There are three primary criteria used to categorize peripheral neuropathy into one of its specific classifications. These are as follows:
- The type of nerve affected: either motor, sensory, or autonomic
- The number and distribution of nerves affected
- The process affecting the nerves: inflammation (neuritis), compression, chemical, etc.
Different combinations of these criteria characterize specific classifications of peripheral neuropathy.
Neuropathy that affects only a single nerve is classified as mononeuropathy; common mononeuropathic disorders are carpal tunnel syndrome and axillary nerve palsy. The most common cause of mononeuropathy is compression (as is the case in carpal tunnel syndrome), but direct injury and inflammation are also possible causes, and not part of the definition of mononeuropathy.
A much larger category is polyneuropathy; this classification is characterized by a nerve damage pattern that affects a large number of nerve cells across a large area spanning various parts of the body. In polyneuropathy, individual nerve cells are affected without regard for the nerve or nerves through which they pass, but only a handful of cells across any specific nerve are affected. This is usually caused by processes that affect the entire body; localized processes and traumatic injury are generally not able to damage individual neurons in multiple nerves across a large area.
Mononeuritis multiplex (sometimes called polyneuritis multiplex) includes disorders where multiple individual nerve trunks are degraded, generally over a long period of time. Eventually this leads to loss of the functions provided by the nerves that make up the failing nerve trunks. Often this leads to sensory and motor loss in certain parts of the body. Although there is no pattern of symmetry to the affected nerve trunks and areas, as a disease progresses and affects a larger number of nerve trunks, the affected areas can begin to appear symmetric; this also makes it more difficult to identify the specific cause and type of disease.
Hellmann, David B., Timothy J. Laing, Michelle Petri, Quinn Whiting-O'keefe, and Gareth J. Parry. "Mononeuritis Multiplex." Medicine 67.3 (1988): n. pag. Web.
Lovelace, R. E. "Mononeuritis Multiplex in Polyarteritis Nodosa." Neurology 14.5 (1964): 434. Web.
"Peripheral Neuropathy Fact Sheet." National Institute of Neurological Disorders and Stroke. National Institutes of Health, 14 Jan. 2016. Web. 25 Feb. 2016. <http://www.ninds.nih.gov>.