Disorders affecting the peripheral elements of the nervous system are the more common causes of numbness and tingling. They also tend to be associated with more localized symptoms and sharper borders between normal and abnormal sensation.
Generalized polyneuropathy as well as more focal entrapment neuropathies are quite common in neurologic practice. The polyneuropathies tend to produce distal and symmetrical sensory loss starting in the feet. Only much later does it tend to affect the fingers. Differential diagnosis for polyneuropathy is discussed in a later chapter though a table of possible causes is provided here.
The peripheral entrapment neuropathies are also quite common. The distribution of symptoms is the principal mode of recognition of the specific nerve injury, and the specific positions and activities that exacerbate symptoms are important clues to mechanism. Pain is variable and is not particularly helpful in diagnosis. Tinel sign may give strong evidence of the precise location of the nerve damage though provocative measures (such as Phalen maneuver for carpal tunnel syndrome) may be more specific.
Radiculopathies tend to produce relatively localized numbness or tingling in the distal extremities. This is because each dermatome is completely overlapped by adjacent dermatomes. There are only relatively small "autonomous zones" where only a single nerve root supplies the region. Therefore, “numbness” tends to be found only in small areas of the distal limbs. Of course there may be associated signs of damage to motor nerve fibers as well.
Brachial plexus lesions affect broader areas of the limb, overlapping the distributions of individual nerves or roots. With rare exception, they have motor signs and symptoms in addition to the sensory loss.