A focal abnormality in the peripheral nervous system can affect one nerve, other than a spinal nerve, which is called mononeuropathy, or one spinal nerve, which has the special name of radiculopathy. These syndromes usually involve somatosensory or lower motor neuron abnormalities of one arm or leg, with pain being a particularly common somatosensory symptom. Autonomic axons also travel in nerves, but autonomic abnormalities are usually absent or subtle with mononeuropathy or radiculopathy, compared to sensory and motor abnormalities. Certain parts of certain nerves are most often affected, usually by the mechanical pathology of compression. This is often from surrounding structures, such as parts of the wrist or spine, or by compression from external objects at places where nerves are vulnerable, such as the elbow or knee.
The most common example of mononeuropathy, and also one of the most common neurological disorders overall, is called median neuropathy. This usually involves gradual compression of the median nerve in the wrist where it passes through a tight canal called the carpal tunnel. Because of this, it is also commonly known as the carpal tunnel syndrome. Patients often develop somatosensory abnormalities of one hand, such as numbness, tingling, or pain. They may also develop lower motor neuron abnormalities such as weakness or atrophy (wasting) of the muscles of one hand. Changes to the bones or soft tissues making up the carpal tunnel decrease its size, causing it to compress the median nerve.
A few cranial nerves commonly develop a focal abnormality, often from different types of pathology than typical mononeuropathies in the limbs. The most common example is facial neuropathy, where the muscles on one side of the face become weak from dysfunction of the facial nerve. The cause of this is unclear for most cases, which is called Bell’s palsy, but there is usually inflammation of the nerve, probably from a viral infection in most cases.
Radiculopathy may occur to any of the spinal nerves, but it most commonly involves a few levels that carry axons to one of the limbs, so that somatosensory or lower motor neuron abnormalities of one arm or leg usually result. These abnormalities usually match the dermatome (area of innervated skin) or myotome (innervated muscles) of the affected spinal nerves. A particular type of pain, called radicular pain, often occurs, which typically moves rapidly (“shoots”) from the spine down the affected limb. Radicular pain of a leg is also called sciatica. Radiculopathy is usually caused by compression of a spinal nerve from parts of the spine that have degenerated from aging or injury. These degenerative changes of the spine are called spondylosis.