Objective Pain Score Faces Criticism

Are you in pain? Are you sure? On a scale from zero to ten, where zero is no pain at all and ten is the worst pain imaginable, how much pain is it?

Invitations to rate your pain on some arbitrary scale of numerals, or on a display of smiley faces that range from happy to sad (if you're a child), remain the standard ways of assessing people's pain. If a doctor wants to know how much pain their patient is in, that's how they'll try to find out. And they do this for good reason: no one knows a person's pain better than the person whose pain it is.

And yet, ratings like these have their limitations. After all, people interpret their pains differently. Some make much of very little, giving high ratings to fairly light pains, while others make little of very much, giving low ratings to pretty bad pains. When a patient rates their pain a five out of ten, who's to say how much pain that five actually stands for (other than the patient)?

Luckily, we are promised that these problems with subjective pain ratings will soon be a matter of the past. Several labs around the world report that they are on the cusp of releasing the first objective pain measurement technology: devices that will be able to determine the type and intensity of a person's pain without having to rely on anyone's rating or interpretation at all.

These measurement devices differ in their specifics, but converge in kind. They track patterns in so-called "biomarkers" that correlate with pain experiences - such as the activation of certain nerve fibres, pupil dilation, or variations in blood flow - and compare these patterns with lots of data from people in pain. Doing so, these devices are meant to measure how much pain a person is in based on their biomarker profile.

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The upshots of this technology are expected to be tremendous: more accurate diagnostics, better testing of analgesic drugs, massive savings for the healthcare system, you name it. Objective pain measurement is meant to transform pain management as we know it.

But there's a catch, and it's a big one. Ask yourself: how did anyone ever figure out that these devices actually work? I mean, how can these researchers be sure that these patterns of nerve activation or those variations in blood flow correspond to that much pain? The answer may surprise you.

To test the accuracy of their devices, pain researchers evaluate their measurements by reference to the only glimpse of people's pain experiences they have access to: subjective pain ratings. That's right. The ultimate test for how good an "objective" pain measurement device truly is is to see how it stacks up against people's subjective ratings - the very ratings that were deemed so problematic that we wanted new ones.

The reason researchers do this is that they're caught in a catch-22: to verify that they have accurately measured a person's pain, they would have to know how much pain the person was in to begin with, which of course they don't - that's why they're developing a measurement device. What they do know, however, is how that person rates their pain, and so that's all they have to evaluate the accuracy of their measurements with.

But a device that predicts people's pain ratings based on their biomarkers is a far cry from an "objective pain measurement" technology. Its measurements can't tell us how much pain a person is in with any more accuracy, any less bias, or any more authority than a person's own rating. Why? Because it's trained on subjective pain ratings we had trouble interpreting in the first place.

A philosophical issue

The problem here has nothing to do with technology. It's not about how sophisticated your algorithms are, how advanced your equipment is, or how much research funding you got. It's about the philosophical issue that pain is a subjective experience with only one person who has access to it: the person in pain. Linking biomarkers to pain ratings will never make that access more public.

Should we be disappointed? I'm not so sure. If objective measures of pain existed, and if they were worth their salt, then they would come apart from people's own assessments of their pain. They'd have to, since they could hardly be any better or any more accurate if they came out just the same.

But if what I make of my pain and what a device makes of it is different, then whose assessment is the more important one here? And who is my doctor going to take more seriously in guiding their treatment recommendations: the flashy objective pain score or my little subjective rating? I'm glad I won't have to find out.

Laurenz Casser receives funding from the Leverhulme Trust.