When you throb with pain...are you feeling the beat?

I think we've all had that kind of pain. A headache, maybe, or an injury. The kind of pain...that THROBS. You know what I mean. When I get one of those throbbing headaches, it's like every beat of my heart is pushing the blood up through my brain, and with the blood comes another throb of pain. I feel like my head throbs in time with my heartbeat, and will even try to relax and slow my heart beat down to at least slow the throbbing a little. It never works.

And it turns out there's a reason that it doesn't work. Because it turns out that pulsatile pain has nothing to do with heartrate. And for this study, all the authors needed was a heart rate monitor...and some very long-suffering dental patients.

Mirza et al. "Is There a Relationship between Throbbing Pain and Arterial Pulsations?" J Neuroscience, 2012.

I have to say the introduction to this paper freaked me out a bit. It describes throbbing pain as being correlated with the most severe types of pain, and correlates with severity of illness as well as progression of things like cancer. I guess I now know that my migraines can be classified as "severe", but...yikes. Ouch.

So what causes the throbbing sensation? The feeling I had of the pain throbbing in time with my heartbeat seems kind of...too simple, but in fact that was the prevailing scientific view: that when an area is in sever pain, the sensory neurons in that area become sensitized, and will respond to the pulsations of nearby blood vessels as blood is pumped from the heart. In fact, this view is so strongly held that dentists use it in practice, to determine whether an area is still viable or not.

But is this mechanism too simple? To find this out, the authors looked at some dental patients. In a previous study, they actually looked at migraineurs (who frequently complain of severe throbbing pain), and showed the throbbing there was not in time with the heartbeat, but since migraine itself is often a difficult thing to generalize (no two migraines are really ever the same, and symptoms vary widely from person to person), they decided to look at dental patients. Dental patients in pain, which I can only imagine made for some pleasant research subjects.

They got 29 dental patients, and asked them to describe their pain, and press a button in time with the throbbing sensation. At the same time, they took an arterial pulse rate from the earlobe (closest to the mouth, I guess). They excluded patients who showed excessively slow or irregular throbbing with no possible relation to heart rate. They then looked at the relationship between the throbbing rate and the pulse rate, including things like oscillation frequency.

What you can see here is one of the many representations of the data. The red line represents the predicted results, that throbbing rate should increase with pulse rate, but the blue line represents what they got: no correlation whatsoever. The average heart rate was 72 beats per minute, while the average throb was 44. The authors checked for any association with frequency (what if, say the ratio was 2:1 or something), but got no results.

However, this is not just a negative results paper, though the negative results are interesting enough. They also observed, by plotting the fractal analyses of the heart rate and throbbing rate, that the throbbing rate...has its own rhythm.

You can see the blue line (the throbbing rate) has it's own fractal law, which means that the rhythmic throbbing is (a) probably real and (b) probably physiologically controlled by something, presumably in the central nervous system. The authors don't know what, yet, but it could be and interesting new way to study throbbing pain, and to find out where it comes from.

I don't know about you guys, but this really makes me want to take up a timer and a heart rate monitor during my next migraine. If I can think to do it, anyway. It would be an unpleasant situation, but a cool bit of science to try on yourself!

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