Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 95. Peripheral Nerve Disorders

Topics: Introduction | Disorders of Muscle Stimulation | Disorders of the Neuromuscular Junction | Plexus Disorders | Thoracic Outlet Syndromes | Mononeuropathy | Mononeuritis Multiplex | Polyneuropathy | Hereditary Neuropathies | Spinal Muscular Atrophies

Mononeuropathy

Mononeuropathy is damage to a single peripheral nerve.

Physical injury is the most common cause of a mononeuropathy. Often, the injury is caused by prolonged pressure on a nerve that runs close to the surface of the body near a bony prominence, such as a nerve in an elbow, a shoulder, a wrist, or a knee. Pressure on a nerve during a long, sound sleep (especially in alcoholics) may be prolonged enough to cause damage. Pressure may result from a misfitting cast, improper use of crutches, or staying in a cramped position for a long time, such as when gardening or when playing cards with the elbows resting on a table. Damage due to pressure may also occur in people who are under anesthesia for surgery, in those who are bedridden (particularly older people), and in those who are paralyzed.

Less commonly, strenuous activities, accidents, prolonged exposure to cold or heat, or radiation therapy for cancer may also damage a nerve. Repeated injuries, such as those due to tight gripping of small tools or to excessive vibration from an air hammer, can also damage nerves. Infections, such as leprosy and Lyme disease, may destroy a nerve, causing mononeuropathy. Cancer may cause mononeuropathy by directly invading a nerve. Some toxic substances and some drugs can cause mononeuropathy.

Certain peripheral nerves are more vulnerable to injury. Examples are the median nerve in the wrist (resulting in carpal tunnel syndrome (see Section 5, Chapter 71)), the ulnar nerve in the elbow, the radial nerve in the upper arm, and the peroneal nerve near the knee.

Ulnar Nerve Palsy: The ulnar nerve passes close to the surface of the skin at the elbow. This nerve is easily damaged by repeatedly leaning on the elbow or sometimes by an abnormal bone growth in the area. The result is ulnar nerve palsy, which consists of a pins-and-needles sensation and weakness in the hand. Severe, chronic ulnar nerve palsy can cause wasting away (atrophy) of muscles and a clawhand deformity (in which muscles tighten, causing the fingers to freeze in a bent position). Nerve conduction studies (see Section 6, Chapter 77) can help locate the damaged nerve. Because surgical repair is often unsuccessful, the disorder is usually treated with physical therapy, which helps prevent tightening of muscles. Avoiding pressure on the elbow is recommended.

Radial Nerve Palsy: The radial nerve passes along the underside of the bone in the upper arm. Prolonged compression of this nerve results in radial nerve palsy. This disorder is sometimes called Saturday night palsy because it occurs in people who drink heavily (often during weekends) and then sleep soundly with an arm draped over the back of a chair or under their partner's head. The nerve damage weakens the wrist and fingers so that the wrist may flop into a bent position with the fingers curved (a condition called wristdrop). Occasionally, the back of the hand may lose feeling. Usually, radial nerve palsy resolves once the pressure is relieved.

Peroneal Nerve Palsy: The peroneal nerve passes close to the surface of the skin on the outer part of the calf near the back of the knee. Compression of this nerve results in peroneal nerve palsy. This disorder weakens the muscles that lift the foot, so that the foot cannot be flexed upward (a condition called footdrop). It is most common among thin people who are bedridden, people who are improperly strapped into a wheelchair, and people who habitually cross their legs for long periods of time. Treatment involves avoiding pressure on the nerve--for example, by not crossing the legs.

See the sidebar When the Foot's Asleep.