Infant in military hospital.

Measuring Pain

Is your pain the same as my pain? And how much worse is a toothache than a mosquito bite? Do some people exaggerate their pain to get more drugs?

In a hospital or clinic you may have been asked to rate your pain on a scale of 1 to 10. Has it struck you that that scale does not answer the questions above?

For many sensations, our psychological impressions are more easily related to a physical stimulus through the findings of psychophysics. There are ways to judge the blackness that is achievable in your computer monitor, for example.

We need an objective measure of pain, too, but the straightforward options have been tried, and anyone who wants to make a better test may spend a decade ironing out wrinkles. Incidentally, among the common pain scales, the 1 to 10 pain scale has received limited validation (good test-retest reliability, p. 7), while the McGill Pain Questionnaire has undergone extensive validation like this. Validation is a tough challenge.

Some people endure pain willingly and it presents us with both a scaling issue and the ethical problem of misjudging the pain that others feel. An overprotected adult might think a sprained ankle rates a “10”, and a catastrophizing friend might do the same, while an athlete desperate to finish a tournament, like Kerri Strug, bears it willingly.

It may be that the only way to bypass such issues is to take pain down to the molecular level, which avoids emotional pain. (Contrary to what some neuroscientists write, though, I don’t think that every behavioral phenomenon can be reduced to the cells responsible.)

There are studies that have used brain images to estimate a person’s pain objectively. As with many other applications of brain imaging, though, early enthusiasm has given way to doubts and skepticism.

One problem is that validating brain measures of pain requires a look at how brain images correlate with other measures of pain, including self-reports. These correlations are weak, and some imaging enthusiasts advise believing the images, not the self-reports. And the question is not just whether brain images are correlated with other pain measures, but whether they are correlated with brain activity! I’d say that fMRI, at least, is building a reliable reputation, but the connection to pain scaling is still problematic. We’re likely to be stuck looking for patterns among multiple measures for a while yet. Meanwhile, the self-reports may help in bearing the pain!

The study of pain has a long history. The current favorite for objective assessment is still brain imaging, and there are promising candidates; but the search is still going on, extending even to genetic causes. (At upwards of 10,000 pounds, do you think an MRI machine might pose problems in the field?)

What we see in a brain image is a map of correlations between recordings at different angles, shown as bright or colored spots to indicate their magnitude. Some of the correlations accurately reflect brain activity that underlies a thought or behavior; some are spuriously generated by the equipment; and some statistically significant correlations are illusory.

Brain imaging has been immensely useful, but we have reasons to approach it critically. It’s possible to misinterpret an image. Experienced researchers and clinicians argue over the images, which can of course be used to turn up absurdities.

Pain can also cause our pupils to dilate, which can be measured objectively. Unfortunately, pain and low light levels are not the only influences to control for. Incidentally, there are “pupillometers” for sale that only measure the interpupillary distance from one pupil to the other: Good to know if you’re buying optical equipment, but pain measurement calls for more, with devices costing much more as well.

For a current review in more depth, please watch this videocast from NIH, entitled: “Measurement of Pain: Behavioral, Social and Biological Factors”. It was posted while this blog was being written, and I suspect that the URL is likely to change soon.

Self-portrait of Van Gogh smoking a pipe

BIO: There are different kinds of pain, and sometimes that nociceptive knee pain, or neuropathic or psychogenic or nociplastic pain, may linger long beyond its original cause and become chronic pain.

Chronic pain is not just acute pain that lasts a long time. Pain like a scraped knee is usually a symptom of something else, though psychological factors contribute. Chronic pain is not just persistence. New pain experiences may appear in the transition from acute to chronic pain, such as wind-up pain and allodynia. Since chronic pain may remain after the original injury has been cured, some pain specialists see pain itself as becoming a disease.

For many years pain was thought to originate in bare nerve endings called nociceptors. But it looks like some pain is sensed first by cells that almost always accompany neurons called glia, such as microglia, astrocytes and Schwann cells (there are others). In skin, for example, Schwann cells are almost certainly the first step in building pain. Neurons then transmit the pain message to the brain, where it can be experienced consciously and sometimes suppressed.

Increasingly we manage our own pain, though people vary widely, some muttering “embrace the suck” or “eat the pain”, while suffering persists.

PSYCHO: The psychophysics problem is to distinguish discriminative, “first” magnitudes of pain from affective, “second” magnitudes. In a hospital, scaling poses a different kind of task. Patients may minimize their pain out of bravado or exaggerate it to get more drugs. Scales are needed for patients with dementia and for children.

A numerical scale is not necessarily an objective scale because numbers, like words, are subject to interpretation. The interpretations reflect one’s culture, and scales can’t be objective when cultural changes rob them of their use*.

Nevertheless, numbers look reliable because non-numerical clinical judgment is always subject to bias. Objective measurement evaluates a person with statistics, charts and tables. Converging lines of evidence. Just not the 1-10 scale, please, because, you know, bias, in patients as well as clinicians.

I suppose we might agree that putting a number to one’s pain is at least better than a scale that flags only pain or no-pain, like finding the point at which you say “ouch”. It would even be desirable to specify the kind of pain we’re measuring and what a number 4 means across different patients. The psychophysical method of magnitude estimation is designed for this, and shows that for thermal pain (radiant heat on the skin–scroll down) people are very accurate judges, with an exponent of 1.

Pain is a sense, but it differs from other senses with its huge emotional component and individual variation in the “why” and “how”. Nevertheless, we can be distracted from pain and we can inhibit the pain message. One mechanism is known as gate control. As the pain message ascends from peripheral nerves to the brain, the brain can often block the message with descending commands.

In part, it’s a problem of attention. Pain grabs our attention, and this is good. Children who are insensitive to pain, or indifferent to pain, suffer for it. But having gotten the message, we don’t need it repeated. A strategy of tricking the brain can be a good one, with several ways to go about it.

Chronic pain (also known as persistent pain) is a particularly stiff challenge for the 1 to 10 pain scale because such pain is multidimensional, and though multi-tiered versions of the scale are being used in some places, such a scale would not address the psychological determinants of chronic pain.

It’s a puzzle why pain persistence is seen chiefly (exclusively?) in humans. Persistence can occur pathologically in other senses such as vision and touch.

The amazing aspect of persistent pain (to me) is that it may not only lodge in the brain but be controlled by the brain.

The complexity allows unscrupulous marketers to operate unchallenged in treating maladies such as persisting back pain**, which is heavily influenced by beliefs and placebos.

Other remedies are often developed using animal testing, and must wait for translational researchers to bridge the gap between discoveries in mice and human needs.

As many writers remind us, pain is private and subjective. Physicians need objective measures of pain, which are bodily biomarkers of distress, including autonomic measures and brain imaging.

Many writers also share a concern with the limitations of measures now in use.

*Because of the opioid crisis it’s harder to measure pain. And because we can’t measure pain, the opioid crisis worsens.

**One scale (p. 15) comprises 10 axes or dimensions, which sounds comprehensive but must take an age to administer.

SOCIAL: Pain is often described as a subjective experience, yet so is embarrassment, which does not prevent a blush from serving as a social signal.

The experience of pain is possibly contagious. Though I haven’t mentioned it before, social pain can be included with other varieties of the overall pain experience. Some drugs reduce both social and physical pain, though there are skeptics.

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