full article 2500 words
There are two well-recognized broad categories of pain: the common sensical sort (the pain of damage), and the somewhat more exotic kind that comes from damage to the system that reports and interprets damage, the nervous system. It’s the difference between engine trouble and trouble with that light on your dashboard that says there’s engine trouble. More specifically:
Obviously these kinds of pain can overlap. Some medical problems, like injuries, can affect both nerves themselves and other tissues, causing both kinds of pain. However, it’s surprising how little overlap there is: look at any list of the most painful conditions [NHS] and they all fit pretty clearly into one category or the other.
Some common kinds of pain are not a great fit for either of the two official categories. The canonical example is the pain of fibromyalgia. [Mayo] Other major examples:
Fibromyalgia is a pain system dysfunction resulting in widespread body pain (and more8), but “dysfunction” of the nervous system is specifically excluded from neuropathic pain, by decree, as of 2011.9 Dysfunction means that fibromyalgia isn’t caused by any obvious damage to the nervous system, but by its misbehaviour, and so it’s not welcome at the neuropathy club. It was before 2011! But not anymore.
Maybe someday we will discover that fibromyalgia is caused by some kind of subtle damage to the nervous system, which would make it just another neuropathy after all, ho hum.10 It probably is a dysfunction, arising from widespread problems in a complex system, and no clear point of failure will ever be discovered.1112 On the other hand — and this is really dang interesting — there is decent evidence that many people with fibromyalgia have slightly pinched spinal cords.13 If true, that would be a game-changer and a slam dunk for classification as a neuropathy. So stay tuned! Science is not finished with fibromyalgia.
Meanwhile, what do we call it? And other miserable pain problems that arise from what seem to be neurological dysfunction?
“Other” is a bit vague, so experts have proposed some more descriptive names for this category. There are issues with all of them. (The first three here all come from the same source.14)
The “noci” terms seem a little misguided to me, because the pain of a misbehaving nervous system is so much closer kin to neuropathic pain.17 Centralization is an important concept and element, but clearly not the whole story18 (and “maladaptive” might be too blamey19). Algopathic gets my vote: suitably neutral and formal, while saying just enough to be better than “other.”
Sensitization is the amplification of danger signalling in any part of the system (which can be driven by both peripheral and central neurology, while centralization is referring only to the latter). It seems like a slam dunk to put these in the “other” category, but not so fast: the nervous system is not being dysfunctional just because it overestimates some danger. Many of us experience normal, healthy centralized sensitization every time we go to the dentist, because our nervous system is not an idiot and is very suspicious of dentists based on past experiences. Our brains think the dentist is a threat, and although they are technically wrong — the dentist is actually there to help — the judgement is not “dysfunctional.”
Sensitization is clearly part of the normal function of the pain system — it’ s not a dumb system where pain is always exactly proportionate to tissue danger. It’s more subtle. Sensitization/centralization is only dysfunctional when it’s chronic and seriously disproportionate. And of course there’s a grey zone a mile wide between normal and dysfuntional. It’s a spectrum.
Sometimes we just don’t understand a problem well enough to classify it. The best example of this is the sensitive spots in soft tissue that so many people have — “trigger points,” the things that motivate most massage therapy appointments. There is a popular hypothesis that they are caused by a problem with muscle tissue, which would make it a clear case of nociceptive pain from a fairly subtle lesion… but it’s just an hypothesis, and some experts have suggested that trigger points are caused by a problem with nerves themselves, which would make them neuropathic.20 No one actually knows, and it’s not likely to be settled for a long time.
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
— Added a nice new Venn diagram depicting the intersection of all three kinds of pain in “hell.” Added section “Do sensitization and centralization belong in the “other” category?”
— Added sidebar about migraine, and converted section about the influence of the brain to a sidebar. Elaborated on overlapping pain problems.
— Extensive next day revisions.
Pain is an output of the brain.
Pain is the end result. Pain is an output of the brain designed to protect you. It’s not something that comes from the tissues of the body.
Lorimer Moseley, from his surprisingly funny TED talk, Why Things Hurt 14:33
The formal definition of pain emphasizes its subjective, experiential nature: “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP Taxonomy). This experience is based on many “inputs,” not just nerve signals about tissue damage.BACK TO TEXT
IASP has recently  published a new definition of neuropathic pain according to which neuropathic pain is defined as ‘pain caused by a lesion or disease of the somatosensory system.’ This definition replaces the 17-year old definition that appeared in the Classification of Chronic Pain published by IASP in 1994, which defined neuropathic pain as ‘pain initiated or caused by a primary lesion, dysfunction, or transitory perturbation of the peripheral or central nervous system’. Even though the definition has not been changed dramatically, there are two important changes in the new version: (1) the word ‘dysfunction’ has been removed and (2) a lesion or disease affecting the nervous system has been specified to be a lesion or disease of the somatosensory system.
The whole paper is excellent, but skipping to the cogent conclusion:
BACK TO TEXT
A definition of neuropathic pain is only useful if it distinguishes conditions in a clinically meaningful way. If the definition does not provide additional benefit in terms of understanding and treating the condition(s), then there is no reason to keep it. Hopefully, the new definition of neuropathic pain will act as a stimulant to discuss the definition in more detail and provide input for studies that can be used to test the value of the definition.
This deep but beautifully readable article explains, with many pictures and apt examples, how “chronic pain is often driven by dysregulation of a ‘supersystem’ that coordinates defensive responses to injury. The supersystem results from dynamic interaction between different subsystems, most notably the nervous system, immune system, and endocrine system.” It’s hard to believe, but the article also manages to make this information seem quite practical.BACK TO TEXT