The International Association for the Study of Pain (IASP) accepts the following definition (paraphrased): Pain is an unpleasant sensory and emotional experience which occurs in the presence of actual or potential damage to our bodies or described in terms of such damage (in the absence of tissue injury). The definition acknowledges three important concepts: 1) pain is always subjective defined by the person who experiences it, 2) pain is unpleasant, so the definition includes how a person feels, and 3) in this definition pain is NOT tied to a specific stimulus.

Neuropathic pain is pain initiated or caused by a primary injury or dysfunction in the peripheral or central nervous system (consisting of our nerves or the spinal cord and brain respectively). Neuropathic pain is "pathological or abnormal pain" in contrast to Nociceptive pain, which is the result of activation of nociceptors, i.e., our pain receptors in muscles, bones, ligaments, the gut, the heart etc. Nociceptive pain is "normal pain" in the sense that it is the pain we feel normally when the nervous system is intact.

Neuropathic pain occurs in some but not all people with injury or dysfunction of the peripheral or central nervous system. In case of injury to nervous tissue, all parts of the nervous system may undergo changes within milliseconds, which may last indefinitely. Specifically, when peripheral nerves get injured, they may fire abnormally generating "ectopic impulses" at the site of injury and becoming supersensitive to multiple neurochemicals. This phenomenon is called "peripheral nociceptor sensitization". Similarly the spinal cord and the brain undergo changes after damage to the peripheral nerves or after spinal cord injury or stroke ("central sensitization"). Some of these abnormalities include pain seemingly coming from body areas larger than the original injury site or generated by stimuli that are normally painless, for example pain on mere touch. Our understanding of neuropathic pain has dramatically increased over the past few years with the invention of neuroimaging, an ingenious way to visualize brain activity in awake people. It is used exclusively for research and has shown many changes in brain activation patterns, even when our everyday tests fail to show abnormalities.

Typical neuropathic symptoms include pain (burning, shooting, stabbing, shock-like etc), occurring spontaneously or generated by touch, pin prick, pressure or movement; paresthesiae and dysesthesiae, numbness, pins and needles sensations and motor dysfunction (weakness, difficulty starting or performing movements etc). Neuropathic signs may include abnormal sensations, weakness, atrophy, changes in colour, temperature or sweating patterns and flickering of the muscles, stiffness or bizarre posturing.

Examples are: 1) brain damage as in the case of stroke (Central post-stroke pain), 2) damage to the Spinal Cord as in the case of trauma after a fall or car accident or in Syringomyelia (when cavities form within the cord), 3) Multiple Sclerosis, 4) after spinal surgeries (failed back surgery syndrome), 5) traumatic brachial plexus injury, 6) after amputations (post amputation pain), 7) diabetic neuropathy, 8) Complex Regional Pain Syndrome (known also as Reflex Sympathetic Dystrophy), 9) trauma or compression of peripheral nerves, 10) trigeminal neuralgia, 11) Post Herpetic Neuralgia after an attack of Shingles, 12) nerve tumours and 13) pain after surgery (like cardiac bypass surgery, breast surgery, thoracotomy, hernia repairs etc).

Many treatments exist, but they do not work in all people. Drugs over the counter are not very useful, while those prescribed by doctors include tricyclic antidepressants (amitriptyline, desipramine etc); antiepileptics (carbamazepine, valproic acid, gabapentin, pregabalin etc); local anesthetic type of drugs (e.g. mexiletine); anti-inflammatory drugs (usually not very effective), opioids (morphine and similar drugs), and a recently approved oral spray derived from marijuana. Non drug treatments are nerve, root and spinal cord blocks; local heat, cold or TENS, acupuncture and exercises; and finally surgery (spinal stimulators or implanted pumps and sometimes procedures like cutting or burning nerves). Chronic pain takes its toll on a patient's emotional and mental state, creating depression, sleep problems, isolation, loss of work and pride and problems with loved ones. The best way to treat chronic pain in general is a "whole person" approach and may require combination of medical and non medical treatments (like counseling, relaxation and cognitive/behavioural techniques).