Defining Pain
And a better way to think about definitions
There have been some recent changes in the terminology used by scientists to describe pain physiology. For example, the term “nociplastic” was recently coined to describe pain caused by excessive central nervous system sensitivity. The term “central sensitization” relates to a similar idea, but has variable meanings according to different authorities. And the definition for pain itself is also in play, as it was recently modified by the the International Association for the Study of Pain (IASP).
In this post I'll briefly review these new conventions, so we can make sure we know what people mean when they use these them. I will also argue that trying to get too precise with your definitions can generate more confusion than it prevents, especially with complex concepts like pain.
What's in a definition?
I've always been skeptical about attempts to improve the way we define pain. One reason is that we don’t need definitions to understand the meanings of commonly known words that describe common experiences like pain. If I say “I have back pain”, or “my shoulder is in pain”, people know what I mean, even young children. Imagine someone asking: “I don't know what you mean by the word pain, can you give me a definition?” That would be weird.
Another problem with definitions is that they use words whose meanings may be harder to understand than the word they are defining. For example, imagine that we define pain as "an emergent experience of embodied suffering by an agent embedded in a world." To understand what this definition means, I might need to look up four more definitions and maybe take a philosophy class. And there would be no assurance that my understanding would match those of other people, which would make communication difficult.
Anyway, here’s the old definition for pain by the IASP:
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
This definition seems basically OK but … isn’t it just a fancy way of saying that pain is the feeling of a body part hurting? Also, the wording is ambiguous. Does the term “sensory and emotional experience” include tastes, smells, sounds, and sights? The definition does not specify. The meaning of the word “associated” is unclear, and so is “potential tissue damage.” Sorry to be picky, but as a former attorney, I can’t help but notice these things. If for some reason I needed to prove in court that stepping in dog poop while running counts as pain, I would be able to do so under the IASP definition, because it's an “unpleasant sensory and emotional experience” that is “associated” with an activity that could “potentially” damage my hamstring.
Of course we know that the IASP definition for pain doesn't intend to include events like stepping in dog poop. But this is only because we already know what pain means before reading the definition. Common sense and context are doing all the work here, and the formal definition appears to provide little additional benefit. So what purpose does it serve?
Here’s the updated version of the IASP definition. Is it better?
An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage, or described in terms of such damage.
I don’t see much meaningful difference here (and I think it still doesn’t pass my dog poop test.) The big difference is that the new definition is accompanied by six notes, which state some important facts about pain1:
Pain is always a personal experience that is influenced to varying degrees by biological, psychological, and social factors.
Pain and nociception are different phenomena. Pain cannot be inferred solely from activity in sensory neurons.
Through their life experiences, individuals learn the concept of pain.
A person’s report of an experience as pain should be respected.
Although pain usually serves an adaptive role, it may have adverse effects on function and social and psychological well-being.
Verbal description is only one of several behaviors to express pain; inability to communicate does not negate the possibility that a human or a nonhuman animal experiences pain.
I think these are accurate and important points. But with the possible exception of #2, they don't serve to define pain in the sense of clarifying what counts as pain and what does not. For example, consider whether the following feelings are examples of pain:
An unpleasant feeling of tightness
An intense sensation caused by deep tissue massage, or a deep stretch (often described as “good pain”)
An unpleasant numbness or tingling
Feelings of anxiety, depression or fatigue, coupled with physical sensations in the chest or stomach.
A spicy but pleasant taste
A nasty itch
Nausea
These feelings all share some of the characteristics of pain, while lacking others. My clients will frequently tell me that some body part is bothering them and I will ask “does it hurt?” Very often, they're not quite sure that pain is the right word, and they might prefer words like tight, stiff, or bad. Maybe even tired, wrong, or dead.
Feelings about the body can be like the flavors of a soup with many ingredients. Back pain might have the taste of sharpness, dullness, heat, electricity, anxiety, fear, exertion, etc. Trying to fit those feelings into neat little boxes and categories created by precise definitions is never going to work perfectly. And this is one reason why we don't learn to tell one thing from another and name things correctly by looking into the dictionary for definitions. So how do we actually do it?
Family resemblances and prototypes
Ludwig Wittgenstein argued that the conceptual categories created by words are usually blurred and overlapping. He used the word “game” as an example. Can we identify any essential quality shared by card games, ball games, boardgames, and ring around the rosie? The answer is no, but these activities share a “family resemblance” that we all start to recognize over time with sufficient experience. That is, games share at least several (but not necessarily all) of the following characteristics: a set of rules, competition, skill, luck, creativity, fun, physical activity, balls, playing fields, specialized equipment, etc. When activities have all these qualities, like recreational soccer, everyone agrees that it is a game. Other activities will have maybe just one or two, and the word game may or may not be useful depending on context. Perhaps the word can be used in a metaphorical sense, as in “game of love” or “game of thrones.”
