Nerve injury-induced pain in the trigeminal system.

This article reviews some recent findings on peripheral mechanisms related to the development of oro-facial pain after trigeminal nerve injury. Chronic injury-induced oro-facial pain is not in itself a life-threatening condition, but patients suffering from this disorder undoubtedly have a reduced quality of life. The vast majority of the work on pain mechanisms has been carried out in spinal nerve systems. Those studies have provided great insight into mechanisms of neuropathic spinal pain, and much of the data from them is obviously relevant to studies of trigeminal pain. However, it is now clear that the pathophysiology of the trigeminal nerve (a cranial nerve) is in many ways different to that found in spinal nerves. Whereas some of the changes seen in animal models of trigeminal nerve injury mimic those occurring after spinal nerve injury (e.g., the development of spontaneous activity from the damaged axons), others are different, such as the time-course of the spontaneous activity, some of the neuropeptide changes in the trigeminal ganglion, and the lack of sprouting of sympathetic terminals in the ganglion. Recent findings provide new insights that help our understanding of the etiology of chronic injury-induced oro-facial pain. Future investigations will hopefully explain how data gained from these studies relate to clinical pain experience in man and should enable the rapid development of new therapeutic regimes.