Neuropathic pain is caused by aberrant signal processing in the peripheral or central nervous system. In other words, neuropathic pain reflects nervous system injury or impairment. Common causes of neuropathic pain include trauma, inflammation, metabolic diseases (e.g., diabetes), infec-tions (e.g., herpes zoster), tumors, toxins, and primary neurological diseases. Neuropathic pain can be broadly categorized as peripheral or central in origin.
Painful peripheral mononeuropathy and polyneuropathy, deafferentation pain, sympathetically maintained pain, and central pain are subdivisions of these categories. Neuropathic pain is sometimes called pathologic pain because it serves no purpose. Achronic pain state may occur when pathophysiologic changes become independent of the inciting event. Sensitization plays an important role in this process. Although central sensitization is relatively short lived in the absence of continuing noxious input, nerve injury triggers changes in the CNS that can persist indefinitely.
Thus, central sensitization explains why neuropathic pain is often disproportionate to the stimulus (e.g., hyperalgesia, allodynia) or occurs when no identifiable stimulus exists (e.g., persistent pain, pain spread). Neuropathic pain may be continuous or episodic and is perceived in many ways (e.g., burning, tingling, prickling, shooting, electric shock-like, jabbing, squeezing, deep aching, spasm, or cold).