Mononeuropathies are usually caused by pressure on a nerve that is just under the skin and near the knee, elbow, shoulder, or wrist. Mononeuropathies may also be caused by trapped or injured nerves.
Some nerves close to the surface of the body, such as the median nerve in the wrist, the ulnar nerve in the elbow, the radial nerve in the upper arm, and the peroneal nerve in the calf, are more easily injured than other nerves. The injuries result in the following mononeuropathies: carpal tunnel syndrome, ulnar nerve palsy, radial nerve palsy, and peroneal nerve palsy.
For example, repetitive wrist movements can put pressure on the median nerve in the wrist, causing carpal tunnel syndrome. There are several conditions (e.g., infections, sarcoidosis, connective tissue disorders) that can cause mononeuropathies in several places at a time - these are called multiple mononeuropathies.
Polyneuropathy has many causes, including alcoholism; diseases such as diabetes, cancer, and end-stage kidney failure; and exposure to poisonous toxic chemicals, heavy metals such as lead or mercury, and chemotherapeutic agents such as vincristine.
Hypothyroidism, a hormonal condition in which the thyroid gland doesn't work normally, can also cause neuropathy. Some neuropathies are due to inherited medical conditions that get worse with time, and others can be caused by certain medications (e.g., zalcitabine, didanosine, metronidazole, isoniazid, vincristine, amiodarone). Vitamin deficiencies (e.g., B12, folic acid) can also cause polyneuropathy.
Guillain-Barré syndrome is a very serious form of polyneuropathy. Sometimes people with the disease become paralyzed. It is probably caused by an autoimmune reaction. The body's immune system attacks the nerve's myelin sheath, a coating that helps carry a signal along the nerve. Guillain-Barré syndrome can develop suddenly between 5 days and 3 weeks after a person has had a respiratory infection or a gastroenteritis, a vaccine (the chance is less than 1 per 1,000,000 people vaccinated), or a surgery.