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, the ulnar nerve in the elbow, the radial nerve in the upper arm, and the peroneal nerve near the knee.
When the Foot's Asleep
A sleeping foot can be considered a temporary neuropathy. The foot falls asleep when the nerve supplying it is compressed. Compression interferes with the blood supply to the nerve, making the nerve give off abnormal signals (a pins-and-needles sensation), called a paraesthesia. Moving around relieves the compression and restores the blood supply. As a result, nerve function resumes, and the pins-and-needles sensation stops.