Last updated: Sat, Feb 8, 2025
...it is clear that the word "pain" refers to an endless variety of qualities categorized under a single linguistic label, not to a specific, single sensation that varies only in intensity or affect.
I've now discussed the distinction between healthy and pathological pain and the multiple meanings of chronicity. If you are a chronic pain sufferer, you've been asked about your pain many times and many ways. You've perhaps been asked about the intensity of your pain so many times that you've formed the impression that this is the most important thing about it.
Perhaps it is. But pain has many dimensions and many flavors beyond its intensity that are important to consider. Some of these dimensions and flavors are important because they give a trained care provider insights or at least clues about what is causing your pain. Others are important because they mold your experience as a pain sufferer. Many are important for both reasons.
When your body is injured it is common to experience first a rather sharp, distinct pain, and somewhat later a dull, aching pain. "Fast pain" is felt very soon after the pain-provoking event happens, in as little as a tenth of a second. Fast pain occurs as a result of mechanical or thermal insults to the skin and other shallow tissues of the body. A needle in the skin, a knife cut, or a burn can provoke fast pain. It is sometimes described as sharp, pricking, or electric. Fast pain is not felt in most of the deep body tissues.2
"Slow pain" follows fast pain after a second or more and may build up over seconds or minutes. Slow pain is sometimes described as slow and burning, aching, throbbing, or even nauseating. This slow pain usually accompanies some form of tissue destruction. It follows and usually outlasts fast pain.3
Both fast and slow pain originate with free nerve endings. Fast pain originates with the free nerve endings of what are called "A-delta" fibers, which transmit signals at a speed of six to thirty meters per second. Once these A-delta signals arrive in the spine, they're transmitted to the brain through fast spinal channels. Biologically, their function is to quickly alert you to the existence of a painful event and to locate the event on your body. In some cases, fast pain can trigger withdrawal reflexes in the body as soon as they arrive at the spinal cord.4
Slow pain originates with the free nerve endings of "C" fibers, which carry signals at one-half to two meters per second. While A-delta nerve endings respond primarily to mechanical and thermal events, C nerve endings respond to chemicals. In particular, they respond to the chemicals that are released during inflammation. (Injury, Inflammation, and Healing.) They do also respond to other chemicals and sometimes to mechanical and thermal conditions. Slow pain seems to be the cause of much of the suffering that goes with pain. Its purpose? It persists as long as the conditions that cause it, which may indicate an ongoing unhealthy state. The evidence suggests that slow pain motivates behavioral changes that promote healing.5
Slow pain sounds a lot like chronic pain, and in fact much of the scientific and medical evidence supports that idea. Theories about the link between pain and suffering have largely been the province of psychology in the past, and increasingly these are being challenged by neuroscience. More detail about how fast and slow pain happen can be found in Pain Science 2: Nociceptors and the Spine, Pain Science 3: Neuroscience and the Brain, and Pain Science 4: Partner Systems. Examination of evidence about the link between pain and behavioral and mood changes can be found especially in A Rational Model of Emotion and Pain and The Challenge of Living in Pain.
"What is your pain level from zero to ten?" Or, is the question "One to ten"? It stands to reason that more intense pain is worse, of course, and increases or decreases in pain often have implications for treatment approaches and availablity of medical resources. Pain level became the "fifth vital sign" in medical facilities during the 1990s, when there was a wave of support for the idea that pain is controllable and should be controlled. The one-to-ten scale has since become standard and largely unremarked in science and medicine.
It happens that the question of pain intensity has an important role in shaping pain treatment, in interpreting the results of pain research, and in the accessibility of treatment for pain sufferers. The poor fit between the apparent clarity of "one to ten" and the actual ambiguity of these values isn't just apparent, it's real. This topic is discussed from a scientific viewpoint in Pain Measurement. It is discussed from a patient's point of view in Destructive Pain Myths. Aspects of the problem as they surface in doctor/patient interactions are touched upon in The Pain Institutions.
If pain is more complex than a one-to-ten scale, how do we communicate about pain? (Or, how do we even think about it?) As it turns out, it isn't straightforward. You can point to a green object and say that it's green, and if we two agree it's green, each has at least an approximate idea of what the other means by "green." The precise borders of "green" may be unclear, but the central idea is there.
Pain isn't like that. There isn't a common point of reference that we can point at, to make sure that we understand one another when we talk about pain intensity or other aspects of pain experience. Care providers and researchers seem to refer to the lack of external referents when they say that the pain experience is "subjective." In fact, you may know from your own experiences that patients and particularly chronic pain sufferers often experience pains that most people have no direct experience of.
