Alzheimer diseases of the peripheral nerves fall into this category. Although certain diseases affect the peripheral nerves primarily, many systemic diseases affect the peripheral nervous system secondarily. Diabetes mellitus is the leading cause of peripheral neuropathy in the United States, whereas leprosy is one of the more common causes worldwide. Polyneuropathy generally produces symptoms involving the distal aspects (fartherest from the body) of the extremities, depriving areas of sensation, motor function or both. Involvement of the extremities may be symmetric or asymmetric. Electromyography and nerve conduction studies are useful in diagnosing and defining peripheral neuropathies.