Radial Nerve Compression/Entrapment Neuropathies

Patients unexpectedly assuming a prolonged posture, with an arm resting posteriorly against a hard surface, are liable to develop acute radial nerve palsies. Examples include sleeping with weight on an arm, such as a loved one’s head over the spiral groove (honeymooner’s palsy); in patients lying on a hospital gurney, when an arm becomes inadvertently draped over a side rail; or in individuals becoming stuporous secondary to alcohol/drug intoxication in a posture that compresses the radial nerve at the axilla, such as when seated with an arm draped over the back of a chair. (“Saturday night palsy”) (see Plate 5-13). Continued pressure on the radial nerve sequentially produces focal demyelination, conduction block, and clinical weakness. In these circumstances, on awakening, the patient, or a nurse discovering such a condition, may initially think a stroke occurred. However, clinical evaluation demonstrates weakness limited to radial-innervated musculature, and not the more global character of a cerebral infarct, wherein not only radial but also median/ulnar–innervated muscles are affected. Primary radial nerve lesions are more severe than those found with a C6 or C7 cervical radiculopathy. Neck or intrascapular pain is the more prominent symptom with cervical root lesions.

DISTAL RADIAL NEUROPATHIES

The posterior interosseous nerve (PIN) is analogous to the anterior interosseous nerve, being a distal, predominantly motor branch of a major peripheral nerve trunk. The extensor carpi radialis longus/brevis (C6, 7) and brachioradialis (C5, 6) muscles are innervated by radial nerve branches exiting the main trunk before the PIN origin in the upper forearm; therefore, fingerdrop, rather than wristdrop, is the predominant manifestation of a PIN neuropathy. Because the extensor carpi ulnaris is affected and not the extensor carpi radialis, radial hand deviation occurs during wrist extension. There are no clinical PIN sensory accompaniments. However, the one exception is pain near the lateral humerus epicondyle, which extends distally as the PIN gives off sensory fibers supplying the interosseous membrane and hand joints near the forearm.

Posterior interosseous neuropathies are quite uncommon, rarely occurring acutely with fractures of the proximal radius Very exceptionally, the PIN is chronically compromised by a soft tissue mass or ligamentous structure near or within the supinator muscle. Posterior interosseous neuropathies may develop precipitously in patients performing repetitious and strenuous pronation/supination movements, such as recurrent hammering or serving at tennis. Occasionally, instances of this activity lead to intermittent PIN entrapment by the fibrous arcade of Frohse at the proximal supinator muscle or a hypertrophied or anomalous supinator.

Predominant Sensory Radial Neuropathies. The superficial terminal radial primary sensory branch may be injured in isolation with external pressure at the wrist, for example, with handcuff injuries. These lesions are readily recognized by the distribution of sensory symptoms on the posterolateral portion of the hand, particularly the thumb. Weakness does not occur. An intermittent radial sensory occupational neuropathy, characterized by recurrent numbness of the thumb and fingers, may occur when the wrist is dorsiflexed, such as by an artist holding a paint brush for a prolonged period of time, leading to a temporary entrapment. Sensory symptoms on the posterior forearm from isolated injuries to the posterior cutaneous nerve of the forearm are equally rare.

In children, more than 50% of radial neuropathies occur secondary to trauma, either fractures or lacerations. Forty percent are due to compression, either intrauterine from prolonged labor or varied mechanisms similar to those in adults. Very occasionally, benign tumors, such as lipomas, ganglia, fibromas, neuromas, and hemangiomas, may affect the radial nerve.

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