Family resemblances are not just for everyday language, but help to define biological categories. The category of bird is not defined by an essential quality that all birds share. Instead, there are many different qualities that we associate with birds, such as wings, feathers, beaks, the ability to fly, and common ancestry with other animals already called bird. Sparrows and robins have all of these different qualities and are therefore great examples of birds. Penguins and emus are called birds but recognized as borderline cases. The same is true about food categories like fruits and vegetables. They have no defining essence, and are grouped together by family resemblances.
A related concept is prototype theory, developed by the philosopher Eleanor Rosch. According to Rosch, we don't form conceptual categories based on Aristotelian specification of the necessary and sufficient conditions for membership. Instead, we tend to think of some prototype that is the best example of that category, and then allow other things membership based on their degree of similarity to the prototype.
Prototypes veggies would be broccoli or lettuce. Sparrows and robins are at the center of the bird category. Ted Cruz is a poster boy Republican. “Bad” examples of each category would be mushrooms, penguins, and Mitt Romney.
One important implication of these ideas is that membership in some category is not absolute, but exists more on spectrum. Whether or not borderline cases should be included depends on context.
I find this idea appealing, because it acknowledges the inability of words to fully capture the complexity of the world. We need them to be fuzzy, flexible and sometimes metaphoric, or else we get locked into simplistic binaries. This doesn’t mean that we don’t need to choose our words carefully. You can’t just say that black is white, or that hamburgers are vegetables. (It does, unfortunately, mean that you can make an argument that ketchup is a vegetable.)
And here's the relevance for pain: it’s a complex entity and therefore nebulous. The same is true about other biological concepts that we use to understand pain, such as sensation, perception, memory, cognition, emotion, immune system activity, and nervous system activity. None of these terms can be defined by an essential quality. Instead, they refer to a whole network of different qualities, which are hard to summarize in tidy definitions.
For example, some of the qualities we associate with pain are: conscious experience of unpleasantness; a perception that the feelings are “about” a specific location in the body; motivation to take action to protect the affected body part; a feeling that there is some kind of damage or pathology in the affected body part; there is in fact some form of pathology related to that body part. A prototypical pain that has all these qualities would be the feeling we have when we stub our toe. Some feelings, such as an itch, or the feeling of tightness, or the emotional “pain” of loneliness, meet some but not all of these criteria, and are therefore borderline cases.
Well, I apologize for all this lecturing. Your reward for reading this far is the list of definitions I promised to clarify at the start of this long post. (Keep some of the lecturing in mind as you review them.)
Nociceptive pain: Pain caused by tissue damage to non-neural tissue. Prototype examples: pain caused by broken bones, torn ligaments, muscle sprains.
Neuropathic pain: Pain caused by damage to the nervous system. Prototype examples: sciatica, diabetic neuropathy.
Nociplastic pain: Pain caused by excess sensitivity of the central nervous system. Prototype examples: chronic nonspecific low back pain, fibromyalgia.2
Central sensitization: Increased responsiveness of nociceptive neurons in the central nervous system to afferent input.3 Prototype example: dorsal horn sensitivity immediately following injury.4
Keep this post in mind when you see semantic disputes on social media.
References
Pain Redefined: Inside the IASP’s Updated Definition. Retrieved from https://www.practicalpainmanagement.com/resources/news-and-research/pain-redefined-inside-iasp-updated-definition
Nijs J, Lahousse A, Kapreli E, et al. Nociplastic Pain Criteria or Recognition of Central Sensitization? Pain Phenotyping in the Past, Present and Future. J Clin Med. 2021;10(15):3203. Published 2021 Jul 21. doi:10.3390/jcm10153203; Fitzcharles MA, Cohen SP, Clauw DJ, Littlejohn G, Usui C, Häuser W. Nociplastic pain: towards an understanding of prevalent pain conditions. Lancet. 2021 May 29;397(10289):2098-2110. doi: 10.1016/S0140-6736(21)00392-5. PMID: 34062144.
Wall and Melzack’s The Textbook of Pain, 6th Ed., p.65.
Latremoliere A, Woolf CJ. Central sensitization: a generator of pain hypersensitivity by central neural plasticity. J Pain. 2009;10(9):895-926. doi:10.1016/j.jpain.2009.06.012