Neuropathic pain is a form of chronic pain defined as "Pain arising as a direct consequence of a lesion or disease affecting the somatosensory system.''1 The spectrum of neuropathic pain is associated with a variety of disease states (Table 1.1),2'3 but it is important to recognize that neuropathic pain is a relatively frequent, but unusual and by no means inevitable, consequence of those disorders.
Various patterns of neuropathic pain are recognized and it may be spontaneous in nature (continuous or paroxysmal) or evoked by sensory stimuli. These patterns may coexist in the same patient and are not necessarily unique to any disease entity. Neuropathic pain is also usually associated with various phenomena associated with disturbances in sensory function and it is possible to broadly classify neuropathic pain patients on the basis of their sensory phenotype, for example in postherpetic neuralgia.4 Therefore, pain may exist in the context of sensory loss (anesthesia dolorosa) or more unusually in the presence of hypersensory phenomena (e.g. allodynia (Figure 1.1), hyperalgesia (Figure 1.1), and hyperpathia). Occasionally, a mixed picture of disordered sensory function may be evident depending on which areas are tested.
While the biological advantage to the organism of nociceptive pain is readily identifiable, it is less easy to do so for neuropathic pain and it is probable that, in broad terms, neuropathic pain is a result of a pathological process representing a disordered regenerative response to neuronal damage. For example, in patients with the hyper-sensory subtype of neuropathic pain, the mechanistic implication of allodynia is that elements of the sensory nervous system which normally signal innocuous
Table 1.1 A classification of the more frequent disorders associated with neuropathic pain, with examples.
Cause of neuropathy Examples
Phantom limb Spinal cord injury Surgical
Peripheral nerve injury Postherpetic neuralgia HIV
Invasion/compression of neural structures by tumor Vinca alkaloids Taxols Ethanol
Antiretroviral drugs Poststroke pain
Multiple sclerosis Charcot-Marie-Tooth context of responses to nerve injury which are possibly, but not certainly, related to pain. Third, the vast majority of research into neuropathic pain mechanisms has concentrated on changes in the peripheral nerve or spinal cord following peripheral nerve injury. Although knowledge is accumulating regarding alterations in the brain following peripheral nerve injury, much less is known about the significance of these changes. Therefore, this chapter will focus mainly on peripheral and spinal mechanisms of neuropathic pain.
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