This creates some hurdles to understanding. Robert Melzack, the psychologist on the Melzack and Wall team, and collaborators at McGill University used techniques from sociology and cognitive psychology to investigate how we talk about pain and, importantly, how effective such natural communication can be. The result of their research was the McGill Pain Questionnaire, which asks the sufferer to describe their experience using words that they found to have reliable shared meanings among patients and care providers. (More about this can be found in The McGill Pain Questionnaire.)
For now, I'll use the questionnaire as a source of pain characteristics that seem to be reliably accepted. The team selected about eighty terms that fall into three main categories:
A complete list is included in The McGill Pain Questionnaire. The questionnaire also asks the sufferer to characterized the intensity of their pain on a six-item scale where 0 is no pain, 1 is mild, 2 is discomforting, 3 is distressing, 4 is horrible, 5 is excruciating.
The MPQ has been evaluated in a number of ways. Importantly, it's been found that patients with the same medical diagnosis (that is, what the doctor believes is causing the pain) have similar answers to the questionnaire. So, in a sense, it is possible to derive a fairly reliable diagnosis from the patient's description of their own pain.
The MPQ also allows a pain rating index (PRI) to be determined from the sufferer's answers. The result is based on which one of the graded items in a group of terms the subject chose. Based on this score, the researchers have offered a comparative ranking of different types of pain. Mothers may not be surprised to learn that a first birth with no preparatory training is one of the most painful of the ranked pains, with a PRI of 37. Chronic back pain received a 30 in this study, while a fractured bone got 20.
I've never been asked, over forty years, to complete the MPQ. Care providers use shorter forms. But the MPQ research does provide some important information about pain and our ability to communicate about it. First, it identified multiple dimensions or attributes of pain that are widely recognized. Second, it supports the notion that we can communicate verbally about our pains if we're careful how we do it.
Inflammation is a physiological response to the damge of body tissue. It occurs in response to minor cuts, piercings, or burns, and also to major insults such as bone or joint damage, surgeries, and so on. It is detected by C-fiber free nerve endings that are common throughout the body, and is a "slow pain" of the type that was discussed above. Non-steroidal anti-inflammatory drugs (NSAIDs) target inflammatory pain by workr pain isLing to suppress the intensity of the inflammatory process. The inflammatory process is discussed in the context of injury and healing in Injury, Inflammation, and Healing.
A blood pressure cuff is inflated to restrict blood flow to your elbow and forearm when you blood pressure is taken. When the cuff is highly inflated, you feel a particular type of pain called "ischemic pain." Ischemic pain can occur for other reasons. When a muscle is in spasm, tightly contracted because of irritation, overuse, or other reasons, you may feel ischemic pain as part of the experience.
The location of pain is always important, although it can also be misleading. Care providers will either ask you where it hurts, ask you to point to where it hurts, or ask you to mark up a line-person to show where you hurt. The MPQ includes such line-people for the patients to mark.
Your pain system is designed to tell you where you are hurt, evidently as an aid to self-protection and learning. This seems to be the purpose of the A-delta fast pain channels. Several conditions, however, can cause the pain system to give misleading impressions. Among these are prolonged or recurrent pain, physical and chemical irritation of nerves, within and outside of the central nervous system, disease of the nerves, or cutting of nerves. These conditions are touched upon further in Pain Science 2: Nociceptors and the Spine and Pain Science 3: Neuroscience and the Brain. Pains that appear to come from tissues that aren't damaged are generally called "referred pains," as in the phrase "the pain is referred there from elsewhere."
Slow pain doesn't give clear signals about precisely where it is. It is sensed as an ache in an area of the body, not at a distinct point. This is just the wayf your pain system works.
The McGill Pain Questionnaire, which I described above, includes a number of questions about pain affect: the mood, emotion, or feeling state that accompanies other sensations that we consider to be pain. Much evidence exists that pain affect is an important factor in pain suffering. Depression, irritability, degraded social functioning, other cognitive changes, and other features accompany long-term pain. The link between pain and these other conditions is probably not coincidental, yet these other features are often treated as if they are separate conditions. This raises some issues related to the medical needs of pain sufferers and about what pain "truly" is. See the upcoming section Co-morbidities, as well as The Challenge of Living in Pain and A Rational Model of Emotion and Pain to understand what the issues are and why they are important.
Pain is a complex phenomenon and there is much diversity in the types and flavors of pain. Pain that is transmitted by fast A-delta pathways has fundamental differences with pain transmitted through C pathways. Pain intensity is clearly important, but is not the only useful thing to know about someone's pains. Although pain isn't easy to talk about clearly, it can be communicated using standard English terms in a careful way, and through well-designed questionnaires. While some sensations are widely agreed to be effects of pain, other sensations and feeling states are not as clearly included when we say "pain."