Neuropathic pain and injured nerve: central mechanisms.

A satisfactory explanation of neuropathic pain must include mechanisms capable of generating three types of pain: ongoing, episodic and allodynic. It must explain why many such pains develop very soon after injury while others occur after long delays. It must take into account the many painless neuropathies and the unpredictable relationship of the pain to the pathology in the painful neuropathies. While these diseases clearly start in the periphery and peripheral changes must contribute to the pain, there are also three types of central change. First changes in the afferent impulse barrage can induce long term shifts of central synaptic excitability. Second, changes of the chemical substances transported from the periphery to the cord produce alterations of cord cell excitability. Third, central control mechanisms can change into a pathological state permitting hyperexcitability. The combined peripheral and central pathology offers more than explanation since each factor could be a target for prevention as well as cure.