Many disorders commonly cause focal or diffuse dysfunction of the peripheral nervous system. This may lead to sensory, motor, or autonomic abnormalities of body parts connected to the involved axons in nerves. The symptoms of numbness, tingling, and pain are particularly common sensory abnormalities that often occur early in the course of lesions involving somatosensory axons in nerves. Common disorders that cause focal peripheral neurological syndromes usually cause somatosensory or motor abnormalities of one arm or one leg. Certain parts of certain nerves are most often affected, usually by the mechanical pathology of compression by surrounding structures, such as parts of the wrist or spine, or by compression from external objects at places where nerves are vulnerable, such as at the elbow or knee. A focal abnormality of one nerve is called mononeuropathy, but a focal abnormality of one spinal nerve has a different name of radiculopathy.
A diffuse abnormality of nerves is called polyneuropathy, which usually involves widespread sensory, motor, or autonomic abnormalities on both sides of the body. Most types of polyneuropathy usually start in the feet and then the hands, at the end of the longest nerves that are most vulnerable to these disorders; these abnormalities may then progress further up the limbs over time. A diffuse abnormality of skeletal muscle is called myopathy, which usually involves widespread weakness on both sides of the body. Most types of myopathy usually start with weakness around the hips and shoulders, which may then progress to involve other muscle groups over time. Many types of pathology commonly cause polyneuropathy or myopathy, including genetic, idiopathic, immune, metabolic, infectious, nutritional, and toxic disorders.