercarpal, and carpometacarpal joints. Patients describe an aching, burning discomfort in the mobile extensor wad musculature, which may radiate distally down the forearm.

The radial nerve rises from the posterior cord of the brachial plexus. About 4 cm proximal or distal to the radiocapitellar joint, the radial nerve divides into two large terminal branches: the radial sensory and posterior interosseous nerves. Anatomic com-pressive factors in posterior interosseous syndrome include the fibrous bands from the anterior capsule, origin of the extensor carpi radialis brevis, radial recurrent leash of vessels, and the arcade of Frohse. These structures compress the nerve in elbow extension, passive forearm pronation with wrist flexion, and resisted supination. Active forearm supination has been reported to produce five times greater pressure than passive forearm pronation.

Unlike radial tunnel syndrome, many causes of posterior interosseous nerve syndrome other than purely anatomic have been described. Trauma primarily resulting in radial head fracture or dislocation may result in palsy of the posterior in-terosseous nerve. Any space-occupying lesions may compress or injure the nerve in the radial tunnel. Ganglions, lipomas, and fibromas have all been reported to compress the posterior interosseous nerve with resulting muscle weakness. Magnetic resonance imaging may be helpful if a mass is suspected. The posterior interosseous nerve may become compressed in patients with rheumatoid arthritis secondary to inflammation or subluxation of the radial head. This can be differentiated by the tenodesis effect by flexion and extension of the wrist.

Pain is frequently the primary initial complaint followed by muscle weakness or paralysis, which may develop over a period of several weeks. Occasionally, weakness

may occur dramatically overnight following unaccustomed exercise. The sequence of muscular involvement follows no set pattern. Patients with motor involvement may notice weakness and radial deviation with wrist extension. This is because the radial nerve proximal to the radial tunnel innervates the extensor carpi radialis longus and brevis. Patients maintain the ability to extend the interphalangeal joints of the fingers, but may be unable to extend the metacarpophalangeal joints beyond 45 degrees. Partial paralysis may result in the loss of extension of the small and ring fingers alone, producing the false appearance of an ulnar claw hand. However, the metacarpophalangeal joints would not be hyperextended and the ulnar nerve intrinsic musculature would be intact. Thumb extension may or may not be involved. There is no sensory loss.

Differential Diagnosis

Other conditions that must be differentiated from posterior interosseous nerve syndrome include cervical radiculopathy, inflammatory arthritis, lead poisoning, and hysterical wrist drop. Radial tunnel syndrome is defined when there is lack of motor involvement.

Examination of the elbow should always include an evaluation of the cervical spine. A Spurling's maneuver is helpful to evaluate for cervical radiculopathy along with a thorough neurologic examination. Cervical radiographs should be performed if there is any question of cervical involvement. Patients with cervical pathology also usually present with sensory symptoms.

Patients with rheumatoid arthritis may present with sudden loss of metacarpopha-langeal extension secondary to acute extensor tendon rupture or posterior interosseous nerve palsy. Patients with posterior interosseous nerve palsy may be differentiated from ruptured extensor tendons by examining the tenodesis effect of wrist flexion-extension. The metacarpophalangeal joints extend with wrist flexion if the extensor tendons are intact, suggesting that the lack of extension is secondary to posterior interosseous nerve palsy. Electromyographic studies also are helpful in this situation.

Lead poisoning may produce a high radial nerve palsy without sensory loss. Patients would then present with involvement of the brachioradialis and radial wrist extensors. Lead poisoning is further confirmed by classical gingival discoloration and by blood and urine studies. Hysterical wrist drop presents with inability to extend both the interphalangeal and metacarpophalangeal joints (Table 17—1).

Table 17-1 Diagnosis and Differentiating Findings

Diagnosis

Differentiating Findings

Radial tunnel syndrome

No motor involvement

Cervical radiculopathy

Spurling's maneuver

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