Mononeuropathy is damage to a single peripheral nerve.
Physical injury is the most common cause of a mononeuropathy. Injury is commonly caused by pressure on a nerve, such as the following:
Pressure may also injure nerves when people cannot move for long periods, as when they are under anesthesia for surgery, are confined to bed (particularly older people), or are paralyzed.
Less commonly, nerve injury results from the following:
If the pressure on the nerve is mild, people may feel only pins-and-needles sensations without any weakness. For example, people may hit their elbow (funny bone), or a foot may fall asleep. These episodes can be considered temporary mononeuropathies.
Nerves that run close to the body's surface near a bone are more vulnerable to injury. Examples are the median nerve in the wrist (resulting in carpal tunnel syndrome—see Carpal Tunnel Syndrome), the ulnar nerve in the elbow, the radial nerve in the upper arm, and the peroneal nerve near the knee.
Abnormal sensations, including pins-and-needles or loss of sensation, occur in the area supplied by the injured nerve. Pain and weakness may or may not be present. Occasionally, weakness results in paralysis, which can lead to permanent shortening of muscles (contractures).
Carpal tunnel syndrome:
The median nerve passes through a narrow passageway at the wrist. Pressure on this nerve causes pain and abnormal sensations in some fingers, the palm side of the hand and wrist, and sometimes the arm (see Carpal Tunnel Syndrome).
Ulnar nerve palsy:
The ulnar nerve passes close to the surface of the skin at the elbow. The nerve is easily damaged by repeatedly leaning on the elbow or by hitting the elbow (funny bone). Sometimes the nerve is damaged by an abnormal bone growth in the area. Usually, people feel a tingling, pins-and-needles sensation in the little and ring fingers. Ulnar nerve palsy that results from more severe injury makes the muscles in the hand weak. Severe, chronic ulnar nerve palsy can cause muscles to waste away (atrophy), resulting in a clawhand deformity (the fingers are frozen in a bent position because the muscles become tight). 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 pressure on 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. If crutches fit incorrectly and press on the inside of the arm near the armpit, they can cause this disorder. 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, lower part of the knee. Pressure on 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 confined to bed, people who are incorrectly strapped into a wheelchair, and people (especially thin people) who habitually cross their legs for long periods of time. Avoiding pressure on the nerve—for example, by not crossing the legs—usually relieves the symptoms.
Usually, doctors can diagnose mononeuropathies based on symptoms and results of a physical examination. Sometimes electromyography and nerve conduction studies are done to rule out other possible causes, to determine where the nerve is damaged, or to determine how severe the disorder is.
If the cause is a disorder, it is treated. For example, a tumor may be surgically removed.
Usually, when temporary pressure is the cause, the following can help relieve symptoms:
Some people with carpal tunnel syndrome benefit from corticosteroid injections.
Braces or splints are often used to prevent contractures until the symptoms resolve. Surgery may be done to relieve pressure on a nerve if the disorder progresses despite other treatments.
For severe, chronic ulnar nerve palsy, physical therapy helps prevent tightening of muscles. Surgical repair is often unsuccessful.
Last full review/revision September 2012 by Michael Rubin, MDCM