Neuropathic pain may arise following damage to a nerve although in individual cases the exact mechanism may be unclear. Causes include trauma, toxins, metabolic compounds, immune reactions, ischemia and infection. However, only a minority of those with nerve damage develop chronic neuropathic pain. The development of neuropathic pain is associated with a genetic predisposition and is influenced by emotional factors such as fear and anger. Chronic pain does not necessarily emerge immediately after an injury or operation and may take days or even weeks to develop leading to a lower level of clinical recognition.
Neuropathic pain is typically described as sharp, gnawing, shooting, or burning. It may be associated with an easily demonstrable sensory loss and paradoxical hypersensitivity. Hyperalgesia describes an exaggerated response to a painful stimulus, such as a pinprick. Allodynia describes an unpleasant or painful response associated with an innocuous stimulus, such as light touch. Both of these are temporarily present in areas of sunburned skin, but these abnormal responses may be permanent in neuropathic pain. Phantom limb pain is another well-known form of neuropathic pain. Silas Weir Mitchell, a physician during the American Civil War, famously described this condition. Patients may report incredibly vivid sensations in their missing limb. There are also numerous patients with phantom breasts, noses, penises and anuses. Remarkably, there are cases of people who have phantom limbs even though their limb was congenitally absent.
Kehlet has proposed diagnostic criteria for neuropathic pain (see below). However, many clinicians would not require all factors to be present.
• Pain in a neuroanatomical defined area- i.e. corresponding to a peripheral or central innervation territory
• A history of relevant disease or lesion in the nervous system, which is temporarily related to development of pain
• Partial or complete sensory loss in all or part of the painful area
• Confirmation of a lesion or disease by a specific test- e.g. surgical evidence, imaging, clinical neurophysiology, biopsy